Spectral Analysis M.Tech in Nanoscience and Technology, Kuvempu University An information for fulfilling the needs of the Master of Technology students of Nanoscience and Technology on the Principles, Instrumentation, Samples Graphs and Application areas of various Spectral Analysis Techniques. Useful for budding scientists and engineers in the field of Material Science- an indispensable guide for anybody wanting to know what-is-what in the study of scientific journals. This book compilation has around 30 techniques of most current utility in the field of Nanoscience. All the Information in this are compilation from various sources in internet. Compilation was done by 13 students of 2011-2013 batch.
Spectral Analysis
Contents AGAROSE GEL ELECTROPHORESIS Separation Agarose gel DNA BAND/SPECTRAL ANALYSIS Scoring pattern BET AND MULTIPOINT BET
7 7 7 8 9 10
Introduction
10
Theory
13
How does BET work?
14
Type I isotherm
14
Type II isotherm
15
Type III isotherm
15
Type IV isotherm
16
Type V isotherm
16
Calculations
16
Multi-point BET
17
Shortcomings of BET
19
The surface area determination of metal-organic frameworks
19
Concept
22
Cement paste
24
Activated Carbon
24
ELECTRON ENERGY LOSS SPECTROSCOPY (EELS)
26
Pressure measurements
29
Principle and Application of Imaging Electron Energy Loss Spectroscopy
30
Quantification of Elemental Mapping
32
ENERGY-DISPERSIVE X-RAY SPECTROSCOPY (EDS)
34
Principle
34
How does it work
34
Strengths
34
Limitations
35
Spectra: ELECTRON PARAMAGNETIC RESONANCE (EPR) Instrumentation
35 37 37
1
Spectral Analysis
Principle:
38
Spectral parameters
38
The g factor
39
Advantages
40
Spectral analysis:
41
EXAFS Theory of EXAFS: Edge:
42 42 42
EXAFS:
43
Experimental Procedure:
44
Sample Spectra of EXAFS
45
FLUORESCENCE SPECTROSCOPY
46
Why Use Fluorescence Spectroscopy?
46
Advantages of fluorescence:
46
Applications of fluorescence:
46
Fluorescence spectra:
47
FOURIER TRANSFORM INFRARED SPECTROSCOPY
48
What is FT-IR?
48
PRINCIPLE:
48
THEORY:
49
Number of vibrational modes: EXPERIMENTAL DETAILS OF IR spectroscopy:
49 50
Sample preparation:
50
Absorption bands:
51
Introduction
51
ATTENUATED TOTAL REFLECTANCE (ATR)
53
How an ATR accessory works
53
DIFFUSE REFLECTANCE
58
GAS CHROMTOGRAPHY
60
Schematic of a gas Chromatography
60
Working principle
60
HPLC(HIGH PERFORMANCE LIQUID CHROMATOGRAPHY)
63
Principle :
63
Instrumentation
63
2
Spectral Analysis
IMPEDANCE SPECTROSCOPY
67
Introduction
67
Sinusoidal representation of current and voltage
67
Measuring Technique
68
Principle
68
Theory
69
Plotting Impedance
70
Impedance Analysis for the Corrosion Potential
71
Applications
73
NEUTRON ACTIVATION ANALYSIS
74
Introduction:
74
The NAA Method
74
Neutrons
76
Measurement of Gamma Rays
77
Using Gamma-ray Counts to Calculate Element Concentration
80
Sensitivities Available by NAA
81
NEUTRON DIFFRACTION
82
Introduction to neutron scattering.
82
Basic properties of the Neutron.
82
Neutron scattering facilities.
83
Neutron scattering instruments and detectors.
83
Instrument.
84
Five reasons for using neutrons.
84
Spectra.
85
Neutron diffraction spectrum of a stainless steel temperature controlled membrane.
85
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
86
Instrumentation of NMR-Spectroscopy
86
Working principle:
87
Result and discussion
88
Acetophenone:
89
NON-DESTRUCTIVE EVALUATION/TESTING - NDE/NDT
93
Introduction to NDT
93
Definition of NDT
93
Uses of NDT Methods
93
3
Spectral Analysis
NDT/NDE Methods
94
Common Application of NDT
102
Reason for Choice of Non-destructive Analysis
104
MASS SPECTROMETRY Diagram of mass spectrometer WORKING PRINCIPLE The spectra of mass spectroscopy:(for aliphatic and aromatic compounds) Mร SSBAUER SPECTRA
105 106 106 107 110
PRINCIPLE
110
Mรถssbauer spectra of iron oxide sample prepared by different methods
113
Strengths
113
Mossbauer Spectra:
114
PHOTOLUMINESCENCE & FLUORESCENCE SPECTROSCOPY
116
Working Principle:
116
Instrumentation
118
Components of Spectrometer: Photoluminescense Uses:
118 119
Band Gap Determination:
119
Impurity Levels and Defect Detection:
119
Recombination Mechanisms:
119
Material Quality:
119
Special Features of Photoluminescence spectroscopy:
120
Photoluminescence Spectra :
120
POLYMERASE CHAIN REACTION
122
THEORY:
122
PCR principles and procedure:
122
Procedure: RADIATION DETECTORS
124 126
Radiation
126
Radioactivity
126
Ionizing Radiation
126
Alpha Rays:
126
Beta rays:
127
Gamma Rays:
128
4
Spectral Analysis
Half-Life:
129
Formulas for half-life in exponential decay
130
DETECTORS OF RADIATION
131
Scintillators:
131
Photon Electron Rejecting Alpha Liquid Scintillation (PERALS) spectrometry.
132
General Principles of Radiation Detection
133
Types of detectors:
133
Working Principle of Geiger-Muller Counters:
135
Scintillation Detectors
135
Working Principle of Scintillation Detector:
136
Spectroscopy
136
Semiconductor Detectors
137
Applications
137
WAVELENGTH-DISPERSIVE X-RAY SPECTROSCOPY (WDS)
139
Strengths
140
Limitations
140
Spectra
141
X-RAY FLUORESCENCE (XRF)
143
What is X-Ray Fluorescence (XRF)
143
Fundamental Principles of X-Ray Fluorescence (XRF)
143
X-Ray Fluorescence (XRF) Instrumentation
144
Applications
145
Strengths
146
Limitations
146
Spectra of XRF
147
X-RAY ABSORPTION SPECTROSCOPY Introduction: XANES
148 148 150
Theory:
150
Principle:
151
Experimental Procedure:
151
XANES Spectrum for Sulphur:
152
XANES Energy range:
152
Applications of XANES:
153
5
Spectral Analysis
X-RAY PHOTO ELECTRON SPECTROSCOPY (XPS)
154
Introduction
154
Principle
154
XPS Instrument
155
General setup
155
The X-ray source
156
The cylindrical mirror analyzer
156
The ion gun
157
The sample holder and stage
157
Identification of XPS peaks
157
XPS Spectrum
158
Applications
159
ZETA POTENTIOMETER
160
Introduction
160
Principle
160
Factors Affecting Zeta Potential
161
Instrumentation
161
Principle
161
Spectral Analysis
164
Application
164
6
Spectral Analysis
AGAROSE GEL ELECTROPHORESIS
Principle and Theory: Gel electrophoresis is a method used in clinical chemistry to separate proteins by charge and or size (IEF agarose, essentially size independent) and in biochemistry and molecular biology to separate a mixed population of DNA and RNA fragments by length, to estimate the size of DNA and RNA fragments or to separate proteins by charge. Nucleic acid molecules are separated by applying an electric field to move the negatively charged molecules through an agarose matrix. Shorter molecules move faster and migrate farther than longer ones because shorter molecules migrate more easily through the pores of the gel. This phenomenon is called sieving. Proteins are separated by charge in agarose because the pores of the gel are too large to sieve proteins. Gel electrophoresis can also be used for separation of nanoparticles. Gel electrophoresis uses a gel as an anticonvective medium and or sieving medium during electrophoresis, the movement of a charged particle in an electrical field. Gels suppress the thermal convection caused by application of the electric field, and can also act as a sieving medium, retarding the passage of molecules; gels can also simply serve to maintain the finished separation, so that a post electrophoresis stain can be applied. DNA Gel electrophoresis is usually performed for analytical purposes, often after amplification of DNA via PCR, but may be used as a preparative technique prior to use of other methods such as mass spectrometry, RFLP, PCR, cloning, DNA sequencing, or Southern blotting for further characterization.
Separation Agarose gel
1. Gel electrophoresis is a widely used technique for the analysis of nucleic acids and proteins.
Agarose gel electrophoresis is routinely used for the preparation and
analysis of DNA.
7
Spectral Analysis
2. Gel electrophoresis is a procedure that separates molecules on the basis of their rate of movement through a gel under the influence of an electrical field. We will be using agarose gel electrophoresis to determine the presence and size of PCR products 3. DNA are nagetively charged. 4. When placed in an electrical field, DNA will migrate toward the positive pole (anode). 5. An agarose gel is used to slow the movement of DNA and separate by size. 6. Small DNA move faster than large DNA.
DNA BAND/SPECTRAL ANALYSIS
Figure 1:Electrophoresis of PCR-amplified DNA fragments. (1) Father. (2) Child. (3) Mother. The child has inherited some, but not all of the fingerprint of each of its parents, giving it a new, unique fingerprint.
8
Spectral Analysis
Scoring pattern
1. Scoring pattern can be done by considering O(Zero) for absence & 1(One) for presence by visualizing the obtained gel pattern. 2. Analysis was conducted on STATISTICA 4.5 software. The program used was cluster analysis joining (tree clustering) with raw input data. The main parameters which guided the joining (tree clustering) process linkage rule is unweighted pair group average (UPGA) and the distance was computed from raw data using Euclidean distance.
Scored data from gel pattern RM264 A
0
0
0
0
0
0
0
1
1
B
1
1
1
1
1
1
1
0
0
A
0
0
0
0
1
0
0
0
0
B
1
1
1
1
0
1
1
1
1
1
1
1
1
1
1
1
1
1
RM212
RM 171 A
Figure 2:This table shows the scoring pattern is done from the obtained DNA band from given above picture and hence used for the Diversity analysis of Landraces of Rice.
9
Spectral Analysis
BET AND MULTIPOINT BET
Introduction During the past decade, spectacular progress has been made in the development of new materials. Among these are nanostructured materials. It is well known that nanometre-sized particles, because of their small size and high specific surface area, display many unique properties; similarly, mesoporous solids, due to their large internal surface area and small pore size, have found great utility . If we assemble the nano-scaled particles (semiconductors, compounds or metals) into the pores of mesoporous solids, a new material will be formed. This nanoparticle-loaded porous solid will undoubtedly possess some unique properties of both the nanoparticle and the mesoporous solid. This new material, however, is significantly different from the recently extensively reported glass–metal colloid composites (films), organic–inorganic Nano composites (films) or other Nano composite films. The large number of pores in the mesoporous solid, of the order of 1019 pores g−1, results in a large surface area, in some cases reaching 900 m2 g−1 . Because the pores are small, interactions at the pore– nanoparticle interface are likely to be extensive and may significantly alter the physicochemical properties of the particles within pores. On the other hand, because all the pores in the porous solids are interconnected and open to the ambient, the nanosized particles located within the pores are also in contact with the ambient air. The nanoparticles within the pores are small in size and chemically active .Therefore; there inevitably exist the interactions between the ambient and the nano-scale phase, especially metal particles. So this new type of composite material will have properties that neither the nanoparticle nor the mesoporous solid possess. For example, the dispersion of semiconductor ultrafine particles in the pores of porous glass produces optical switching and optical nonlinearity effects ; polyaniline filaments within the mesoporous channel host (aluminosilicate) have significant conductivity, and this demonstration of conjugated polymer with mobile charge carriers in nanometre channels represents a step toward the design of nanometre electronic devices. A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase. Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic head group and a hydrophobic tail. Since the 1980s a large number of other materials have been employed to produce Langmuir monolayers, some of which are semi-amphiphilic, including macromolecules such as polymers. Langmuir monolayers are extensively studied for the fabrication of Langmuir-Blodgett film (LB films), which are
10
Spectral Analysis
formed by transferred monolayers on a solid substrate. A Gibbs monolayer or soluble monolayer is a monolayer formed by a compound that is soluble in one of the phases separated by the interface on which the monolayer is formed.
The Langmuir equation (also known as the Langmuir isotherm, Langmuir adsorption equation or Hill-Langmuir equation) relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature. The equation was developed by Irving Langmuir in 1916. The equation is stated as:
θ is the fractional coverage of the surface, P is the gas pressure or concentration, α is a constant. The constant α is the Langmuir adsorption constant and increases with an increase in the binding energy of adsorption and with a decrease in temperature. The Langmuir theory relates the monolayer adsorption of gas molecules (Figure 3), also called adsorbates, onto a solid surface to the gas pressure of a medium above the solid surface at a fixed temperature to Equation 1, where θ is the fractional cover of the surface, P is the gas pressure and α is a constant. Equation 1:
Figure 3: Schematic of the adsorption of gas molecules onto the surface of a sample showing (a) the monolayer adsorption model assumed by the Langmuir theory and (b) s the multilayer adsorption model assumed by the BET theory.
11
Spectral Analysis
The Langmuir theory is based on the following assumptions: All surface sites have the same adsorption energy for the adsorbate, which is usually argon, krypton or nitrogen gas. The surface site is defined as the area on the sample where one molecule can adsorb onto. Adsorption of the solvent at one site occurs independently of adsorption at neighboring sites.
Activity of adsorbate is directly proportional to its concentration.
Adsorbates form a monolayer.
Each active site can be occupied only by one particle.
The Langmuir theory has a few flaws that are addressed by the BET theory. Surface area [Brunauer– Emmett–Teller (BET)] measurement is generally a good surrogate for determining average particle size. The BET theory extends the Langmuir theory to multilayer adsorption (Figure 3) with three additional assumptions: 1.Gas molecules will physically adsorb on a solid in layers infinitely. 2.The different adsorption layers do not interact. 3.The theory can be applied to each layer.
Figure 4:BET surface area plot
12
Spectral Analysis
Figure 5: FCC catalyst adsorption isotherm
Theory
Figure 6:BET region with both adsorption and desorption isotherm
13
Spectral Analysis
How does BET work? Adsorption is defined as the adhesion of atoms or molecules of gas to a surface. It should be noted that adsorption is not confused with absorption, in which a fluid permeates a liquid or solid. The amount of gas adsorbed depends on the exposed surface area but also on the temperature, gas pressure and strength of interaction between the gas and solid. In BET surface area analysis, nitrogen is usually used because of its availability in high purity and its strong interaction with most solids. Because the interaction between gaseous and solid phases is usually weak, the surface is cooled using liquid N2 to obtain detectable amounts of adsorption. Known amounts of nitrogen gas are then released stepwise into the sample cell. Relative pressures less than atmospheric pressure is achieved by creating conditions of partial vacuum. After the saturation pressure, no more adsorption occurs regardless of any further increase in pressure. Highly precise and accurate pressure transducers monitor the pressure changes due to the adsorption process. After the adsorption layers are formed, the sample is removed from the nitrogen atmosphere and heated to cause the adsorbed nitrogen to be released from the material and quantified. The data collected is displayed in the form of a BET isotherm, which plots the amount of gas adsorbed as a function of the relative pressure. There are five types of adsorption isotherms possible. Type I isotherm
Type I is a pseudo-Langmuir isotherm because it depicts monolayer adsorption (Figure 7). A type I isotherm is obtained when P/Po< 1 and c > 1 in the BET equation, where P/Po is the partial pressure value and c is the BET constant, which is related to the adsorption energy of the first monolayer and varies from solid to solid. The characterization of microporous materials, those with pore diameters less than 2 nm, gives this type of isotherm.
14
Spectral Analysis
Figure 7: The isotherm plots the volume of gas adsorbed onto the surface of the sample as pressure increases. Adapted from S. Brunauer L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. Soc., 1940, 62, 1723. Type II isotherm
A type II isotherm (Figure 8) is very different than the Langmuir model. The flatter region in the middle represents the formation of a monolayer. A type II isotherm is obtained when c > 1 in the BET equation. This is the most common isotherm obtained when using the BET technique. At very low pressures, the micropores fill with nitrogen gas. At the knee, monolayer formation is beginning and multilayer formation occurs at medium pressure. At the higher pressures, capillary condensation occurs.
Figure 8: The isotherm plots the volume of gas adsorbed onto the surface of the sample as pressure increases. Adapted from S. Brunauer, L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. Soc., 1940, 62, 1723. Type III isotherm
A type III isotherm (Figure 9) is obtained when the c < 1 and shows the formation of a multilayer. Because there is no asymptote in the curve, no monolayer is formed and BET is not applicable.
15
Spectral Analysis
Figure 9: Brunauer, L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. Soc., 1940, 62, 1723. Type IV isotherm
Type IV isotherms (Figure 10) occur when capillary condensation occurs. Gases condense in the tiny capillary pores of the solid at pressures below the saturation pressure of the gas. At the lower pressure regions, it shows the formation of a monolayer followed by a formation of multilayers. BET surface area characterization of mesoporous materials, which are materials with pore diameters between 2 - 50 nm, gives this type of isotherm.
Figure 10: Brunauer, L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. Soc., 1940, 62, 1723. Type V isotherm
Type V isotherms (Figure 11) are very similar to type IV isotherms and are not applicable to BET.
Figure 11: Brunauer L. S. Deming, W. E. Deming, and E. Teller, J. Am. Chem. Soc., 1940, 62, 1723.
Calculations The BET equation, Equation 2, uses the information from the isotherm to determine the surface area of the sample, where X is the weight of nitrogen adsorbed at a given relative
16
Spectral Analysis
pressure (P/Po), Xmis monolayer capacity, which is the volume of gas adsorbed at standard temperature and pressure (STP), and C is constant. STP is defined as 273 K and 1 atm. Equation 2:
Multi-point BET Ideally five data points, with a minimum of three data points, in the P/P0 range 0.025 to 0.30 should be used to successfully determine the surface area using the BET equation. At relative pressures higher than 0.5, there is the onset of capillary condensation, and at relative pressures that are too low, only monolayer formation is occurring. When the BET equation is plotted, the graph should be linear with a positive slope. If such a graph is not obtained, then the BET method was insufficient in obtaining the surface area.
The slope and y-intercept can be obtained using least squares regression.
The monolayer capacity Xm can be calculated with Equation 3.
Once Xm is determined, the total surface area St can be calculated with the following
equation, where Lav is Avogadro’s number, Am is the cross sectional area of the adsorbate and equals 0.162 nm2 for an absorbed nitrogen molecule, and Mv is the molar volume and equals 22414 mL, Equation 4. Equation 3:
Equation 4:
Single point BET can also be used by setting the intercept to 0 and ignoring the value of C. The data point at the relative pressure of 0.3 will match up the best with a multipoint BET. Single point BET can be used over the more accurate multipoint BET to determine the appropriate relative pressure range for multi-point BET. Sample preparation and experimental setup Prior to any measurement the sample must be degassed to remove water and other contaminants before the surface area can be accurately measured. Samples are degassed in a vacuum at high temperatures. The highest temperature possible that will not damage the sample’s structure is usually chosen in order to shorten the degassing time. IUPAC
17
Spectral Analysis
recommends that samples be degassed for at least 16 hours to ensure that unwanted vapors and gases are removed from the surface of the sample. Generally, samples that can withstand higher temperatures without structural changes have smaller degassing times. A minimum of 0.5 g of sample is required for the BET to successfully determine the surface area. Samples are placed in glass cells to be degassed and analyzed by the BET machine. Glass rods are placed within the cell to minimize the dead space in the cell. Sample cells typically come in sizes of 6, 9 and 12 mm and come in different shapes. 6 mm cells are usually used for fine powders, 9 mm cells for larger particles and small pellets and 12 mm are used for large pieces that cannot be further reduced. The cells are placed into heating mantles and connected to the outgas port of the machine. After the sample is degassed, the cell is moved to the analysis port (Figure 12). Dewars of liquid nitrogen are used to cool the sample and maintain it at a constant temperature. A low temperature must be maintained so that the interaction between the gas molecules and the surface of the sample will be strong enough for measurable amounts of adsorption to occur. The adsorbate, nitrogen gas in this case, is injected into the sample cell with a calibrated piston. The dead volume in the sample cell must be calibrated before and after each measurement. To do that, helium gas is used for a blank run, because helium does not adsorb onto the sample.
Figure 12: Schematic representation of the BET instrument. The degasser is not shown.
18
Spectral Analysis
Figure 13:Typical commercial BET measuring instrument
Shortcomings of BET The BET technique has some disadvantages when compared to NMR, which can also be used to measure the surface area of nanoparticles. BET measurements can only be used to determine the surface area of dry powders. This technique requires a lot of time for the adsorption of gas molecules to occur. A lot of manual preparation is required.
The surface area determination of metal-organic frameworks The BET technique was used to determine the surface areas of metal-organic frameworks (MOFs), which are crystalline compounds of metal ions coordinated to organic molecules. Possible applications of MOFs, which are porous, include gas purification and catalysis. An isoreticular MOF (IRMOF) with the chemical formula Zn4O(pyrene-1,2-dicarboxylate)3 (Figure 14) was used as an example to see if BET could accurately determine the surface area of microporous materials. The predicted surface area was calculated directly from the
19
Spectral Analysis
geometry of the crystals and agreed with the data obtained from the BET isotherms. Data was collected at a constant temperature of 77 K and a type II isotherm (Figure 15) was obtained.
Figure 14: The structure of catenated IRMOF-13. Orange and yellow represent noncatenated pore volumes. Green represents catenated pore volume.
Figure 15: The BET isotherms of the zeolites and metal-organic frameworks. IRMOF13 is symbolized by the black triangle and red line. Adapted from Y.S. Bae, R.Q. Snurr, and O. Yazaydin, Langmuir, 2010, 26, 5478. The isotherm data obtained from partial pressure range of 0.05 to 0.3 is plugged into the BET equation, Equation 2, to obtain the BET plot (Figure 16).
20
Spectral Analysis
Figure 16: BET plot of IRMOF-13 using points collected at the pressure range 0.05 to 0.3. The equation of the best-fit line and R2 value are shown. Adapted from Y.S. Bae, R.Q. Snurr, and O. Yazaydin, Langmuir, 2010, 26, 5479. Using Equation 5, the monolayer capacity is determined to be 391.2 cm3/g. Equation 5:
Now that Xm is known, then Equation 6 can be used to determine that the surface area is 1702.3 m2/g. Equation 6:
A plot of relative pressure vs. volume adsorbed obtained by measuring the amount of N2 gas that adsorbs onto the surface of interest (the 'sorbate'), and the subsequent amount that desorbs at a constant temperature gives absorption and desorption isotherms BET theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of a material. In 1938, Stephen Brunauer, Paul Hugh Emmett, and Edward Teller published an article about the BET theory in a journal for the first time; "BET" consists of the first initials of their family names.
21
Spectral Analysis
Concept
The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses: (a) gas molecules physically adsorb on a solid in layers infinitely; (b) there is no interaction between each adsorption layer; and (c) the Langmuir theory can be applied to each layer. The resulting BET equation is expressed by (Equation 7): Equation 7
and
are the equilibrium and the saturation pressure of adsorbates at the temperature of
adsorption, is the adsorbed gas quantity (for example, in volume units), and is the monolayer adsorbed gas quantity. is the BET constant, which is expressed by (Equation 8): Equation 8
is the heat of adsorption for the first layer, and is that for the second and higher layers and is equal to the heat of liquefaction.
22
Spectral Analysis
Figure 17:Equation (1) is an adsorption isotherm and can be plotted as a straight line with
on the y-axis and
on the x-axis according to
experimental results. This plot is called a BET plot. The linear relationship of this equation is maintained only in the range of . The value of the slope
and the y-intercept
of the line are used to calculate the monolayer
adsorbed gas quantity
and the BET constant . The following equations can be used:
Equation 9
Equation 10
The BET method is widely used in surface science for the calculation of surface areas of solids by physical adsorption of gas molecules. A total surface area and a specific surface area
are evaluated by the following equations:
Equation 11
where
is in units of volume which are also the units of the molar volume of the adsorbate
gas
23
Spectral Analysis
Equation 12
: Avogadro's number, : adsorption cross section of the adsorbing species, : molar volume of adsorbate gas : mass of adsorbent (in g)
Cement paste By application of the BET theory it is possible to determine the inner surface of hardened cement paste. If the quantity of adsorbed water vapor is measured at different levels of relative humidity a BET plot is obtained. From the slope
and y-intersection
on the plot it
is possible to calculate and the BET constant . In case of cement paste hardened in water (T=97°C), the slope of the line is
and the y-intersection
;
from this follows
From this the specific BET surface area
can be calculated by use of the above
mentioned equation (one water molecule covers
). It follows thus
which means that hardened cement paste has an inner surface of 156 square meters per g of cement.
Activated Carbon For example, activated carbon, which is a strong adsorbate and usually has an adsorption cross section
of 0.16 nm2 for nitrogen adsorption at liquid nitrogen temperature, is revealed
from experimental data to have a large surface area around 3000 m² g-1. Moreover, in the field of solid catalysis, the surface area of catalysts is an important factor in catalytic activity. Porous inorganic materials such as mesoporous silica and layer clay minerals have high
24
Spectral Analysis
surface areas of several hundred m² g-1 calculated by the BET method, indicating the possibility of application for efficient catalytic materials.
25
Spectral Analysis
ELECTRON ENERGY LOSS SPECTROSCOPY (EELS) The technique was developed by James Hillier and RF Baker in the mid 1940s but was not widely used over the next 50 years, only becoming more widespread in research in the 1990s due to advances in microscope instrumentation and vacuum technology. With modern instrumentation becoming widely available in laboratories worldwide, the technical and scientific developments from the mid 1990s have been rapid. The technique is able to take advantage of modern aberration-corrected probe forming systems to attain spatial resolutions down to ~0.1 nm, while with a monochromated electron source and/or careful deconvolution the energy resolution can be 100 meV or better. This has enabled detailed measurements of the atomic and electronic properties of single columns of atoms, and in a few cases, of single atoms. In electron energy loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic energies.Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected. The amount of energy loss can be measured via an electron spectrometer and interpreted in terms of what caused the energy loss. Inelastic interactions include phonon excitations, inter and intra band transitions, Plasmon excitations, inner shell ionizations, and Ä&#x152;erenkov radiation. The inner-shell ionizations are particularly useful for detecting the elemental components of a material. For example, one might find that a larger-than-expected number of electrons come through the material with 285 eV less energy than they had when they entered the material.
26
Spectral Analysis
Figure 18:Schematic diagram of an EELS It so happens that this is about the amount of energy needed to remove an inner-shell electron from a carbon atom. This can be taken as evidence that there is a significant amount of carbon in the part of the material that is being hit by the electron beam. With some care, and looking at a wide range of energy losses, one can determine the types of atoms, and the numbers of atoms of each type, being struck by the beam. The scattering angle (that is, the amount that the electron's path is deflected) can also be measured, giving information about the dispersion relation of whatever material excitation caused the inelastic scattering. EDX excels at identifying the atomic composition of a material, is quite easy to use, and is particularly sensitive to heavier elements. EELS has historically been a more difficult technique but is in principle capable of measuring atomic composition, chemical bonding, valence and conduction band electronic properties, surface properties, and element-specific pair distance distribution functions. EELS tends to work best at relatively low atomic numbers, where the excitation edges tend to be sharp, well-defined, and at experimentally accessible energy losses (the signal being very weak beyond about 3 keV energy loss). EELS is perhaps best developed for the elements ranging from carbon through the 3d transition metals (from scandium to zinc).For carbon, an experienced spectroscopist can tell at a glance the differences among diamond, graphite, amorphous carbon, and "mineral" carbon (such as the
27
Spectral Analysis
carbon appearing in carbonates). The spectra of 3d transition metals can be analyzed to identify the oxidation states of the atoms.
Figure 19:Typical spectra obtained in EELS Cu(I), for instance, has a different so-called "white-line" intensity ratio than does Cu(II). This ability to "fingerprint" different forms of the same element is a strong advantage of EELS over EDX. The difference is mainly due to the difference in energy resolution between the two techniques (~1 eV or better for EELS, perhaps a few times ten eV for EDX).There are several basic flavors of EELS, primarily classified by the geometry and by the kinetic energy of the incident electrons (typically measured in kiloelectron-volts, or keV). Probably the most common today is transmission EELS, in which the kinetic energies are typically 100 to 300 keV and the incident electrons pass entirely through the material sample.Usually this occurs in a transmission electron microscope (TEM), although some dedicated systems exist which enable extreme resolution in terms of energy and momentum transfer at the expense of spatial resolution.Other flavors include reflection EELS (including reflection high-energy electron energy-loss spectroscopy (RHEELS), typically at 10 to 30 keV) and aloof EELS (sometimes called near-field EELS, in which the electron beam does not in fact strike the sample but instead interacts with it via the long-ranged Coulomb interaction Aloof EELS is particularly sensitive to surface properties but is limited to very small energy losses such as those associated with surface plasmons or direct interband transitions).Within transmission EELS, the technique is further subdivided into valence EELS (which measures plasmons and interband transitions) and inner-shell
28
Spectral Analysis
ionization EELS (which provides much the same information as x-ray absorption spectroscopy, but from much smaller volumes of material). The dividing line between the two, while somewhat ill-defined, is in the vicinity of 50 eV energy loss.High resolution electron energy loss spectroscopy, in which the electron beam is 1eV to 10eV, and highly monochromatic. EELS allows quick and reliable measurement of local thickness in transmission electron microscopy The most efficient procedure is the following Measure the energy loss spectrum in the energy range about 200 eV (wider better). Such measurement is quick (milliseconds) and thus can be applied to materials normally unstable under electron beam. Analyse the spectrum: (i) extract zero-loss peak (ZLP) using standard routines; (ii) calculate integrals under the ZLP (I0) and under the whole spectrum (I). The thickness t is calculated as mfp*ln(I/I0). Here mfp is the mean free path of electron inelastic scattering, which has recently been tabulated for most elemental solids and oxides The spatial resolution of this procedure is limited by the plasmon localization and is about 1 nm, meaning that spatial thickness maps can be measured in scanning transmission electron microscopy with ~1 nm resolution.
Pressure measurements The intensity and position of low-energy EELS peaks are affected by pressure. This fact allows mapping local pressure with ~1 nm spatial resolution. •
Peak shift method is reliable and straightforward. The peak position is calibrated by independent (usually optical) measurement using a diamond anvil cell. However, the spectral resolution of most EEL spectrometers (0.3-2 eV, typically 1eV) is often too crude for the small pressure-induced shifts. Therefore, the sensitivity and accuracy of this method is relatively poor. Nevertheless, pressures as small as 0.2 GPa inside helium bubbles in aluminum have been measured.
29
Spectral Analysis
ď&#x201A;ˇ
Peak intensity method relies on pressure-induced change in the intensity of dipoleforbidden transitions. Because this intensity is zero for zero pressure the method is relatively sensitive and accurate. However, it requires existence of allowed and forbidden transitions of similar energies and thus is only applicable to specific systems, e.g., Xe bubbles in aluminum
Principle and Application of Imaging Electron Energy Loss Spectroscopy When electron beam is incident into specimen, a part of the electrons is inelastically scattered and loses a part of the energy. Elemental composition and atomic bonding state can be determined by analysing the energy with the spectroscope attached under the electron microscope (Electron Energy Loss Spectroscopy). Because the analysing region can be selected from a part of the enlarged electron microscopic image, one can analyse very small region. Moreover, by selecting electrons with a specific loss energy by a slit so as to image them, element distribution in specimen can be visualized (Elemental Mapping). Elemental Analysis by EELS
Figure 20:Elemental analysis in spectrum of EELS
30
Spectral Analysis
This shows EELS spectrum from the region of 1 Îźm diameter of iron tetraphenylporphyrin monochloride crystal. Integral peak intensity is proportional to the number of each atom in the measured region. From the intensity and the cross-section of inelastic scattering, the elemental ratio is determined as C:N:Cl:Fe = 42.9:3.9:1.1:0.9. This value is corresponding well to the expected molecular composition (43:4:1:1). This result is obtained from the sample weight of 1X10-13 g. An Example of Elemental Analysis by EELS
Figure 21:This shows EELS spectrum from the region of 1 Îźm diameter of iron tetraphenylporphyrin monochloride crystal. Integral peak intensity is proportional to the number of each atom in the measured region. From the intensity and the cross-section of inelastic scattering, the elemental ratio is determined as C:N:Cl:Fe = 42.9:3.9:1.1:0.9. This value is corresponding well to the expected molecular composition (43:4:1:1). This result is obtained from the sample weight of 1X10-13 g.
31
Spectral Analysis
Quantification of Elemental Mapping
Figure 22:The intensity of elemental map of thin film specimen is proportional to the number of existence atom. Photo (a) is the elastic high resolution electron microscopic image of carbon nanotube. (b) is the carbon map at the same region.As shown in the energy loss spectrum of (c), the intensity of carbon K-shell exitation superposes on the monotonous background component. This background component is removed by an extrapolation, which was decided from two different energy images taken at energyes before the absorption edge. The intensity profile of carbon map perpendicular to the tube axis is shown in figure (d). The intensity profile corresponds well to the calculated number distribution of carbon atom (solid line) based on the size and the shape of nanotube. The intensity dip at center part is corresponding to 20 carbon atoms.
32
Spectral Analysis
Table 1: Energies Ek and types of edge in loss range 50 to 2000 eV. Element
Z
EK(eV)
Element
Z
EL23(eV)
Li
3
55
Al
13
73
Be
4
110
Si
14
100
B
5
188
P
15
136
C
6
284
S
16
165
N
7
400
Cl
17
202
O
8
532
Ar
18
250
F
9
684
K
19
296
Ne
10
865
Ca
20
348
Na
11
1075
Sc
21
405
Mg
12
1305
Ti
22
459
Al
13
1510
V
23
517
Si
14
1832
Cr
24
580
Mn
25
645
Fe
26
714
Co
27
786
Ni
28
863
Cu
29
941
Zn
30
1031
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Spectral Analysis
ENERGY-DISPERSIVE X-RAY SPECTROSCOPY (EDS) Principle Interaction of an electron beam with a sample target produces a variety of emissions, including x-rays. An energy-dispersive (EDS) detector is used to separate the characteristic xrays of different elements into an energy spectrum, and EDS system software is used to analyze the energy spectrum in order to determine the abundance of specific elements. EDS can be used to find the chemical composition of materials down to a spot size of a few microns, and to create element composition maps over a much broader raster area. Together, these capabilities provide fundamental compositional information for a wide variety of materials.
How does it work EDS systems are typically integrated into either an SEM or EPMA instrument. EDS systems include a sensitive x-ray detector, a liquid nitrogen dewar for cooling, and software to collect and analyze energy spectra. The detector is mounted in the sample chamber of the main instrument at the end of a long arm, which is itself cooled by liquid nitrogen. The most common detectors are made of Si(Li) crystals that operate at low voltages to improve sensitivity, but recent advances in detector technology make available so-called "silicon drift detectors" that operate at higher count rates without liquid nitrogen cooling. An EDS detector contains a crystal that absorbs the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The xray absorption thus converts the energy of individual x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element. Strengths ď&#x201A;ˇ
When used in "spot" mode, a user can acquire a full elemental spectrum in
only a few seconds. Supporting software makes it possible to readily identify peaks, which makes EDS a great survey tool to quickly identify unknown phases prior to quantitative analysis. ď&#x201A;ˇ
EDS can be used in semi-quantitative mode to determine chemical
composition by peak-height ratio relative to a standard.
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Spectral Analysis
Limitations
There are energy peak overlaps among different elements, particularly those
corresponding to x-rays generated by emission from different energy-level shells (K, L and M) in different elements. For example, there are close overlaps of MnKα and Cr-Kβ, or Ti-Kα and various L lines in Ba. Particularly at higher energies, individual peaks may correspond to several different elements; in this case, the user can apply deconvolution methods to try peak separation, or simply consider which elements make "most sense" given the known context of the sample.
Because the wavelength-dispersive (WDS) method is more precise and
capable of detecting lower elemental abundances, EDS is less commonly used for actual chemical analysis although improvements in detector resolution make EDS a reliable and precise alternative.
EDS cannot detect the lightest elements, typically below the atomic number
of Na for detectors equipped with a Be window. Polymer-based thin windows allow for detection of light elements, depending on the instrument and operating conditions.
Spectra:
Figure 23: Metallic Au NP's synthesized using aqueous clove buds solution.
35
Spectral Analysis
A typical EDS spectrum of Au nanoparticles as a plot of x-ray counts vs. energy in keV. They are narrow and readily resolved. Presence of carbon and oxygen indicates that the extracellular organic molecules are adsorbed on the surface of the metallic nanoparticles. The appearance of aluminium (Al) is because of the aluminium grid base used for the analysis.
36
Spectral Analysis
ELECTRON PARAMAGNETIC RESONANCE (EPR) Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion. The basic physical concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of spins of atomic nuclei. Because most stable molecules have all their electrons paired, the EPR technique is less widely used than NMR. However, this limitation to paramagnetic species also means that the EPR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to EPR spectra.
Instrumentation EPR spectroscopy can be carried out by either by varying the magnetic field and holding the frequency constant by varying the frequency and holding the magnetic field constant. The majority of EPR spectrometers are in the range of 8-10 GHz (X-band), though there are spectrometers which work at lower and higher fields: 1-2 GHz (L-band) and 2-4 GHz (S-band), 35 GHz (Q-band) and 95 GHz (W-band). EPR spectrometers work by generating microwaves from a source (typically a klystron), sending them through an attenuator, and passing them on to the sample, which is located in a microwave cavity(Figure 24). Microwaves are reflected back from the cavity and routed to the detector diode, where the signal comes out as a decrease in current at the detector analogous to absorption of microwaves by the sample.
. Figure 24:Block diagram of a typical EPR spectrometer. An unpaired electron can move between the two energy levels by either absorbing or emitting electromagnetic radiation of energy ε = hν such that the resonance condition, ε = ΔE, is
37
Spectral Analysis
obeyed. Substituting in ε = hν and ΔE = geμBB0 leads to the fundamental equation of EPR spectroscopy: hν = geμBB0. Experimentally, this equation permits a large combination of frequency and magnetic field values, but the great majority of EPR measurements are made with microwaves in the 9000 – 10000 MHz (9 – 10 GHz) region, with fields corresponding to about 3500 G (0.35 T).
Principle: EPR spectra can be generated by either varying the photon frequency incident on a sample while holding the magnetic field constant, or doing the reverse. In practice, it is usually the frequency which is kept fixed. A collection of paramagnetic centers, such as free radicals, is exposed to microwaves at a fixed frequency. By increasing an external magnetic field, the gap between the ms = +1/2 and ms = −1/2 energy states is widened until it matches the energy of the microwaves, as represented by the double-arrow in the diagram above. At this point the unpaired electrons can move between their two spin states. Since there typically are more electrons in the lower state, due to the Maxwell-Boltzmann distribution, there is a net absorption of energy, and it is this absorption which is monitored and converted into a spectrum. For a single electron, the two spin states have the same energy in the absence of a magnetic field. The energies of spin states diverge linearly as the magnetic field increases. Therefore, without a magnetic field, there is no energy difference to measure. The measured energy difference depends linearly on the magnetic field. The energy differences studied in EPR spectroscopy are due to the interaction of unpaired electrons in the sample with an external magnetic field produced by the EPR spectrometer. This effect is called Zeeman Effect. A strong magnetic field B is applied to a material containing paramagnetic species.
Spectral parameters In real systems, electrons are normally not solitary, but are associated with one or more atoms. There are several important consequences of this: 7.
An unpaired electron can gain or lose angular momentum, which can change
the value of its g-factor, causing it to differ from ge. This is especially significant for chemical systems with transition-metal ions. 8.
If an atom with which an unpaired electron is associated has a non-zero
nuclear spin, then its magnetic moment will affect the electron. This leads to the
38
Spectral Analysis
phenomenon of hyperfine coupling, analogous to J-coupling in NMR, splitting the EPR resonance signal into doublets, triplets and so forth. 9.
Interactions of an unpaired electron with its environment influence the shape
of an EPR spectral line. Line shapes can yield information about, for example, rates of chemical reactions. 10.
The g-factor and hyperfine coupling in an atom or molecule may not be the
same for all orientations of an unpaired electron in an external magnetic field. This anisotropy depends upon the electronic structure of the atom or molecule (e.g., free radical) in question, and so can provide information about the atomic or molecular orbital containing the unpaired electron. The g factor The g-factor can give information about a paramagnetic center's electronic structure. An unpaired electron responds not only to a spectrometer's applied magnetic field B0 but also to any local magnetic fields of atoms or molecules. The effective field Beff experienced by an electron is thus written Equation 13
where σ includes the effects of local fields (σ can be positive or negative). Therefore, the hν = geμBBeff resonance condition (above) is rewritten as follows: Equation 14
The quantity ge(1 – σ) is denoted g and called simply the g-factor, so that the final resonance equation becomes Equation 15
This last equation is used to determine g in an EPR experiment by measuring the field and the frequency at which resonance occurs. If g does not equal ge the implication is that the ratio of the unpaired electron's spin magnetic moment to its angular momentum differs from the free electron value. In general, the g factor is not a number but a second-rank tensor represented by nine numbers arranged in a 3×3 matrix. The principal axes of this tensor are determined by the local fields,
39
Spectral Analysis
for example, by the local atomic arrangement around the unpaired spin in a solid or in a molecule.
Advantages 1.
EPR spectra are simplified due to the reduction of second-order effects at high
fields. 2.
Increase in orientation selectivity and sensitivity in the investigation of
disordered systems. 3.
Accessibility of spin systems with larger zero-field splitting due to the larger
microwave quantum energy hν. 4.
The higher spectral resolution over g-factor, which increases with irradiation
frequency ν and external magnetic field B0. This is used to investigate the structure, polarity, and dynamics of radical microenvironments in spin-modified organic and biological systems through the spin label and probe method. The figure shows how spectral resolution improves with increasing frequency. 5.
Saturation of paramagnetic centers occurs at a comparatively low microwave
polarizing field B1, due to the exponential dependence of the number of excited spins on the radiation frequency ν. This effect can be successfully used to study the relaxation and dynamics of paramagnetic centers as well as of super slow motion in the systems under study. 6.
The cross-relaxation of paramagnetic centers decreases dramatically at high
magnetic fields, making it easier to obtain more-precise and more-complete information about the system under study.
40
Spectral Analysis
Spectral analysis:
Figure 25:EPR spectra of complexes (1) and (3) in CH2Cl2 solution at room temperature. The EPR spectrum of (1) is shown above. EPR lines in solution are sharp, indicting an average anisotropies in both, the g tensor and the nuclear coupling due to the rapid tumbling in solution. On the contrary the EPR spectra of (3) in CH2Cl2 solution appears most broad as is shown, In the case of (3) the broadening can be due to an unresolved hyperfine coupling of the bromine atom
41
Spectral Analysis
EXAFS EXAFS is sensitive to atoms that are only 4-5
away. The orientation of this molecule does
not matter in this scenario since EXAFS does not give any information other than the radial distances. EXAFS mainly uses amorphous types of materials such as frozen solutions, powders, gases and solutions. This selectivity for materials comes from its predecessor, XAS and its love for crystals. Since EXAFS prefers amorphous analytes, and gives only radial information, this technique works well for molecules with structurally similar molecules such as tetrahedralâ&#x20AC;&#x2122;s, octahedralâ&#x20AC;&#x2122;s and even buckyballs. This can be done by looking at the conjunction between the EXAFS region and the edge region to yield geometrical structural information. However, this is a poor technique to use for the heterogeneous population since the structural environment is averaged afterwards giving false information to the untrained.
Theory of EXAFS: Edge:
Before studying EXAFS, we have to understand the edge region. In this region ( in conjugation with EXAFS), information can be gathered on an absorbed atom, such as ionization potential energies, geometry of the molecule, and radial information. The edge occurs when an x-ray photon is absorbed by an atom, giving rise to sharp spike given in below figure(1):
Figure 26:Edge region of EXAFS The intensity of the spike is proportionate to the ionization potential of a bound electron. When scanning a molecule, in its entirety, it will give rise to several absorption peaks on the
42
Spectral Analysis
spectrum. This occurs when the photon energies (hν) are between 2-30 keV. Currently, it is quite a feat to be able to use 2 keV because if it were to be done, it would require the sample to be used on a windowless beam line under ultrahigh vacuum conditions making it very impractical to use. The 30 keV limit exists because the most powerful synchrotron can only emit photon energies at the highest intensity of 30 keV. The edge energy is also a function of its atomic number as shown in Figure 27:
Figure 27:Graph of atomic number versus edge energy The letters used to show this diagram are relative to their bohr atom shells, for example K edges for n=1 and L edges for n=2. This graph shows the simple relationship offered by EXAFS. In general, as the atomic number increases, the energy spike at the edge increases as well. This is mainly caused by the oxidation state or, the amount of valence electrons in the outer shells. In other words, there is a proportional relationship between the intensity of the edge energy levels and the oxidation state of the absorbing atom. This can be explained on the basis of electrostatics, electrons find it harder to dissociate from an atom with a large positive charge.
EXAFS: The key components of an EXAFS exists in its photo dissociated core electron and the density of electrons surrounding the absorbing atom. These atoms usually absorb x-rays whose energies are in excess. This gives rise to scattering photoelectron energies with
43
Spectral Analysis
neighbouring atoms which in turn gives information on the structural composition. Depending on where the scattering photoelectron energy hits when it comes back, there it will modulate the EXAFS intensity peaks. When interpreting data for the EXAFS, it is general practice use the photoelectron wave electron, k, which is an independent variable that is proportional to momentum rather than energy. We can solve for k by first assuming that the photon energy E will be greater than
(the initial X-ray absorption energy at the edge). Since energy is conserved, excess
energy given by E-
is conserved by being converted into the kinetic energy of the
photoelectron wave. Since wavelengths are dependent of kinetic energies, the photoelectron wave(de Broglie wavelength) will propagate through the EXAFS EXAFS region with a velocity of ν where the wavelength of the photoelectron will be scanned. This gives the relation, (E-
)=
. One of the identities for the de Broglie wavelength is that it is
inversely proportional to the photoelectrons momentum (
ν): Ν =
. Using simple
algebraic manipulations, we obtain the following relation:
Equation 16 k= =
=[
(
)
].
Experimental Procedure:
Figure 28:Experimental setup of EXAFS
44
Spectral Analysis
Sample Spectra of EXAFS
Figure 29:EXAFS Spectrum for Copper
45
Spectral Analysis
FLUORESCENCE SPECTROSCOPY Fluorescence is a photoluminescence process in which atoms or molecules are excited by absorption of electromagnetic radiation. The excited species then relax to the ground state, giving up their excess energy as photons. Fluorescence methods are much less widely applicable than absorption methods because of the relatively limited number of chemical systems that show appreciable fluorescence.
Why Use Fluorescence Spectroscopy? •
Sensitivity to local electrical environment – polarity, hydrophobicity
•
Track (bio-)chemical reactions
•
Measure local friction (microviscosity)
•
Track solvation dynamics
•
Measure distances using molecular rulers: fluorescence resonance energy transfer (FRET)
Advantages of fluorescence: Very good detection: measurable at low concentrations Fluorescence is sensitive to the environment Another advantage is the large linear concentration range of fluorescence methods, which is significantly greater than those encountered in absorption spectroscopy.
Applications of fluorescence: •
Clinical: Analysis of Pb, Hg, As, Sb, Bi, Ge, Se, in blood, urine, tissue, nail, hair; Cu, Zn and Pb in blood serum and urine samples.
•
Environmental: Determination of Hg at 1 ng/L levels; As, Se, Sb, and Te with detection limits between 10 and 50 ng/L; in environmental samples. Mercury in air can be determined at levels as low as 10 pg. Determination of femtogram (10-15 g) quantities of elements in samples by graphite furnace laser-excited AFS is also done.
46
Spectral Analysis
•
Agricultural: Analysis of dairy, wine, feed, meat, cigarettes, and other products for As, Hg, Pb, Sb, Se.
•
Geological and Metallurgical: Analysis of ore, rock, mineral, metals for Ge, Hg, Se, As in Sb, Se, Te in Cu.
•
Pharmaceutical: Determination of Hg, Pb, As, Se in active ingredients and fillers.
•
Petrochemical: Quantitative determination of Pb, Hg, Cd, As, Sn, Zn in fuels, lubricant, crude oil.
Fluorescence spectra:
Figure 30:Typical fluorescence spectra. The fluorescence intensity decreases linearly with decreasing concentration units of gold nanorods. •
Compared to shorter rods with lengths 30–120 nm, the longer rods showed very strong fluorescence bands that are red-shifted in the emission spectrum.
•
The fluorescence intensity of gold nanorods decreased linearly with decreasing concentration of the nanorod solution.
•
Gradual increase in the concentration of ions in the solution, there is increased fluorescence emission taking place.
47
Spectral Analysis
FOURIER TRANSFORM INFRARED SPECTROSCOPY
What is FT-IR? FT-IR stands for Fourier Transform Infrared, the preferred method of infrared spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some of it is passed through (transmitted). The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. Like a fingerprint no two unique molecular structures produce the same infrared spectrum. This makes infrared spectroscopy useful for several types of analysis.
Figure 31:Schematic diagram of FT-IR spectroscopy operating principle
PRINCIPLE: The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The higherenergy near-IR, approximately 14000–4000 cm−1 (0.8–2.5 μm wavelength) can excite overtone or harmonic vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5– 25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared, approximately 400–10 cm−1 (25–1000 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties.
48
Spectral Analysis
THEORY: Number of vibrational modes:
In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes in the dipole. A permanent dipole is not necessary, as the rule requires only a change in dipole moment.
Symmetrical
Antisymmetrical
stretching
stretching
Rocking
Wagging
Scissoring
Twisting
Figure 32: Dangling Bonds (These figures do not represent the "recoil" of the C atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter H atoms).
49
Spectral Analysis
EXPERIMENTAL DETAILS OF IR spectroscopy: The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This can be achieved by scanning the wavelength range using a monochromator. Alternatively, the whole wavelength range is measured at once using a Fourier transform instrument and then a transmittance or absorbance spectrum is generated using a dedicated procedure. Analysis of the position, shape and intensity of peaks in this spectrum reveals details about the molecular structure of the sample. Sample preparation:
Gaseous samples require a sample cell with a long pathlength to compensate for the diluteness. The pathlength of the sample cell depends on the concentration of the compound of the interest. A simple glass tube with length of 5 to 10 cm equipped with infrared windows at the both ends of the tube can be used for concentrations down to several hundred ppm. Sample gas concentrations well below ppm can be measured with a White's cell in which the infrared light is guided with mirrors to travel through the gas. White's cells are available with optical pathlength starting from 0.5 m up to hundred meters.
Figure 33: schematic diagram of FT-IR spectroscopy
50
Spectral Analysis
Absorption bands:
Figure 34:Absorption band scale
Introduction Mid-Infrared (IR) spectroscopy is an extremely reliable and well recognized fingerprinting method. Many substances can be characterized, identified and also quantified. One of the strengths of IR spectroscopy is its ability as an analytical technique to obtain spectra from a very wide range of solids, liquids and gases. However, in many cases some form of sample preparation is required in order to obtain a good quality spectrum. Traditionally IR spectrometers have been used to analyze solids, liquids and gases by means of transmitting the infrared radiation directly through the sample. Where the sample is in a liquid or solid form the intensity of the spectral features is determined by the thickness of the sample and typically this sample thickness cannot be more than a few tens of microns. Table 2:IR-absorption AQA
51
Spectral Analysis
Figure 35:IR Spectra for cyclobutanol
Figure 36:IR Spectra for 2-methylpropanal
52
Spectral Analysis
ATTENUATED TOTAL REFLECTANCE (ATR) The technique of Attenuated Total Reflectance (ATR) has in recent years revolutionized solid and liquid sample analyses because it combats the most challenging aspects of infrared analyses, namely sample preparation and spectral reproducibility.
How an ATR accessory works •
Internal reflection spectroscopy passes infrared radiation through an infraredtransmitting crystal of high refractive index, allowing the radiation to reflect in the crystal one or more times
•
An attenuated total reflection accessory measures the totally reflected infrared beam when the beam comes in contact with a sample
•
In this way, an evanescent wave penetrates into the sample in contact with the crystal, producing a spectrum of the sample
Figure 37: Schematic diagram of ATR operation
53
Spectral Analysis
Table 3:Saturated aliphatic (alkane/alkyl) group frequencies
t
54
Spectral Analysis
Table 4:Olefinic (alkene) group frequencies.
55
Spectral Analysis
Figure 38: ATR spectrum of 1-hexane
Table 5:Aromatic ring (aryl) group frequencies
56
Spectral Analysis
Figure 39:ATR spectra of xylene isomers (a)o-xylene, 1,2- dimethylbenzene; (b) mxylene, 1.3-dimethylbenzene; (c) p-xylene. 1,4-dimethylbenzene.
57
Spectral Analysis
DIFFUSE REFLECTANCE
In external reflectance, incident radiation is focused onto the sample and two forms of reflection can occur â&#x20AC;&#x201C; diffuse and specular. Energy from the incident beam that penetrates one or more particles is reflected in all directions and this component of light is called diffuse reflectance. On a rough or irregular surface material, such as a powder, specularly reflected light is a minor contributor to the overall signal. Therefore Specac accessories are optimised to increase collection of the diffuse reflectance component and decrease the specular component. Collection of the diffusely scattered light can be made directly from a sample or by using an abrasive sampling pad for intractable samples. Many samples will give diffusely reflected spectra including powders, fibers or matt surfaced samples such as textiles.
Figure 40: Operating principle of diffused reflectance
58
Spectral Analysis
A and C, Spectra from KBr pellets of caffeine and aspirin respectively.B and D, Reflectance spectra from TLC plates ofcaffeine and aspirin respectively. Figure 41: Transmission Spectra from KBr Pellets and Reflectance Spectra from TLC Plates ofcaffeine andaspirin
It is seen that there is a distinct difference in the form of the spectra taken from the TLC plate compared with that from the KBr pellet. It follows that reference spectra that are to be used for solute identification should also be obtained from the TLC plate in the same manner. The mass of solute in each spot examined was about 10 Îźg but it was estimated that about 1 Îźg would be sufficient for a recognizable Spectrum to be obtained.
59
Spectral Analysis
GAS CHROMTOGRAPHY Gas Chromatography (GC) is a commonly used analytic technique in many research and industrial laboratories. A broad variety of samples can be analyzed as long as the compounds are sufficiently thermal stable and volatile enough.
Schematic of a gas Chromatography
Figure 42:Schematic diagram of gas chromatography
Working principle Like for all other chromatographic techniques, a mobile and a stationary phase are required. The mobile phase (=carrier gas) is comprised of an inert gas e.g. helium, argon, nitrogen, etc. The stationary phase consists of a packed column where the packing or solid support itself acts as stationary phase, or is coated with the liquid stationary phase (=high boiling polymer). More commonly used in many instruments are capillary columns, where the stationary phase coats the walls of a small-diameter tube directly (e.g. 0.25 mm film in a 0.32 mm tube). The main reason why different compounds can be separated this way is the interaction of the compound with the stationary phaseâ&#x20AC;&#x153;(like-dissolves-likeâ&#x20AC;?-rule). The stronger the interaction is the longer the compound remains attached to the stationary phase, and the more time it takes to go through the column (=longer retention time).
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Spectral Analysis
Cromatogram no.1 Analysis of Phenols Phase: BP20, 0.25 μm film Column: 30 m x 0.25 mm ID Initial Temp: Isothermal at 155 °C Detector: FID Sensitivity: 32 x 10-12 AFS Injection Mode: Split
Figure 43:Chromatography of phenols 1. 2,6-Xylenol
5. 2,5-Xylenol
2. o-Cresol
6. p-Cresol
3. Phenol Ethylphenol
7. 2,4-Xylenol
9. 2-iso Propylphenol 10. 2,3-Xylenol 11. 3,5-Xylenol + p-Ethyl phenol
4. o-
8. m-Cresol
Cromatogram no.2 Analysis of Aliphatic Alcohols Phase: BP 1, 3.0μm film Column: 12 m x 0.53 mm ID Initial Temp: 100 °C
61
Spectral Analysis
Rate: 10 °C/min Final Temp.: 260 °C Carrier Gas: Nitrogen Injection Volume: 0.1μL
Figure 44:Chromatography of aliphatic alcohols 1. Octanol
4. Dodecanol
2. Decanol
5. Tetradecanol
3. Undecanol
7. Eicosanol
6. Hexadecanol
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Spectral Analysis
HPLC(HIGH PERFORMANCE LIQUID CHROMATOGRAPHY) Principle : ď&#x201A;ˇ
HPLC is a form of liquid chromatography used to separate compounds that are dissolved in solution. HPLC instruments consists of a reservoir of mobile phase, apump, an injector, a separator column, and a detector.
ď&#x201A;ˇ
Compounds are separated by injecting a sample mixture onto the column. The different components in the mixture pass through the column and differentiate due to differences in their partition behavior between the mobile and the stationary phase. The mobile phase must be degassed to eliminate the formation of air bubbles.
Instrumentation
Figure 45:Block diagram of HPLC system Stationary phase: solid materials such as beads of silica or polymer can be used as stationary phase. These are fixed, they will not involve any movement so the name stationary phase.They are nonporous , spherical having the size around 30-40 mm. Mobile phase: Aqueous solutions containing methanol, water-miscible organic solvents and also contains ionic species, in the form of a buffer. Solvent strength and selectivity are
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Spectral Analysis
determined by type and concentration of added ingredients. Ions in this phase compete with analyte ions for the active site in the packing. Properties of the mobile phase
It must dissolve the sample
It must have a strong solvent strength leads to reasonable retention times
It must be interact with solutes in such a way as to lead to selectivity.
Elution methods: Isocratic elution: single solvent of constant composition. Gradient elution: 2 or more solvents of differing polarity used. Pumping system Provide a continuous constant flow of the solvent through the injector Requirements
Pressure outputs to 6000 psi
Pulse-free output
Flow rates ranging from . 1-10 mL/min
Flow control and flow reproducibility of 0.5% or better
Corrosion-resistant components.
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Spectral Analysis
Chromatagram no :1: Analysis of Nucleic acid bases
Figure 46:Chromatogram of Nucleic acid bases Chromatagram no 2: Analysis of oligosaccharides
Figure 47:Chromatogram of oligosaccharides
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Spectral Analysis
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Spectral Analysis
IMPEDANCE SPECTROSCOPY Introduction It is also called Electrochemical Impedance spectroscopy. It is the response of an electrochemical system (cell) to an applied potential. The response of electrochemical systems is very nonlinear. The complex response of the system is usually displayed in Nyquist format with the reactance inverted (since such systems are inherently capacitive) It is a relatively new and powerful method of characterizing many of the electrical properties of materials and their interfaces with electronically conducting electrodes. It may be used to investigate the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material: ionic, semi conducting mixed electronic-ionic and even insulators (dielectrics). Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and then measuring the current through the cell. Assume that we apply a sinusoidal potential excitation. The response to this potential is an AC current signal. This current signal can be analyzed as a sum of sinusoidal functions (a Fourier series). It is normally measured using a small excitation signal. This is done so that the cell's response is pseudo-linear. In a linear (or pseudo-linear) system, the current response to a sinusoidal potential will be a sinusoid at the same frequency but shifted in phase.
Sinusoidal representation of current and voltage E(t) = E0cos(ωt), ω=2πf I(t) = I0 cos(ωt-φ) In complex notation E(t) = E0 exp(iωt) I(t) = I0 exp(iωt - iφ) Z(t) = Z0 exp(iθ) = Z0 (cos θ + isin θ) Electrical Circuit Elements Resistor E= IR Z = R
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Spectral Analysis
Inductor E = L di/dt Z = jĎ&#x2030;L Capacitor I = C dE/dt Z = 1/jĎ&#x2030;C
Measuring Technique
Figure 48:Block diagram of a potentiostatic frequency response analyzer
Principle In an electrochemical system for equilibrium a small time dependent potential or current signal acts. Then response of the system is measured. The signal can be a single sinus wave or consist of a sum of such waves with different amplitudes, frequencies and phases Electrochemical systems are linear at signal amplitudes of 10 mV or less. The overall equivalent circuit at high frequencies can be assumed as a series combination of the linear solution resistance and the predominantly capacitive interface.
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Theory The system for generating the test signal (small signal) must be linear over the entire frequency and amplitude scale and the noise amplitude must be low too. The sum of signals at the output of the operations amplifier A1 is the polarization signal of the electrochemical cell. The amplifiers A2 and A3 together with the cell compose a potentiostat. The potential of the reference electrode (RE) is compared with the polarization signal and the control loop with A2 changes the potential at the counter electrode (CE) until the potential at RE is equal the sum from the test signal and the DC polarization signal. The output potential of the amplifier A4 is proportional to the cell current (I-U converter). Then, the input potential of A4 is controlled to be zero (virtual earth). Then current can be measured by determining the potential drop on a small resistance between the working electrode and the earth. The output signal of A4 is applied to a phase sensitive detector. Here, it is compared with reference signals 1 and 2 in phase or 90â&#x2014;Ś shifted to the test signal, respectively. The accuracy of the method is only affected by the elements and the sensitivity and stability of the amplitude and phase measuring methodologies and techniques. The drawback here is that this method is very time consuming. Time domain techniques mostly use step and ramp signals. For electrochemical impedance spectroscopy a number of excellent commercial measuring systems exist. In some cases it is favorable to use precision impedance analyzers designed especially for accurate impedance measurements of electronic components and materials in a broad-frequency scale.
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Plotting Impedance
Figure 49: Complex plane impedance spectrum â&#x20AC;&#x201C;series resistance, capacitance
Figure 50:Complex plane impedence spectrum-parallel resistance, capacitance
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Figure 51:Impedence spectra of the resistance network
Impedance Analysis for the Corrosion Potential
Figure 52:Nyquist plots at selected times for the impedance response of the cylinder electrode to variable amplitude galvanostatic modulation about zero applied current.
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Figure 53: Bode plots of the negative imaginary component as a function of frequency, at selected times, for the cylinder electrode in response to variable amplitude galvanostatic modulation about zero applied current.
The impedance data generated during this experiment are presented as Nyquist plots in Fig 1. Where the system appeared to reach steady state within about 20 hours, the impedance results demonstrated that the system was still evolving after several days. Increases in the magnitude of the impedance, after the potential steadied, are consistent with films making the surface more resistive to charge transfer reactions and to diffusion of oxygen, leading to decreases in the rates of iron dissolution and oxygen reduction. The gaps in the trace of Figure 52, correspond to times when impedance scans were performed and the DC potential was not recorded. Between gaps, no peaks in the potential measurement were observed; The semicircle observed in the Nyquist plot represents the capacitive behavior of the cell. Upon inspection of the Bode plot for the negative of the imaginary component as a function of frequency, presented in Figure 53, it was observed that the characteristic frequency, where the magnitude of the imaginary component was a maximum, decreased with time. The reciprocal of the characteristic frequency has units of time according to Equation 17
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Spectral Analysis
The time constant τ, was proportional to a characteristic diffusion length or layer thickness. Increases in the characteristic time constant, over the course of the experiment, give evidence supporting the evolution of film growth.
Each individual impedance scan was considered to be a snap shot of the state of the system at the time the scan was conducted. At early times in the experiment, during the initial potential transient, generation of complete spectra could not be achieved since sweeping down to the 1 mHz range required several hours. To observe changes in the impedance during this time of highly non-stationary behavior, shorter scans were conducted by sweeping to the 10 mHz range, which required about 20 minutes to complete. As the potential stabilized, sweeps to lower frequencies were accomplished. The beginnings of a second semicircle or capacitive loop were observed to develop in the low frequency range of the spectra presented in the Nyquist plots of Fig 2. A complete semicircle would correspond to a second local maximum in the Bode plot. This result suggested the presence of two diffusion regions from the bulk of the electrolyte to the surface of the WE.
Applications
Determine corrosion rates of materials
Probe linearity of electrical/electrochemical reactions
Characterize electrical activity across interfaces to determine carrier concentration, etc.
Determine diffusion rate of counter ions in conducting polymer/carbon nanotube composites
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NEUTRON ACTIVATION ANALYSIS
Introduction: Neutron Activation Analysis (NAA) is a sensitive analytical technique useful for performing both qualitative and quantitative multi-element analysis of major, minor, and trace elements in samples from almost every conceivable field of scientific or technical interest. For many elements and applications, NAA offers sensitivities that are superior to those attainable by other methods, about parts per billion or better. In addition, because of its accuracy and reliability, NAA is generally recognized as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. Worldwide application of NAA is widespread. It is estimated that approximately 100,000 samples undergo analysis each year. Neutron activation analysis was discovered in 1936 when Hevesy and Levi found that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons. From this observation, they quickly recognized the potential of employing nuclear reactions on samples followed by measurement of the induced radioactivity to facilitate both qualitative and quantitative identification of the elements present in the samples. The basic essentials required to carry out an analysis of samples by NAA are ď&#x192;&#x2DC; A source of neutrons, ď&#x192;&#x2DC; Instrumentation suitable for o detecting gamma rays, and o the reactions that occur when neutrons interact with target nuclei.
The NAA Method The sequence of events occurring during the most common type of nuclear reaction used for NAA, namely the neutron capture or (n,gamma) reaction, is illustrated in Figure 54. When a neutron interacts with the target nucleus through a non-elastic collision sequence, a
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compound nucleus forms in an excited state. The excitation energy of the compound nucleus is due to the binding energy of the neutron with the nucleus. The compound nucleus will almost instantaneously de-excite into a more stable configuration through emission of one or more characteristic prompt gamma rays. In many cases, this new configuration yields a radioactive nucleus, which also de-excites (or decays) by emission of one or more characteristic delayed gamma rays, but at a much slower rate according to the unique half-life of the radioactive nucleus. Depending upon the particular radioactive species, half-lives can range from fractions of a second to several years.
Figure 54: Diagram illustrating the process of neutron capture by a target nucleus followed by the emission of gamma rays.
In principle, therefore, with respect to the time of measurement, NAA falls into two categories: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common. Thus, when one mentions NAA, it is generally assumed that measurement of the delayed gamma rays is intended. About 70% of the elements have properties suitable for measurement by NAA.
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Spectral Analysis
Neutrons There are several types of neutron sources: reactors, accelerators, and radio-isotopic neutron emitters. Nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes due to the materials used to moderate (or reduce the energies of) the primary fission neutrons. However, as shown in Figure 55, most neutron energy distributions are quite broad and consist of three principal components (thermal, epithermal, and fast).
Figure 55. A typical reactor neutron energy spectrum showing the various components used to describe the neutron energy regions.
The thermal neutron component consists of low-energy neutrons (energies below 0.5 eV) in thermal equilibrium with atoms in the reactor's moderator. At room temperature, the energy spectrum of thermal neutrons is best described by a Maxwell-Boltzmann distribution with a mean energy of 0.025 eV and a most probable velocity of 2200 m/s. In most reactor irradiation positions, 90-95% of the neutrons that bombard a sample are thermal neutrons. In general, a one-megawatt reactor has a peak thermal neutron flux of approximately 1013 neutrons per square centimeter per second. The epithermal neutron component consists of neutrons (energies from 0.5 eV to about 0.5 MeV) which have been only partially moderated. A cadmium foil 1 mm thick absorbs all thermal neutrons but will allow epithermal and fast neutrons above 0.5 eV in energy to pass through. In a typical unshielded reactor irradiation position, the epithermal neutron flux represents about 2% the total neutron flux. Both thermal and epithermal neutrons induce (n,gamma) reactions on target nuclei. An NAA technique that employs only epithermal neutrons to induce (n,gamma) reactions by irradiating the samples being analyzed inside either cadmium or boron shields is called epithermal neutron
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Spectral Analysis
activation analysis (ENAA). The fast neutron component of the neutron spectrum (energies above 0.5 MeV) consists of the primary fission neutrons, which still have much of their original energy following fission. Fast neutrons contribute very little to the (n,gamma) reaction, but instead induce nuclear reactions where the ejection of one or more nuclear particles - (n,p), (n,n'), and (n,2n) - are prevalent. In a typical reactor irradiation position, about 5% of the total flux consists of fast neutrons. An NAA technique that employs nuclear reactions induced by fast neutrons is called fast neutron activation analysis (FNAA).
Measurement of Gamma Rays The instrumentation used to measure gamma rays from radioactive samples generally consists of a semiconductor detector, associated electronics, and a computer-based, multichannel analyzer (MCA/computer). Most NAA labs operate one or more hyperpure or intrinsic germanium (HPGe) detectors which operate at liquid nitrogen temperatures (77 K) by mounting the germanium crystal in a vacuum cryostat, thermally connected to a copper rod or "cold finger". Although HPGe detectors come in many different designs and sizes, the most common type of detector is the coaxial detector, which in NAA, is useful for measurement of gamma-rays with energies over the range from about 60 keV to 3.0 MeV.
Figure 56: Typical internal structure of Neutron Activation Analysis
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Spectral Analysis
The two most important performance characteristics requiring consideration when purchasing a new HPGe detector are resolution and efficiency. Other characteristics to consider are peak shape, peak-to-Compton ratio, crystal dimensions or shape, and price. The detector's resolution is a measure of its ability to separate closely spaced peaks in a spectrum. In general, detector resolution is specified in terms of the full width at half maximum (FWHM) of the 122-keV photopeak of Co-57 and the 1332-keV photopeak of Co-60. For most NAA applications, a detector with 1.0-keV resolution or below at 122 keV and 1.8 keV or below at 1332 keV is sufficient. Detector efficiency depends on the energy of the measured radiation, the solid angle between sample and detector crystal, and the active volume of the crystal. A larger volume detector will have a higher efficiency. In general, detector efficiency is measured relative to a 3-inch by 3-inch sodium iodide detector using a Co-60 source (1332-keV gamma ray) at a distance of 25 cm from the crystal face. A general rule of thumb for germanium detectors is 1 percent efficiency per each 5 cc of active volume. As detector volume increases, the detector resolution gradually decreases. For most NAA applications, an HPGe detector of 15-30 percent efficiency is adequate. Typical gamma-ray spectra from an irradiated pottery specimen are shown in Figures 3-5 using two different irradiation and measurement procedures.
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Spectral Analysis
Figure 57: Gamma-ray spectrum showing several short-lived elements measured in a sample of pottery irradiated for 5 seconds, decayed for 25 minutes, and counted for 12 minutes with an HPGe detector.
Figure 58. Gamma-ray spectrum from 0 to 800 keV showing medium- and long-lived elements measured in a sample of pottery irradiated for 24 hours, decayed for 9 days, and counted for 30 minutes on a HPGe detector.
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Spectral Analysis
Figure 59. Gamma-ray spectrum from 800 to 1600 keV showing medium- and longlived elements measured in a sample of pottery irradiated for 24 hours, decayed for 9 days, and counted for 30 minutes on a HPGe dectector.
Using Gamma-ray Counts to Calculate Element Concentration The procedure generally used to calculate concentration (i.e., ppm of element) in the unknown sample is to irradiate the unknown sample and a comparator standard containing a known amount of the element of interest together in the reactor. If the unknown sample and the comparator standard are both measured on the same detector, then one needs to correct the difference in decay between the two. One usually decay-corrects the measured counts (or activity) for both samples back to the end of irradiation using the half-life of the measured isotope. The equation used to calculate the mass of an element in the unknown sample relative to the comparator standard is Equation 18
where A= activity of the sample (sam) and standard (std), Msam= mass of the element, Mstd= decay constant for the isotope and Td = decay time. When performing short irradiations, the irradiation, decay and counting times are normally fixed the same for all samples and standards such that the time dependent factors cancel. Thus the above equation simplifies into Equation 19
where C = concentration of the element and W= weight of the sample and standard.
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Spectral Analysis
Sensitivities Available by NAA The sensitivities for NAA are dependent upon the irradiation parameters (i.e., neutron flux, irradiation and decay times), measurement conditions (i.e., measurement time, detector efficiency), nuclear parameters of the elements being measured (i.e., isotope abundance, neutron cross-section, half-life, and gamma-ray abundance). The accuracy of an individual NAA determination usually ranges between 1 to 10 percent of the reported value. Table 6 lists the approximate sensitivities for determination of elements assuming interference free spectra. Table 6. Estimated detection limits for INAA using decay gamma rays. Assuming irradiation in a reactor neutron flux of 1x1013 n cm-2 s-1.
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Spectral Analysis
NEUTRON DIFFRACTION The neutron is one of the three principal sub-atomic particles in the atom. Unlike the proton and the electron, the neutron carries no charge. Neutrons have just slightly less mass than protons.
Figure 60: Neutrons were discovered in 1932 and their wave properties was shown in 1936.
Introduction to neutron scattering. Neutron scattering is one of the most powerful and versatile experimental methods to study the structure and dynamics of materials on the nanometer scale. “Neutrons tell you where the atoms are and what the atoms do” For the study of atomic and nanometer-scale structure in materials, X-ray scattering is the technique of choice. X-ray sources are by far more abundant and are, especially for synchrotron X-ray sources, much stronger than neutron sources. Hence, the rule of thumb goes: ”If an experiment can be performed with X-rays, use X-rays”.
Basic properties of the Neutron. The neutron is a nuclear particle with a mass rather close to that of the proton Equation 20 mn = 1.675 x10−27 kg.
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Spectral Analysis
The neutron does not exist naturally in free form, but decays into a proton, an electron, and an anti-neutrino. The neutron lifetime, τ = 886 s is much longer than the time of a neutron within a scattering experiment, where each neutron spends merely a fraction of a second. The neutron is electically neutral but still possess a magnetic moment Equation 21 µ = γµN, where γ = −1.913 is the neutron magnetogyric ratio and the nuclear magneton is given by Equation 22 µN = eh/m ¯ p. The neutron magnetic moment is coupled antiparallel to its spin, which has the value s = 1/2. The neutron interacts with nuclei via the strong nuclear force and with magnetic moments via the electromagnetic force. Most of this text deals with the consequences of these interactions; i.e. the scattering and absorption of neutrons inside materials and reflection from their surfaces.
Neutron scattering facilities. Neutron sources with flux densities adequate for neutron scattering investigations of materials are based on one of two principles. 1) Fission. A high continuous flux of neutrons is produced in the core of a conventional fission reactor. For neutron scattering purposes, research reactors with compact cores are used rather than the more abundant nuclear power plants. 2) Spallation. By bombarding a target of heavy elements with high-energy particles (typically protons), a very large number of neutrons can be produced. Spallation sources are typically pulsed, but can also be pseudo continuous, depending on the proton accelerator.
Neutron scattering instruments and detectors. Neutron scattering instruments are built in many different designs, reflecting that they are specialized for vastly different research purposes. Some instruments deal with the study of the structure of crystals, other with excitations in materials, others again with the properties of thin films, and so forth. This is typically done by a nuclear reaction, which destroys the
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Spectral Analysis
neutron as a result. The charged by-products give rise to an electrical signal, which can be amplified and detected.
Instrument.
Figure 61: Schematic of neutron scattering instrument
Five reasons for using neutrons. Neutrons can be preferred overX-rays. It is commonly agreed in the neutron scattering community that this can be formulated in five general points: 1. Thermal neutrons have a wavelength (2 ) similar to inter-atomic distances, and an energy (20 meV) similar to elementary excitations in solids. One can thus obtain simultaneous information on the structure and dynamics of materials and also measure dispersion relations (energy-wavelength dependence) of excitations. 2. The neutron scattering cross section varies randomly between elements and even between different isotopes of the same element. One can thus use neutrons to study light isotopes. In particular, this is important for hydrogen, which is almost invisible with X-rays. With neutrons, the large difference in scattering between usual hydrogen (1H) and deuterium, (“heavyhydrogen”, 2D) can be used in polymer- and biological sciences to change the contrast in the scattering and also “highlight” selected groups of large molecules. 3. The interaction between neutrons and solids is rather weak, implying that neutrons in most cases probe the bulk of the sample, and not only its surface. In addition, quantitative comparisons between neutron scattering data and theoretical models are possible, since higher-order effects are small and can usually be corrected for or neglected.
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Spectral Analysis
4. Since neutrons penetrate matter easily, neutron scattering can be performed with samples stored in all sorts of sample environment: Cryostats, magnets, furnaces, pressure cells, etc. Furthermore, very bulky samples can be studied, up to 10 cm thickness, depending on its elemental composition. 5. The neutron magnetic moment makes neutrons scatter from magnetic structures or magnetic ďŹ eld gradients. Un-polarized neutrons are used to learn about the periodicity and magnitude of the magnetic order, while scattering of spin-polarized neutrons can reveal the direction of the atomic magnetic moments.
Spectra.
Figure 62: Typical neutron diffraction spectrum
Neutron diffraction spectrum of a stainless steel temperature controlled membrane. The neutron diffraction spectra have shown that the material consists of two phases: a) iron with body centered cubic crystal structure (BCC, ferrite) and b) iron with face centerd cubic structure (FCC, austenite). For illustration Figure 62 shows a neutron diffraction spectrum where the relative intensities of the Bragg peaks correspond to random of crystallites (powder-like case). The Miller indices of Bragg reflections both for the FCC and for the BCC phases are indicated.
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Spectral Analysis
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Nuclear Magnetic Resonance Spectroscopy is the name given to a technique that exploits the magnetic properties of certain nuclei. It is the property of a magnetic nuclei in a magnetic field and applied electromagnetic pulses, which cause the nuclei to absorb energy from the EM pulse and radiate this energy back out, The energy radiated back out is at a specific resonance frequency which depends on the strength of the magnetic field and other factors. This allows the observation of specific quantum mechanical magnetic properties of an atomic nucleus. NMR spectroscopic technique gives us information about the number and types of atoms in a molecule. The experiment is carried out with the analysis of nuclei atoms, not electrons. Types of NMR Spectroscopy
1
H-NMR spectroscopy: Functionality, Presence of symmetry, Number of protons of
each type per signal, Number of neighboring protons per signal
13
C-NMR spectroscopy: Functionality (Chemical Shift), Presence of symmetry,
Presence of non-protonated carbons. Phosphorus using 19
F,
119
Sn,
195
31
P-NMR spectroscopy, Silicon using
29
Si-NMR spectroscopy,
Pt.
Instrumentation of NMR-Spectroscopy
Figure 63: Block diagram of construction of NMR spectroscopy
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Spectral Analysis
Working principle:
Figure 64: Schematic diagram illustrating operating principle of NMR 1. The alignment of the magnetic nuclear spins in an applied, constant magnetic field H0. 2. The perturbation of this alignment of the nuclear spins is achieved by employing an electromagnetic, usually radio frequency (RF) pulse. The required perturbing frequency is dependent upon the static magnetic field (H) and the nuclei of observation. 3. The two fields are usually chosen to be perpendicular to each other as this maximizes the NMR signal strength. The resulting response by the total magnetization (M) of the nuclear spins is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance imaging. 4. Different kinds of protons typically come at different chemical shifts. Shown below most protons appear between 0 and 10 ppm. The reference, tetramethylsilane (TMS) appears at 0 ppm, and aldehydes appear near 10 ppm. 5. These are typical values and that there are lots of exceptions!
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Spectral Analysis
R R
NH
OH R
Ph
R
H
10
(R) HO CH3
R
O
H
9
8
H Me
OH
R
7
Downfield region of the spectrum
6
R
O
R OCH3
H
5
4
TMS = Me Ph CH3 NR2 O
Cl
CH3
CH3
Si Me Me
R CH3
CH3 TMS
3
2
ppm
1
0
Upfield region of the spectrum
Figure 65: Chart of protons on δ scale
Result and discussion 1
H-NMR spectroscopy: NMR spectrum of ethanol
Figure 66: NMR spectra of ethanol The 1H-NMR spectrum of ethanol shows the methyl peak has been split into three peaks (a triplet) and the methylene peak has been split into four peaks (a quartet). This occurs because there is a small interaction (coupling) between the two groups of protons. The spacing between the peaks of the methyl triplet is equal to the spacing between the peaks of the methylene quartet. There are two of them, and each can have one of two possible orientations (aligned with or opposed against the applied field). This gives a total of four possible states;
Figure 67:Illustration of field interaction in methylene protons during NMR
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Spectral Analysis
In the first possible combination, spins are paired and opposed to the field. This has the effect of reducing the field experienced by the methyl protons; therefore a slightly higher field is needed to bring them to resonance, resulting in an upfield shift. Neither combination of spins opposed to each other has an effect on the methyl peak. The spins paired in the direction of the field produce a downfield shift. Hence, the methyl peak is split into three, with the ratio of areas 1:2:1. Similarly, the effect of the methyl protons on the methylene protons is such that there are eight possible spin combinations for the three methyl protons;
Figure 68: Field interactions of methyl protons on methylene protons
Acetophenone: From NMR data: There are three types of protons present in the molecule, in NMR spectra which is due to five protons that is two a-type protons and three b-types protons including the aromatic ring at higher δ values of 7-8 ppm. In aromatic ring for a-type proton, it has one adjacent but due to anisotropic effect of C=O group multiplet is seen, since a-type protons are somewhat near to C=O group compared to b-types. Protons So delta value of a-type protons is slightly higher ,for b-types proton it also have one adjacent proton but due to anisotropic effect of C=O group multiplet is seen and also it â&#x20AC;&#x2DC;s delta value is slightly lower than a-type of proton. since C=O group de-shields b-types protons at lower delta value. Since it is far from C=O group, for c-types that is methyl group, it has no adjacent protons hence it exhibits singlet in NMR spectra. Thus also methyl group is directly attached to C=O, this de-shields c-types protons to higher delta value of 2.5-3 ppm.
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Spectral Analysis
H
H
O CH3C
H H
13
H
C NMR
Figure 69: 1H NMR spectrum of acetophenone â&#x20AC;&#x201C; with integration
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Spectral Analysis
Table 7:
13
Cchemical shift
Example: 2-Bromobutane
Figure 70: NMR spectrum of 2-bromobutane ď&#x201A;ˇ
2-Bromobutane has four C-13 peaks: 11.6ppm, 25.4ppm, 33.6ppm, and 52.9ppm.
ď&#x201A;ˇ
How can we assign these?
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Spectral Analysis
Signal at 51.9ppm is assigned to C2; this carbon will be shifted furthest downfield because of the bromine atom
Signal at 11.6ppm is assigned to C4; this carbon atom is furthest from the bromine.
Signal at 25.4ppm assigned to C3.
Signal at 33.6ppm assigned to C1.
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Spectral Analysis
NON-DESTRUCTIVE EVALUATION/TESTING - NDE/NDT Introduction to NDT Non-destructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. The terms Nondestructive examination (NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are also commonly used to describe this technology.
Definition of NDT NDT is a technique that does not damage or destroy the material or product being tested. It is performed on a finished item instead of on a material sample, it uses infrared radiation, radiography, ultrasound, x-rays, and other techniques to detect fatigue effects, structural flaws and other such defects.
Figure 71: Illustration of typical Non-destructive testing procedure
Uses of NDT Methods
Flaw Detection and Evaluation
Leak Detection
Location Determination
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Spectral Analysis
Dimensional Measurements
Structure and Microstructure Characterization
Estimation of Mechanical and Physical Properties
Stress (Strain) and Dynamic Response Measurements
Material Sorting and Chemical Composition
NDT/NDE Methods Visual Inspection
Penetrant Testing
Magnetic Particle Testing
Electromagnetic or Eddy Current Testing
Radiography
Ultrasonic Testing.
Figure 72: Various NDT techniques
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Spectral Analysis
1. Visual / Optical Testing (VT) Visual inspection is the NDT method used to evaluate the condition or the quality of a weld or component, or to look for defects using an inspector's eyes. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. It is, inexpensive, easily carried out and usually doesn't require special equipment. It requires good vision, good lighting and the knowledge of what to look for.
Figure 73: Visual/optical testing
2. Penetrant Testing (PT) Liquid penetration inspection is used to reveal surface breaking flaws by bleed out of a colored or fluorescent dye from the flaw. Principle Test objects are coated with visible or fluorescent dye solution and allowed time to seep into surface breaking defects
Figure 74: Penetrant testing
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Spectral Analysis
Excess dye is then removed from the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface.
Figure 75: Coating of dyes on the material for inspection With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see.
Figure 76: Bleed-out of developer in the imperfection With fluorescent dyes, ultraviolet light is used to make the bleed out fluoresce brightly, thus allowing imperfections to be readily seen.
Figure 77: Impact of using fluorescent dye on material Penetrant testing is most often used on materials clad in stainless steel, and stainless welded items which cannot be inspected by other methods. It is typically used to inspect non-ferrous metal, plastic or ceramic components.
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Spectral Analysis
3. Magnetic Particle Testing (MT) Magnetic particle inspection (MPI) is used for the detection of surface and near-surface flaws in ferromagnetic materials. Principle A magnetic field is established in a component made from ferromagnetic material. The magnetic lines of force travel through the material, and exit (North) and reenter (South) the material at the poles. Defects such as crack or voids cannot support as much flux, and force some of the flux outside of the part. Magnetic particles or iron particles (either dry or suspended in liquid) distributed over the component will be attracted to areas of flux leakage. Surface and near-surface imperfections distort the magnetic field and concentrate iron particles near imperfections, previewing a visual indication of the flaw.
Figure 78: Impact of magnetic field on defect surface
Figure 79: Look at specimens undergoing MT
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Spectral Analysis
Spectra of Magnetic Particle Testing The surfaces of disk drive parts have magnetic particulate contamination such as Neodymium (Nd)
Figure 80: Typical Magnetic Particle EDX Spectrum Neodymium is used in very powerful permanent magnets. Neodymium magnets appear in products such as in-ear headphones and computer hard drives.
4. Electromagnetic or Eddy Current Testing Eddy current, penetrating radar and other electromagnetic techniques are used to detect or measure flaws, bond or weld integrity, thickness, electrical conductivity. It is the most widely applied electromagnetic NDT technique. Principle: Alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are called eddy currents. Eddy currents produce their own magnetic field that can be measured and used to find flaws and characterize conductivity, permeability and dimensional features.
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Spectral Analysis
Spectra of Eddy Current Testing Pulsed Eddy Current Method Developments for Hidden Corrosion Detection in Aircraft Structures
Figure 81: Transient signals measured by the pick-up coil for three different test parameters. ď&#x201A;ˇ
Radiography Testing (RT)
Radiography technique is based upon exposing the material or object to short wavelength radiations in the form of X-rays or gamma rays to examine internal defects or hidden features. An X-ray machine or radioactive isotope is used as a source of radiation
Figure 82: Electromagnetic spectrum
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Spectral Analysis
Principle: The test object is placed between the radiation source and detector. Radiation is directed through a part and onto film or other detector. The resulting shadow graph shows the internal features and soundness of the part. Material thickness and density changes are indicated as lighter or darker areas on the film or detector. The darker areas in the radiograph above represent internal voids in the component.
Analysis of Spectra of Radiography Testing This x-ray spectrum shows results from radiation-effects tests using a germanium silicon dioxide target (keV = kilo electron volts). The three helium like lines (Heα, Heβ, and Heδ) were generated by germanium ions with all but two electrons stripped by the laser energy. The Lyman-series (Ly) lines were produced by germanium ions with all but one electron stripped.
Figure 83: Spectra of radiography testing
Ultrasonic Testing
Principle In ultrasonic testing, high-frequency sound waves are transmitted into a material to detect imperfections or to locate changes in material properties.
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The most commonly used ultrasonic testing technique is pulse echo, whereby sound is introduced into a test object and reflections (echoes) from internal imperfections or the part's geometrical surfaces are returned to a receiver. Below is an example of shear wave weld inspection.. This indication is produced by sound reflected from a defect within the weld.
Figure 84:Shear wave weld inspection(The indication extending to the upper limits of the screen)
Spectra of Ultrasonic Testing The spectra is got on a display in which the received pulse amplitude is represented as a displacement along one axis (y-axis) and the travel time of the ultrasonic pulse is represented as a displacement along the other axis (x-axis). In a linear amplification system the vertical excursion is proportional to the amplitude of the signal.
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Figure 85: Spectra of ultrasonic testing(The ultrasonic image is from the root of the weld)
Figure 86: Noise ratio of flat bed demonstration weld
Common Application of NDT 1. Inspection of Raw Products Forgings, Castings, Extrusions, etc.
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2. Inspection Following Secondary Processing Machining, Welding, Grinding, Heat treating, Plating etc. 3. In-Services Damage Inspection Cracking, Corrosion, Erosion/Wear, Heat Damage, etc. 4. Pressure Vessel Inspection The failure of a pressure vessel can result in the rapid release of a large amount of energy. To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing.
Figure 87: Pressure vessel inspection 5. Wire Rope Inspection Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope that is used in chairlifts, cranes and other lifting devices.
Figure 88: Wire rope inspection
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Reason for Choice of Non-destructive Analysis NDT is used typically for the following reasons:
Accident prevention and to reduce costs.
To improve product reliability.
To determine acceptance to a given requirement.
To give information on repair criteria.
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MASS SPECTROMETRY Mass spectrometers are the most commonly used tool for studying isotope abundances. Mass spectrometry is a powerful analytical technique used to quantify known materials, to identify unknown compounds within a sample, and to elucidate the structure and chemical properties of different molecules. The complete process involves the conversion of the sample into gaseous ions, with or without fragmentation, which are then characterized by their mass to charge ratios (m/z) and relative abundances. This technique basically studies the effect of ionizing energy on molecules. It depends upon chemical reactions in the gas phase in which sample molecules are consumed during the formation of ionic and neutral species.
Figure 89: Processes undergoing in a mass spectrometer ď&#x201A;ˇ
The physics behind mass spectroscopy is that a charged particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass to charge ratio, m/z.
ď&#x201A;ˇ
In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion.
ď&#x201A;ˇ
If the molecular ion is too unstable then it can fragment to give other smaller ions.
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ď&#x201A;ˇ
The collection of ions is then focused into a beam and accelerated into the magnetic field and deflected along circular paths according to the masses of the ions.
ď&#x201A;ˇ
By adjusting the magnetic field, the ions can be focused on the detector and recorded.
Diagram of mass spectrometer
Figure 90: Schematic diagram of mass spectrometer WORKING PRINCIPLE
Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions. Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy. Stage 3: Deflection
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The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. Stage 4: Detection The beam of ions passing through the machine is detected electrically.
The spectra of mass spectroscopy:(for aliphatic and aromatic compounds) 1. The mass spectra of Hexane.
Figure 91:Mass spectrogram of hexane
Figure 92: Various ligands in the propagating chain ď&#x201A;ˇ
Hexane shows the same fragmentation pattern as other un-branched alkanes.
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ď&#x201A;ˇ
Thus, alkyl carbocations at m/z=15, 29, 43 and 57 Da provide the dominant peaks in the spectrum.
ď&#x201A;ˇ
The m/z=57 butyl cation (M-29) is the base peak, and the m/z=43 and 29 ions are also abundant.
2. The mass spectra of propyle benzene.( for aromatic compounds)
Figure 93: Mass spectrogram of propyl benzene
Figure 94: Mechanisms of aromatic compounds during mass spectroscopy observation
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The phenyl cation will fragment further. One route involves the loss of acetylene yielding a fragment with formula C4H3+ (m/z 51). Another route involves the loss of presumably an alkene diradical with formula C3H2, forming probably the simplest aromatic species of the formula C3H3+ (m/z 39), namely the cyclopropenyl ion.
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MÖSSBAUER SPECTRA PRINCIPLE Mössbauer spectroscopy is a technique in which interaction between the electromagnetic moment of the nuclear charge and electromagnetic fields produced by the extra-nuclear electrons are studied. This interaction gives splitting/shifting of the nuclear energy levels. Mössbauer spectra are described using three parameters: 1. Isomer shift (δ), which arises from the difference in s electron density between the source and the absorber, 2. Quadruple splitting (Δ which is a shift in nuclear energy levels that is induced by an electric field gradient caused by nearby electrons, and 3. Hyperfine splitting (for magnetic materials only). Mössbauer parameters are temperature-sensitive, and this characteristic is sometimes exploited by using lower temperatures to improve peak resolution and induce interesting magnetic phenomena. The Mössbauer effect is used to study many different types of isotopes with long-lived, lowlying excited nuclear energy state such as 99Ru, 151Eu, 155Gd, 193Ir, 195Pt and 197Au. However, among all the elements, the isotope with the strongest recoil-free resonant absorption is 57Fe, and for this reason the vast majority of Mössbauer studies are done using 57Fe.
Figure 95: Mössbauer studies are done using 57Fe.
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The Mössbauer effect, as generally applied to the study of minerals, relies on the fact that 57Fe, which is a decay product of 57Co, is unstable. 57Fe decays by giving off a gamma ray (γ-ray), along with other types of energy. Figure 95 shows the nuclear decay scheme for 57Co → 57Fe. If a nucleus gives off radiation or any other form of energy (in this case, in the form of a γ-ray), the nucleus must recoil (or move) with an equal and opposite momentum to preserve its energy (E), in the same way that a gun (by analogy, the nucleus) recoils when a bullet (the γ-ray) is fired out of it. We describe this general case in terms of energy by saying that: Equation 23 Eγ-ray emission = Etransition - ER , where Eγ-ray emission = the energy of the emitted γ-ray Etransition = the energy of the nuclear transition ER = the energy of the recoil.
Recoilless Energy: The recoil energy associated with absorption or emission of a photon can be described by the conservation of momentum. We find that the recoil energy depends inversely on the mass of the system. For a gas the mass of the single nucleus is small compared to a solid. The solid or crystal absorbs the energy as phonons (quantized vibration states of the solid), but there is a probability that no phonons are created and the whole lattice acts as the mass, resulting in a recoilless emission of the gamma ray. The new radiation is at the proper energy to excite the next ground state nucleus. The probability of recoilless events increases with decreasing transition energy.
Figure 96: Recoilless emission of the gamma ray and resonance absorption
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Doppler Effect: The Doppler shift describes the change in frequency due to a moving source and a moving observer. f is the frequency measured at the observer, v is the velocity of the wave, c is the speed of light ,vr is the velocity of the observer, vs is the velocity of the source which is positive when heading away from the observer, and fo is the initial frequency. Equation 24
Lorentzian (Cauchy) lines:
Figure 97: In the above spectrum the emission and absorption are both estimated by the Lorentzian distribution. Lorentzian (Cauchy) line shapes, used to describe spectral lines resulting from broadened resonance and other phenomena, have been used since the technique was first developed. This line shape gives a good approximation of line shapes in spectra of paramagnetic materials where all of the Fe nuclei are in identical electronic environments.
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Mössbauer spectra of iron oxide sample prepared by different methods This technique of Mössbauer spectroscopy is widely used in mineralogy to examine the valence state of iron, which is found in nature as Fe0 (metal), Fe2+, and Fe3+, as well as the type of coordination polyhedron occupied by iron atoms (trigonal, tetrahedral, octahedral, etc.). It is sometimes used to determine redox ratios in glasses and (less successfully) in rocks. Mössbauer spectroscopy is also used to assist in the identification of Fe oxide phases based on their magnetic properties. If the electrons around the Fe atom create a magnetic field, as in the case of magnetite, then the energy levels in the Fe nucleus will split to allow six possible nuclear transitions, and a sextet (six-peak) spectrum results. The position of the peaks in the sextet defines what is called the hyperfine splitting of the nuclear energy levels. Iron atoms in different local environments and those having different oxidation states absorb at different, diagnostic energies. A typical Mössbauer spectrum thus consists of sets of peaks (usually doublets and sextets), with each set corresponding to an iron nucleus in a specific environment in the sample (a Fe nuclear site). Different sets of peaks appear depending on what the Fe nucleus "sees" in its environment. The nuclear environment depends on a number of factors including the number of electrons (Fe0, Fe2+, Fe3+), the number of coordinating anions, the symmetry of the site, and the presence/absence of magnetic ordering (which may be temperature-dependent). Thus the spectrum of a given mineral consist of a superposition of sextets.
Strengths Along with wet chemistry, Mössbauer spectroscopy remains the "gold standard" for quantitative determination of the valence state of iron in minerals and identification of various iron oxides. It is also well-suited for determination of the coordination number of Fe atoms. Limitations The biggest limitation of the Mössbauer is that it is inherently a bulk technique; it uses powders spread thinly across an absorber to get optimal experimental conditions. In recent years, improvements in electronics and detectors have made it possible to run very small samples (1-5 mg). Another approach to this problem is the Mössbauer milliprobe developed by Catherine McCammon at Bayreuth. This modification, which uses a lead plate to restrict
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gamma rays to a small diameter (~100 μm), can be used to study single grains in thin sections or single crystals.
Mossbauer Spectra:
Figure 98: Mössbauer spectra of Iron Oxides Above figure shows that the iron occupies two local environments, the A-site and B site, and two species (Fe2+ and Fe3+) occupy the B-site. One might expect the spectrum to be a combination of 3 spectra, however delocalization of electrons or electron hopping between Fe2+ and Fe3+ in the B site causes the nuclei to sense an average valence in the B site and hence the spectrum are fitted with two curves accordingly. This is most easily seen in the Mt025 spectrum. The two fitted curves correspond to Fe3+ in the A-site and mixed valance Fe2.5+ in the B-site. The isomer shift of the fitted curves can be used to determined which curve corresponds to which valence. The isomer shift relative to the top fitted curve is reported to be 0.661 and the bottom fitted curve is 0.274 relative to αFe. Thus the top fitted curve corresponds to less s-electron dense Fe2.5+. The magnetic splitting is quite apparent. In each of the spectra, six peaks are present due to magnetic splitting of the nuclear energy states as explained previously. Quadrupole splitting is not so apparent, but actually is present in the spectra. The three peaks to the left of the center
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of a spectrum should be spaced the same as those to the right due to magnetic splitting alone since the energy level spacing between sublevels is equal. This is not the case in the above spectra, because the higher energy I = 3/2 sublevels are split unevenly due to magnetic and quadrupole splitting interactions.
Once the peaks have been fitted appropriately, determination of the extent of B-site vacancy in (Fe3+)A(Fe(1-3x)2+ Fe(1+2X)3+Ox)BO4 is a relatively simple to determine the number of vacancies (x) and to solve the equation: Equation 25
where RAB
or A
= relative area (
) of the curve for the B or A site
respectively.
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PHOTOLUMINESCENCE & FLUORESCENCE SPECTROSCOPY Luminescence: emission of photons from electronically excited states of atoms, molecules, and ions. Fluorescence: Average lifetime from <10—10 to 10—7 sec from singlet states. Phosphorescence: Average lifetime from 10—5 to >10+3 sec from triplet excited states. Photophysics: Jablonski Diagram
Figure 99: One form of Jablonski diagram Photoexcitation from the ground electronic state S0 creates excited states S1, (S2, …, Sn) Kasha’s rule: Rapid relaxation from excited electronic and vibrational states precedes nearly all fluorescence emission.(We can track these processes using femtosecond spectroscopy) Internal Conversion: Molecules rapidly relax (10-14 to 10-11 s) to the lowest vibrational level of S1.(This is why DNA doesn’t emit much fluorescence) Intersystem crossing:
Molecules in S1 state can also convert to first triplet state T1;
emission from T1 is termed phosphorescence, shifting to longer wavelengths (lower energy) than fluorescence. Transition from S1 to T1 is called intersystem crossing. Heavy atoms such as Br, I, and metals promote ISC.
Working Principle: Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or
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luminescence.
In
the
case
of
photo-excitation,
this
luminescence
is
called
photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties. Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a nonradiative process). The energy of the emitted light (photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The quantity of the emitted light is related to the relative contribution of the radiative process.
Figure 100: Partial energy diagram for a photoluminescent system
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Instrumentation
Figure 101: Internal picture of femtosecond spectroscopy and block diagram illustrating its fundamental units Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called "photo-excitation." One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties. Components of Spectrometer:
•
Sources: A more intense source in needed than the tungsten of hydrogen lamp.
•
Lamps: The most common source for filter fluorometer is a low-pressure mercury vapor lamp equipped with a fused silica window. For spectrofluorometers, a 75 to 450-W high-pressure xenon arc lamp in commonly employed.
•
Lasers: Most commercial spectrofluorometers utilize lamp sources because they are less expensive and less troublesome to use.
•
Filters and Monochromators: Both interface and absorption filters have been used in fluorometers for wavelength selection of both the excitation beam and the resulting fluorescence radiation. Most spectrofluorometers are equipped with at least one and sometimes two grating monochromators.
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â&#x20AC;˘
Transducers: Photomultiplier tubes are the most common transducers in sensitive fluorescence instruments.
â&#x20AC;˘
Cell and Cell Compartments: Both cylindrical and rectangular cell fabricated of glass or silica are employed for fluorescence measurements.
The experimental details are given below: Light from an optical fiber enters, reflects off the collimating mirror no is diffracted by the grating no reflects from focusing mirror, then finally is read out by the CCD(Charged coupled device) detector no 9.
Photoluminescense Uses: Band Gap Determination:
The most common radiative transition in semiconductors is between states in the conduction and valence bands, with the energy difference being known as the band gap. Band gap determination is particularly useful when working with new compound semiconductors. Impurity Levels and Defect Detection:
Radiative transitions in semiconductors also involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration. Recombination Mechanisms:
The return to equilibrium, also known as "recombination," can involve both radiative and nonradiative processes. The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. Analysis of photoluminescence helps to understand the underlying physics of the recombination mechanism. Material Quality:
In general, nonradiative processes are associated with localized defect levels, whose presence is detrimental to material quality and subsequent device performance. Thus, material quality can be measured by quantifying the amount of radiative recombination.
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Special Features of Photoluminescence spectroscopy:
Various excitation wavelengths allow for varying penetration depths into the material,
and thus, varying levels of volume excitation.
Detection of photoluminescence from 0.4 to 2.8 micrometers using diffraction and
Fourier-transform-based systems.
Mapping capabilities with 1-micrometer spatial resolution on the Fourier-transform-
based system.
Sample temperatures of 4 to 300 K.
Sensitivity down to the level of parts per thousand, depending on impurity species and
host.
Photoluminescence Spectra :
Figure 102: Photoluminescence spectra Comparison of PL intensity of pure DHFLC (deformed helix ferroelectric liquid crystal) material LAHS19 and GNPs doped DHFLC material, excited at 340 nm, emission wavelength is 411 nm. Figure shows the PL spectra of LAHS19 doped with 20 and 100µl of GNPs solution. One can clearly see seven-fold and nine-fold enhancement in the PL intensity of LAHS19 with 20 and 100µl doping concentrations of GNPs solution, respectively. It is noticeable that the doping of GNPs enhances the PL intensity while they do not affect the PL profile of the LAHS19 material, as can be seen from Figure 102 The enhancement in the PL intensity of the GNPs doped LAHS19 material is due to the constructive interaction between enhanced electromagnetic field generated near
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the GNPs surfaces and molecular fluorophores chiral terphenyl compound present in the LAHS19 material.
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POLYMERASE CHAIN REACTION THEORY: The method relies on thermal cycling, consisting of cycles of repeated heating and cooling of the reaction for DNA melting and enzymatic replication of the DNA. Primers (short DNA fragments) containing sequences complementary to the target region along with a DNA polymerase (after which the method is named) are key components to enable selective and repeated amplification. As PCR progresses, the DNA generated is itself used as a template for replication, setting in motion a chain reaction in which the DNA template is exponentially amplified. PCR can be extensively modified to perform a wide array of genetic manipulations. Almost all PCR applications employ a heat-stable DNA polymerase, such as Taq polymerase, an enzyme originally isolated from the bacterium Thermus aquaticus. This DNA polymerase enzymatically assembles a new DNA strand from DNA building-blocks, the nucleotides, by using single-stranded DNA as a template and DNA oligonucleotides (also called DNA primers), which are required for initiation of DNA synthesis. The vast majority of PCR methods use thermal cycling, i.e., alternately heating and cooling the PCR sample to a defined series of temperature steps. These thermal cycling steps are necessary first to physically separate the two strands in a DNA double helix at a high temperature in a process called DNA melting. At a lower temperature, each strand is then used as the template in DNA synthesis by the DNA polymerase to selectively amplify the target DNA. The selectivity of PCR results from the use of primers that are complementary to the DNA region targeted for amplification under specific thermal cycling conditions.
PCR principles and procedure: The polymerase chain reaction (PCR) is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence or the polymerase chain reaction (PCR) is a technique to amplify a piece of DNA very rapidly outside of a cell.
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Figure 103: A thermal cycler for PCR PCR is used to amplify a specific region of a DNA strand (the DNA target). Most PCR methods typically amplify DNA fragments of up to ~10 kilo base pairs (kb), although some techniques allow for amplification of fragments up to 40 kb in size. A basic PCR set up requires several components and reagents. These components include: •
DNA template that contains the DNA region (target) to be amplified.
•
Two primers that are complementary to the 3' (three prime) ends of each of the sense
and anti-sense strand of the DNA target. •
Taq polymerase or another DNA polymerase with a temperature optimum at around
70 °C. •
Deoxynucleoside triphosphates (dNTPs; nucleotides containing triphosphate groups),
the building-blocks from which the DNA polymerase synthesizes a new DNA strand. •
Buffer solution, providing a suitable chemical environment for optimum activity and
stability of the DNA polymerase. •
Divalentcations, magnesium or manganese ions; generally Mg2+ is used, but Mn2+ can
be utilized for PCR-mediated DNA mutagenesis, as higher Mn2+ concentration increases the error rate during DNA synthesis •
Monovalent cation potassium ions.
The PCR is commonly carried out in a reaction volume of 10–200 μl in small reaction tubes (0.2–0.5 ml volumes) in a thermal cycler. The thermal cycler heats and cools the reaction tubes to achieve the temperatures required at each step of the reaction (see below). Many modern thermal cyclers make use of the Peltier effect, which permits both heating and cooling of the block holding the PCR tubes simply by reversing the electric current. Thinwalled reaction tubes permit favorable thermal conductivity to allow for rapid thermal equilibration. Most thermal cyclers have heated lids to prevent condensation at the top of the
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reaction tube. Older thermo-cyclers lacking a heated lid require a layer of oil on top of the reaction mixture or a ball of wax inside the tube. Procedure:
Typically, PCR consists of a series of 20-40 repeated temperature changes, called cycles, with each cycle commonly consisting of 2-3 discrete temperature steps, usually three. The cycling is often preceded by a single temperature step (called hold) at a high temperature (>90째C), and followed by one hold at the end for final product extension or brief storage. The temperatures used and the length of time they are applied in each cycle depend on a variety of parameters. These include the enzyme used for DNA synthesis, the concentration of divalent ions and dNTPs in the reaction, and the melting temperature (Tm) of the primers. 1. Denaturation at around 94째C : During denaturation, the double strand melts open to single stranded DNA, all enzymatic reactions stop (for example the extension from a previous cycle). 2. Annealing at around 54째C Hydrogen bonds are constantly formed and broken between the single stranded primer and the single stranded template. If the primers exactly fit the template, the hydrogen bonds are so strong that the primer stays attached.
3. EXTENSION at around 72째C :The bases (complementary to the template) are coupled to the primer on the 3' side (the polymerase adds dNTP's from 5' to 3', reading the template from 3' to 5' side, bases are added complementary to the template) .
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Figure 104: Schematic drawing of the PCR cycle. (1) Denaturing at 94–96 °C. (2) Annealing at ~65 °C (3) Extention at 72 °C. Four cycles are shown here. The blue lines represent the DNA template to which primers (red arrows) anneal that are extended by the DNA polymerase (light green circles), to give shorter DNA products (green lines), which themselves are used as templates as PCR progresses.
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RADIATION DETECTORS Radiation
Radiation is the emission and propagation of energy in the form of waves, rays or particles. Examples:
A burning candle emits radiation in the form of heat and light.
Uranium-238 decaying into Thorium-234 emits radiation in the form of alpha particles.
Electrons dropping from one energy state to a lower state emit radiation in the form of a photon.
Radioactivity
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. An unstable nucleus releases energy to become more stable. Ionizing Radiation
Radiations that can transfer enough energy to remove electrons from their atoms are referred to as “Ionizing Radiations” Types of Ionizing Radiation:
Alpha particles
Beta particles
Gamma rays (or photons)
X-Rays (or photons)
Neutrons
Alpha Rays:
•
An alpha particle consists of two neutrons and two protons ejected from the nucleus of an atom. It is identical to the nucleus of a helium atom.
•
Alpha particles are charged and relatively heavy.
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7. Examples of alpha emitters are radium, radon, thorium, and uranium. 8. It has velocity of over 10000 miles/sec. 9. These are easily shielded against and can be stopped by a single sheet of paper.
Figure 105: Radioactive decay leading to alpha particles
Figure 106: Penetration limitation of alpha particles Beta rays:
•
A beta particle is an electron emitted from the nucleus of a radioactive atom.Beta particles are much less massive and less charged than alpha particles
•
Examples of beta emitters commonly used in biological research are: hydrogen-3 (tritium), carbon-14, phosphorus-32, phosphorus-33, and sulfur-35.
•
It has the velocity of over 100,000 to 150,000 miles/sec.
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Figure 107: Radioactive decay leading to beta particles
Figure 108: Penetration limitations of beta rays Gamma Rays:
A gamma ray is a packet (or photon) of electromagnetic radiation emitted from the nucleus during radioactive decay and occasionally accompanying the emission of an alpha or beta particle. Gamma rays are identical in nature to other electro- magnetic radiations such as light or microwaves but are of much higher energy. Examples of gamma emitters are cobalt-60, zinc-65, cesium-137, and radium-226. They travel with the velocity of light.
Like all forms of electromagnetic radiation, gamma rays have no mass or charge.
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Figure 109:Radioactive decay leading to gamma rays
Figure 110: Penetration limitations of gamma rays
Half-Life:
Half-life is the time required for a given amount of some radioactive material to be reduced to one-half of its original activity. Radioactive materials decay at exponential rates unique to each radioisotope. The half-life values for radioisotopes vary widely. For example, the half-lives for radioisotopes are shown beside.
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Figure 111: Radioactive decay and half life measurement
Formulas for half-life in exponential decay
An exponential decay process can be described by any of the following three equivalent formulas: N(t) = N0(1/2)t/t1/2 N(t) = N0e-t/τ N(t) = N0e-λt where N0 is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc.), N(t) is the quantity that still remains and has not yet decayed after a time t, t1/2 is the half-life of the decaying quantity, τ is a positive number called the mean lifetime of the decaying quantity, λ is a positive number called the decay constant of the decaying quantity. Units of Radioactivity: The Becquerel (Bq) - International Unit 1 Bq
= 1 disintegration per second
1 MBq = 1,000,000 disintegrations per second 1 GBq = 1,000,000,000 disintegrations per second The Curie (Ci) – Commonly used in the United States 1 Ci = 3.7x1010 disintegrations per second
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1 Ci = 2.2x1012 disintegrations per minute 1 Ci = 1000 millicurie (mCi) = 1,000,000 microcurie (μCi) 1 Bq = 2.7x10-8 mCi
RAD •
The RAD is the unit commonly used in the United States for Absorbed Dose (D)
It is determined by the energy that is actually deposited in matter •
1 Rad = 100 ergs of deposited energy per gram of absorber
Gray •
International Unit for Absorbed Dose 1 Gray = 100 Rads
REM
The REM is the unit commonly used in the United States for the Dose Equivalent
Determined by Multiplying the absorbed dose (D) times a quality factor (Q)
Q equals 1 for beta, gamma and x-rays, 5-20 for neutrons, and 20 for alpha
SIEVERT
International Unit for absorbed dose 1 Sievert = 100 REM
DETECTORS OF RADIATION Scintillators:
Inorganic scintillators. The most common scintillation crystal used as a radiation detector is NaI(Tl). They are used for the detection of Alpha radiation with medium resolution. Photoelectric absorption or Compton scattering in the crystal leads to scintillation light that is usually converted to an electrical pulse by a photomultiplier tube. Organic (plastic) scintillators. These low mass detectors are used for the detection of many types of radiation with generally low energy resolution. They are often used in coincidence
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systems where a particle or gamma-ray loses a small part of its energy to the detector. The scintillation light is usually converted to an electrical pulse by a photomultiplier tube.
Photon Electron Rejecting Alpha Liquid Scintillation (PERALS) spectrometry.
Selective solvent (liquidliquid) extraction methods (phase-transfers), setting the nuclide of interest into the scintillator, simplifies sample preparation and provides important additional information for nuclide identification. Such rapid, selective procedures have been developed for radium, uranium, thorium, plutonium, polonium, and the trivalent transplutonium elements. Presently, available energy resolution (230 keV FWHM) allows identification of many of the isotopes of these nuclides.
Figure 112: Probabilistic identification from sample energy
Alpha decay is a change from the ground state of an original nucleus to an excited or ground state of a daughter nucleus with the expulsion of an alpha particle. The energy released in this change will be shared as kinetic energy by the alpha particle and the recoiling daughter nucleus. Since the total momentum is initially zero, the alpha particle and the daughter nuclues must move in exactly opposite directions with the same magnitude of momentum Thus the fixed total kinetic energy will always be split between the alpha and the daughter nuclues in the same way, depending on the mass of each. This quenching variation can usually be corrected by measuring the radiation produced in that sample by a known external or internal source. quench corrections of this type cannot be used with alpha spectra. Initial small amounts of quenching do not reduce the counts under an alpha peak but simply shift the alpha peak to a lower voltage/ energy scale position. More severe quenching can push the alpha peak out of the detectable region, but the reduction in
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count is not a simple function of the amount of quencher added. First the count reduction is small, then, with more quencher, large, and then, with still more quencher, small again, as the left edge, median and right edge of the bell-shaped curve pass below the detection threshold.
Figure 113: From left to right the peaks are due to 209Po, 210Po, 239Pu and 241Am. The fact that such as
239
Pu
and 241Am have more than one alpha line indicates that
the nucleus has the ability to be in different discrete energy levels (like a molecule can).
General Principles of Radiation Detection Types of detectors:
Gas-filled detectors consist of a volume of gas between two electrodes
Scintillation detectors, the interaction of ionizing radiation produces UV and/or visible light
Semiconductor detectors are especially pure crystals of silicon, germanium, or other materials to which trace amounts of impurity atoms have been added so that they act as diodes
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Spectral Analysis
Radiation detecting instruments
Figure 114: Geiger Muller counters
Figure 115: Scintillation detector Instrument
Gas Ionization Regions Pulse Amplitude vs. Applied Voltage Ion Saturation, Proportional/Limited Proportional and Geiger-Muller Gas filled detectors operate in either 1. Current mode: The electrical signals from individual interactions are averaged together, forming a net current signal 2. Pulse mode: The signal from each interaction is processed individually
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Geiger counter:
Figure 116: Geiger-Muller Counters
Working Principle of Geiger-Muller Counters: This is the radiation detector instrument. It consists of a cylindrical tube filled with a noble gas. Charged particles travel through the â&#x20AC;&#x153;thin windowâ&#x20AC;? and ionize gas atoms, which frees electrons. These electrons are attracted to the positive wire. While they accelerate towards the wire, the electrons crash into other atoms, ionizing them as well. The free electrons reaching the wire anode create a voltage pulse in the wire. This pulse is amplified. The electric counter keeps track of how many particles are detected.
Scintillation Detectors
Figure 117: Scintillation Detectors
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Spectral Analysis
Working Principle of Scintillation Detector:
The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube. The first part of the photomultiplier tube is made of another special material called a photocathode. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument.
Spectroscopy • •
Most spectrometers operated in pulse mode A pulse height spectrum is usually depicted as a graph of the number of interactions depositing a particular amount of energy in the spectrometer as a function of energy
Figure 118:Idealized Gamma-Ray Spectrum in NaI
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Spectral Analysis
Semiconductor Detectors When a photon strikes a semiconductor, it can promote an electron from the valence band (filled orbitals) to the conduction band (unfilled orbitals) creating an electron(-) - hole(+) pair. The concentration of these electron-hole pairs is dependent on the amount of light striking the semiconductor, making the semiconductor suitable as an optical detector.
Figure 119: Illustration of the operating principles of semiconductor detector
Applications 1.
Medical Diagnoses and Treatment
2.
Research Applications
3.
Industrial/Manufacturing Applications
4.
Food Irradiation
5.
Consumer Products/Safety and Security
6.
Spacecraft Power Supply
7.
Electric Power Generation
In Medical Diagnostic Uses X-radiation: Radiographs, Fluoroscopy, CT scan Nuclear Medicine Industrial Use Moisture/density gauges
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ď&#x192;&#x2DC; Radiography ď&#x192;&#x2DC; Sterilization Medical instruments, Cosmetics, Food/Spices and more
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WAVELENGTH-DISPERSIVE X-RAY SPECTROSCOPY (WDS) A wavelength-dispersive (WD) spectrometer is used to isolate the X-rays of interest for quantitative analysis. There may be a single WD spectrometer horizontally mounted on an electron column (more typical in SEM instruments) or 4-5 spectrometers may be mounted vertically in sequence around the sample chamber (more typical of EPMA).
Figure 120: Schematic diagram of WDS operation WDS analysis involves four steps that must work together to achieve optimal results.
A beam of electrons is accelerated in an evacuated electron column of an EPMA or SEM to the sample surface with sufficient energy (typically with a potential difference of 15-20 kV) to generate characteristic X-rays for the elements to be analyzed.
Once X-rays are generated in the sample, they are selected using an analytical crystal(s) with specific lattice spacing(s). The geometry of the X-ray generating sample and the analytical crystal is such that they maintain a constant take-off angle. When X-rays encounter the analytical crystal at a specific angle Θ, only those X-rays that satisfy Bragg's Law are reflected and a single wavelength is passed on to the detector. The wavelength of the X-rays reflected into the detector may be varied by changing the position of the analyzing crystal relative to the sample i.e. the X-ray source-crystal distance is a linear function of the wavelength.
Within a given sample, once the x-ray intensities of each element of interest are "counted" in a detector at a specific beam current, the count rates are compared to
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those of standards containing known values of the elements of interest. Thus, measurement of an element's abundance requires exciting an atom to produce X-rays, focusing the X-rays through a crystal spectrometer to a detector, converting the Xrays to photoelectrons, which in turn generate an electrical signal whose magnitude is proportional to the abundance of the element! This multi-step process involves many potential breakdowns, but works reliably well to allow for routine analysis.
Strengths •
WDS works well in a variety of natural and synthetic solid materials,
including minerals, glasses, tooth enamel, semi-conductors, ceramics, metals, etc. •
The high spatial resolution of WDS not only allows quantitative analyses to
be performed on small phases but also to detect chemical zoning on a small scale within a material (e.g. mineral). •
When the electron beam is rastered, the WD spectrometers can allow x-ray
image maps of individual elements to be constructed.
Limitations Because WDS cannot determine elements below atomic number 5 (boron), several geologically important elements cannot be measured with WDS (e.g., H, Li, and Be). Despite the improved spectral resolution of elemental peaks, some peaks exhibit significant overlaps that result in analytical challenges (e.g., VKα and TiKβ). WDS analyses are not able to distinguish among the valence states of elements (e.g. Fe2+ vs. Fe3+) such that this information must be obtained by other techniques (e.g.Mossbauer spectroscopy). The multiple masses of an element (i.e. isotopes) cannot be determined by WDS, but rather are most commonly obtained with a mass spectrometer (see stable and radiogenic isotope techniques).
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Spectral Analysis
Spectra
Figure 121: WDX spectra of Molybdenite Comparison of resolution of Mo and S spectral lines in EDS (yellow) vs. WDS (blue). In the EDS spectrum the molybdenum and sulfur lines are overlapped, but can be resolved in the WDS spectrum which shows the sharper peaks. Generally, •
The XRF spectrum is just a plot of how frequently an X-ray is received for each energy level.
•
An XRF spectrum normally displays peaks corresponding to the energy levels or wavelength for which the most X-rays had been received.
•
Each of these peaks is unique to an atom, and therefore corresponds to a single element.
•
The higher a peak in a spectrum, the more concentrated the element is in the specimen.
•
Analysis of the resulting x-ray energy spectrum at each position provides plots of the relative elemental concentration variance for each element as a function of those position-point values along the given path.
•
Peak positions are specific to a particular element, (for example intensity (height) is proportional to its relative abundance).
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â&#x20AC;˘
The resulting spectral distribution is automatically compared with those from actual standards or synthetic X-ray fluorescence spectra of material formulations and displays the results as a function of the weight % of the oxides or elements.
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Spectral Analysis
X-RAY FLUORESCENCE (XRF) What is X-Ray Fluorescence (XRF) An X-ray fluorescence (XRF) spectrometer is an x-ray instrument used for routine, relatively non-destructive chemical analyses of rocks, minerals, sediments and fluids. It works on wavelength-dispersive spectroscopic principles that are similar to an electron microprobe (EPMA). However, an XRF cannot generally make analyses at the small spot sizes typical of EPMA work (2-5 microns), so it is typically used for bulk analyses of larger fractions of geological materials. The relative ease and low cost of sample preparation, and the stability and ease of use of x-ray spectrometers make this one of the most widely used methods for analysis of major and trace elements in rocks, minerals, and sediment.
Fundamental Principles of X-Ray Fluorescence (XRF) The XRF method depends on fundamental principles that are common to several other instrumental methods involving interactions between electron beams and x-rays with samples, including: X-ray spectroscopy (e.g., SEM - EDS), X-ray diffraction (XRD), WDS etc.
The analysis of major and trace elements in geological materials by x-ray fluorescence is made possible by the behavior of atoms when they interact with radiation. When materials are excited with high-energy, short wavelength radiation (e.g., X-rays), they can become ionized. If the energy of the radiation is sufficient to dislodge a tightly-held inner electron, the atom becomes unstable and an outer electron replaces the missing inner electron. When this happens, energy is released due to the decreased binding energy of the inner electron orbital compared with an outer one. The emitted radiation is of lower energy than the primary incident X-rays and is termed fluorescent radiation. Because the energy of the emitted photon is characteristic of a transition between specific electron orbitals in a particular element, the resulting fluorescent X-rays can be used to detect the abundances of elements that are present in the sample.
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Spectral Analysis
X-Ray Fluorescence (XRF) Instrumentation
Figure 122: Block diagram of XRF instrumentation The analysis of major and trace elements in geological materials by XRF is made possible by the behavior of atoms when they interact with X-radiation. An XRF spectrometer works because if a sample is illuminated by an intense X-ray beam, known as the incident beam, some of the energy is scattered, but some is also absorbed within the sample in a manner that depends on its chemistry. The incident X-ray beam is typically produced from a Rh target, although W, Mo, Cr and others can also be used, depending on the application.
Figure 123: Origin of X rays from the atom When this primary X-ray beam illuminates the sample, it is said to be excited. The excited sample in turn emits X-rays along a spectrum of wavelengths characteristic of the types of atoms present in the sample. How does this happen? The atoms in the sample absorb X-ray energy by ionizing, ejecting electrons from the lower (usually K and L) energy levels. The ejected electrons are replaced by electrons from an outer, higher energy orbital. When
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this happens, energy is released due to the decreased binding energy of the inner electron orbital compared with an outer one. This energy release is in the form of emission of characteristic X-rays indicating the type of atom present. If a sample has many elements present, as is typical for most minerals and rocks, the use of a Wavelength Dispersive Spectrometer much like that in an EPMA allows the separation of a complex emitted X-ray spectrum into characteristic wavelengths for each element present. Various types of detectors (gas flow proportional and scintillation) are used to measure the intensity of the emitted beam. The flow counter is commonly utilized for measuring long wavelength (>0.15 nm) Xrays that are typical of K spectra from elements lighter than Zn. The scintillation detector is commonly used to analyze shorter wavelengths in the X-ray spectrum (K spectra of element from Nb to I; L spectra of Th and U). X-rays of intermediate wavelength (K spectra produced from Zn to Zr and L spectra from Ba and the rare earth elements) are generally measured by using both detectors in tandem. The intensity of the energy measured by these detectors is proportional to the abundance of the element in the sample. The exact value of this proportionality for each element is derived by comparison to mineral or rock standards whose composition is known from prior analyses by other techniques.
Applications
research in igneous, sedimentary, and metamorphic petrology
soil surveys
mining (e.g., measuring the grade of ore)
cement production
ceramic and glass manufacturing
metallurgy (e.g., quality control)
environmental studies (e.g., analyses of particulate matter on air filters)
petroleum industry (e.g., sulfur content of crude oils and petroleum
products)
field analysis in geological and environmental studies (using portable, hand-
held XRF spectrometers)
bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na,
K, P) in rock and sediment
bulk chemical analyses of trace elements (in abundances >1 ppm; Ba, Ce,
Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, Zn) in rock and
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sediment - detection limits for trace elements are typically on the order of a few parts per million.
Strengths X-Ray fluorescence is particularly well-suited for investigations that involve: 1.
bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na,
K, P) in rock and sediment 2.
bulk chemical analyses of trace elements (>1 ppm; Ba, Ce, Co, Cr, Cu, Ga,
La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, Zn) in rock and sediment
Limitations In theory the XRF has the ability to detect X-ray emission from virtually all elements, depending on the wavelength and intensity of incident x-rays. However... ď&#x201A;ˇ
In practice, most commercially available instruments are very limited in
their ability to precisely and accurately measure the abundances of elements with Z<11 in most natural earth materials. ď&#x201A;ˇ
XRF analyses cannot distinguish variations among isotopes of an element,
so these analyses are routinely done with other instruments . ď&#x201A;ˇ
XRF analyses cannot distinguish ions of the same element in different
valence states, so these analyses of rocks and minerals are done with techniques such as wet chemical analysis or Mossbauer spectroscopy.
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Spectra of XRF
Figure 124:109Cd Excitation source The specra of
109
Cd shows the characteristic X-ray of individual elements. The plot shows
stronger peaks for Mn, Cu and Mo in larger amount. Weaker peak for Fe is observed but only small amount is sufficient to sample.
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Spectral Analysis
X-RAY ABSORPTION SPECTROSCOPY Introduction: When x-rays hits a sample, the oscillating electric field of the electromagnetic radiation interacts with the electrons bound in an atom. Either the radiation will be scattered by these electrons or absorbed and excite the electrons. X-ray absorption spectroscopy is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation sources, which provide intense and tunable x-ray beams. Samples can be in the gas phase, solution or solids. XAS data are obtained by tuning the photon energy using a crystalline monochromator to a range where core electrons can beexcited(0.1-100 keV photon energy). The name of the edges depends upon the core electron which is excited; the principal quantum numbers n=1, 2, and 3 correspond to the K-, L-, and M-edges respectively. There are three main regions found on a spectrum generated by XAS are shown below:
Figure 125: Typical XAS with distinctive features
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Spectral Analysis
The dominant feature is called XANES(X-ray Absorption Near Edge Structure) or NEXAFS(Near Edge X-ray Absorption Fine Structure). The pre edge region is at energies lower than XANES. The EXAFS(Extended X-ray absorption Fine Structure) region is at energies above the XANES, and corresponds to the scattering of the ejected photoelectrons off neighbouring atoms. The combination of XANES and EXAFS is referred to as XAFS.
Figure 126: Illustration of operating principles in XAFS
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Spectral Analysis
XANES XANES data indicate the absorption peaks due to the photoabsorption cross-section in the Xray Absorption Spectra(XAS)
Theory: The fundamental phenomenon underlying NEXAFS is the absorption of an x-ray photon by a core level of an atom in a solid and the consequent emission of a photoelectron. The resulting core hole is filled either via an Auger process or by capture of an electron from another shell followed by emission of a fluorescent photon. The difference between NEXAFS and traditional photoemission experiments is that in photoemission, the initial photoelectron is measured, while in NEXAFS the fluorescent photon or Auger electron or an inelastically scattered photoelectron may also be measured. The distinction sounds trivial but is actually significant; in photoemission the final state of the emitted electron captured in the detector must be an extended, free-electron state. By contrast in NEXAFS the final state of the photoelectron may be a bound state such as an exciton since the photoelectron itself need not be detected. The effect of measuring fluorescent photons, Auger electrons, and directly emitted electrons is to sum over all possible final states of the photoelectrons, meaning that what NEXAFS measures is the total joint density of states of the initial core level with all final states, consistent with conservation rules. The distinction is critical because in spectroscopy final states are more susceptible to many-body effects than initial states, meaning that NEXAFS spectra are more easily calculable than photoemission spectra due to the summation over final states. Various sum rules are helpful in the interpretation of NEXFAS spectra. When the x-ray photon energy resonantly connects a core level with a narrow final state in a solid, such as an exciton, readily identifiable characteristic peaks will appear in the spectrum.
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Spectral Analysis
Principle: In synchrotron Cyclic in particle accelerator which the particle is confined to its orbit by magnetic field, the strength of the magnetic field increases as the particles moments increases. An alternating electric field in synchrony with the orbital frequency of the particle produces acceleration. Synchrotrons are named according to the particles they accelerate.
Experimental Procedure:
Figure 127:Experimental setup for XANES. NEXAFS spectral are usually measured either through the fluorescent yield, in which emitted photons are monitored, or total electron yield, in which the sample is connected to ground through an ammeter and the neutralization current is monitored. Because NEXAFS measurements require an intense tunable source of soft x-rays, they are performed at synchrotrons. Because NEAXFS soft x-rays are absorbed by air, the synchrotron radiation travels from the ring in an evacuated beam-line to the end-station where the specimen to be studied is measured. Specialized beam-lines intended for NEXAFS studies often have additional capabilities such as heating a sample or exposing it to a dose of reactive gas.
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XANES Spectrum for Sulphur:
Figure 128: Analysis of sulphur oxidation states by XANES
XANES Energy range: In the XANES region, starting about 5eV beyond the absorption threshold, because of the low kinetic range (5-150eV) the photoelectron backscattering amplitude by neighbour atoms is very large so that multiple scattering events become dominant in the XANES spectra. The difference energy range between XANES and EXAFS can be also explained in a very simple manner by the comparison between the photoelectron wavelength ‘λ’ and the inter-atomic distance of the photoabsorber-backscattered pair. The photoelectron kinetic energy is connected with the wavelength λ by the following relation:
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Spectral Analysis
Equation 26 =h谓-
=
=
(
)
That means for high wavelength in the EXAFS region corresponds to a single scattering regime; while for lower E, 位 is larger than interatomic distances and the XANES region is associated with a multiple scattering regime.
Applications of XANES: 1.
Speciation of metals in soils, sediments and organisms.
2.
Grazing incidence studies of cations and anions on surfaces.
3.
Time-resolved studies of reactions on surfaces and interfaces.
4.
High temperature studies(trace elements in melts).
5.
Oxidation states of planetary materials.
6.
High pressure phases(diamond anvil cell).
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Spectral Analysis
X-RAY PHOTO ELECTRON SPECTROSCOPY (XPS)
Introduction The X-Rays penetrate the sample to a depth on the order of a micrometer. Irradiate the sample surface, hitting the core electrons of the atoms. Useful electrons signal is obtained only from a depth of around 10 to 100 Å on the surface. The X-Ray source produces photons with certain energies:
Mg-Kα photon with energy of 1253.6 eV with
Al-Kα photon with energy of 1486.6 eV
Normally, the sample will be radiated photons of a single energy (Mg-Kα or Al-Kα). This is known as a monoenergetic X-Ray beam. The core electrons have a higher probability of matching the energies of Mg-Kα and Al-Kα .
Principle Electrons are liberated from the specimen as a result of a photoemission process. An electron is ejected from an atomic energy level by an X-ray photon, mostly from an Al-Kα or Mg-Kα primary source, and its energy is analyzed by the spectrometer. The XPS process is schematically represented in Figure 129. for the emission of an electron from the 1s shell of an atom. The characteristic parameter for the electron is its binding energy this can be calculated by the relation i.e. BE = hν – KE – W Where
BE
and
KE
are
respectively
the
binding
and
the
kinetic
energy
the emitted photoelectron and hν is the photon energy, W is the spectrometer work function.
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Spectral Analysis
Figure 129: Schematic representation of the XPS process
XPS Instrument XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis). The technique is widely used because it is very simple to use and the data is easily analyzed. XPS works by irradiating atoms of a surface of any solid material with X-Ray photons, causing the ejection of electrons. The instrument uses different pump systems to reach the goal of an Ultra High Vacuum (UHV) environment. The Ultra High Vacuum environment will prevent contamination of the surface and aid an accurate analysis of the sample. After the first chamber is at low vacuum the sample will be introduced into the second chamber in which a UHV environment exists.
General setup The technique contains mainly the following parts:
An X-ray source,
An electron energy analyzer, combined with a detection system
A sample stage
All contained within a vacuum chamber. As for most techniques the system is operated and controlled by a computer,
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Spectral Analysis
X-Ray source Ion source Detector
SIMS Analyzer Sample introduction Chamber
Axial Electron Gun
Sample Holder sample Roughing Pump
CMA Slits Ion Pump
Figure 130: Diagram of the side view of XPS System The X-ray source
The X-ray source energy depends on the choice of the anode material, resulting in the availability of a number of discrete energies rather than a continuous variation of the energy, as exists for electron and ion guns. Al-KÎą and Mg-KÎą photons with line energies/line widths of respectively 1486.6 eV/ 0.85 eV and 1253.6 eV/0.70 eV respectively are the popular choices. The X-ray line width can be reduced by using a monochromator. The X-rays are dispersed by diffraction on the crystal and refocused on the sample surface. The cylindrical mirror analyzer
The electrons ejected will pass through a device called a CMA. The CMA has two concentric metal cylinders at different voltages. One of the metal cylinders will have a positive voltage and the other will have a 0 voltage. This will create an electric field between the two cylinders. The CMA not only can detect electrons from the irradiation of X-Rays, it can also detect electrons from irradiation by the electron gun (it is located inside the CMA while the X-Ray source is located on top of the instrument). With a change in cylinder voltage the acceptable kinetic energy will change and then you can count how many electrons have that KE to reach the detector.
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Spectral Analysis
The ion gun
Sample sputtering can be performed by means of beams of energetic primary ions. Sputtering is useful for two reasons. 1. Clean the sample prior to the analysis, because often the surface is contaminated with dirt or residual gas from the atmosphere. 2. Sputter a sample is to record depth profiles, where the composition is probed in depth by the collection of AES or XPS spectra as a function of the sputtering time.
The sample holder and stage
The mounting of the samples on the sample holder should be on an electrical conductor. This is achieved by using metallic clips or bolt-down assemblies. Alternatively a metal loaded tape may also be used. In the case of powders, the particles can be pressed into an indium foil. The sample holder is mounted on a sample stage which allows for high resolution positioning in the x, y, z and θ directions. The sample size is limited in area to a few cm2 and in thickness to a few mm
Identification of XPS peaks The plot has characteristic peaks for each element found in the surface of the sample. There are tables with the KE and BE already assigned to each element. After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface. Core level peaks have different intensities and widths. The relative intensities are governed by the ionization efficiencies of the different core shells. The line width defined as the full width at half-maximum intensity (FWHM), is a convolution of several contributions: the natural width of the core level, the width of the X-ray line and the resolution of the analyzer.
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Spectral Analysis
XPS Spectrum
Figure 131: Fe 2p core level x-ray photoelectron spectra for Fe3O4
The measured Fe 2p3/2 and Fe 2p1/2binding energy are 709.2 eV and 722.4 eV. For Fe2+ and Fe3+ are 711.1 eV and 724.5 eV .
Figure 132- XPS spectrum of Ag. Conditions: Al-KÎą at 350 W, pass energy = 58.7 eV. In the above spectrum, peaks are ejected from XPS and Auger electrons. In XPS peaks, the first digit indicates the atomic shell i.e. K, L, M, N. Second letter tells about sub shells l=s p d f... Third one indicates total angular momentum J (sum of spin and orbital angular momentum). Except s sub shells ( l=0 ) all the rest occur in doublet peaks.
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Spectral Analysis
Electronic configuration of Ag (Z = 47) 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s1 For p sub shell l=1 3p1/2 (J= 1-1/2) 3p3/2 (J=1+1/2) d sub shell l=2 3d3/2 (J= 2-1/2) 3d5/2 (J=2+1/2) The Auger peaks in the spectrum, MNN peaks in the case of Ag, originate from the decay of the ionized atoms to their ground state. M - Level of ionization N - Where the electron involved for the transition. N -From which the auger electron is emitted.
Applications Polymer surface, Catalyst, Corrosion, Adhesion, Semiconductors, Dielectric materials, Electronics packaging, Magnetic media, Thin film coatings
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ZETA POTENTIOMETER Introduction Zeta Potential analysis is a technique for determining the surface charge of nanoparticles in solution (colloids). Nanoparticles have a surface charge that attracts a thin layer of ions of opposite charge to the nanoparticle surface. Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle in a colloidal system, it can be used to predict dispersion stability. The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system. If all the particles in suspension have a large negative or positive zeta potential then they will tend to repel each other and there will be no tendency for the particles to come together. However, if the particles have low zeta potential values then there will be no force to prevent the particles coming together and flocculating. The general dividing line between stable and unstable suspensions is generally taken at either +30 or 30mV. Particles with zeta potentials more positive than +30 mV or more negative than -30 mV are normally considered stable. However, if the particles have a density different form the dispersant, they will eventually sediment forming a close packed bed (eg: hard cake).
Principle The liquid layer surrounding the particle exists as two parts; an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g. due to gravity), ions within the boundary move it. Those ions beyond the boundary stay with the bulk dispersant. The potential at this boundary (surface of hydrodynamic shear) is the zeta potential (Figure 133).
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Spectral Analysis
Figure 133: Schematic representation of zeta potential
Factors Affecting Zeta Potential • Changes in pH of the sample: Dissociation of acidic groups on the surface of a particle will give rise to a negatively
charged surface.Conversely, a basic surface will take on a
positive charge • The conductivity of the medium (concentration and type of salts). • The concentration of a particular additive in contact with the molecules such as, ionic surfactant, polymers.
Instrumentation PALS Technique ( Phase Analysis Light Scattering)
Principle
System has a laser beam which passes through the sample in a cell which carries two electrodes to provide the electric field. The light which is scattered by the particles is Doppler shifted because the scattering particles are moving in the electric field.
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Spectral Analysis
Figure 134: Optical configuration of the Zetasizer Nanoseries for zetapotential measurements
A zeta potential measurement system comprises of six main components (figure). Firstly, a laser 1 is used to provides a light source to illuminate the particles within the sample. For zeta potential measurements, this light source is split to provide an incident and reference beam. The incident laser beam passes through the centre of the sample cell 2, and the scattered light at an angle of about 13 degree is detected 3. When an electric field is applied to the cell, any particles moving through the measurement volume will cause the intensity of light detected to fluctuate with a frequency proportional to the particle speed and this information is passed to a digital signal processor 4 and then to a computer 5. The Zetasizer Nano software produces a frequency spectrum from which the electrophoretic mobility and hence zeta potential is calculated. The intensity of the detected, scattered light must be within a specific range for the detector to successfully measure it. This is achieved using an attenuator 6, which adjusts the intensity of the light reaching the sample and hence the intensity of the scattering. To correct for any differences in the cell wall thickness and dispersant refraction, compensation optics 7 are installed to maintain optimum alignment.
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Spectral Analysis
Figure 135: Inside of the cell, migration of paricles towards opposite electrode
The particles move with a characteristic velocity which is dependent on: – Field Strength – Dielectric constant of the medium – Viscosity of the medium - Zeta Potential A positive mobility of a particle means it's surface is positively charged and A negative mobility means the surface is negatively charged Vs=average drift velocity, toward the electrode of opposite charge (μ/s) μe=the electrophoretic mobility (μ/s)/(V/cm) E=electric field (V/cm) Henry´s equation • μe=the electrophoretic mobility (μ/s)/(V/cm) • ζ= zeta Potential (mV) • ε= dielectric constant of the medium • η= viscosity of the medium • F(ka)= function of particle radius. Ka • For particles in polar media the maximun value of f(ka) is 1.5 Smoluchowski Limit μe = eζ/η for κa >>1 •For particles in non-polar media the minimun value of f(ka) is 1 Hückel Limit μe = (2eζ)/(3η) for κa <<1
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Figure 136: Huckel and Smoluchowski limits
Spectral Analysis
Figure 137: Optimizing a process for preparing human serum albumin nanoparticles When the pH is less than 6 ,then the dispersion is stable and it will be 1000 nm in diameter and its zeta potential is 30mV. When the pH is 6, the particles tend to agglomerate and it will be in 2500nm in diameter and its zeta potential will be 50mV. When the pH is >6, then the dispersion is stable and it will be 500nm in diameter and its zeta potenial will be -30 mv.
Application ď&#x201A;ˇ
THE PREPARATION OF COLLOIDAL DISPERSIONS for useful purposes as in paints, inks, pharmaceutical and cosmetic preparations, food products, drilling muds, dyestuffs, foams and agricultural chemicals.
ď&#x201A;ˇ
THE DESTRUCTION OF UNWANTED COLLOIDAL DISPERSIONS in water purification, the fining of wines and beers, sewage disposal, the breaking of oil etc.
164