Analytical methods for dating modern writing instrument inks on paper by T Sh

Forensic Science International 197 (2010) 1–20

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Forensic Science International journal homepage: www.elsevier.com/locate/forsciint

Review

Analytical methods for dating modern writing instrument inks on paper Magdalena Ezcurra a,*, Juan M.G. Go´ngora a, Itxaso Maguregui b, Rosa Alonso a a b

Analytical Chemistry Department, Faculty of Science and Technology, University of Basque Country (UPV-EHU), P.O. Box 644, 48080 Bilbao, Spain Paint Department, Faculty of Fine Arts, University of Basque Country (UPV-EHU), P.O. Box. 644, 48080 Bilbao, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 March 2009 Received in revised form 16 November 2009 Accepted 18 November 2009 Available online 12 January 2010

This work reviews the different analytical methods that have been proposed in the field of forensic dating of inks from different modern writing instruments. The reported works have been classified according to the writing instrument studied and the ink component analyzed in relation to aging. The study, done chronologically, shows the advances experienced in the ink dating field in the last decades. ß 2009 Elsevier Ireland Ltd. All rights reserved.

Keywords: Forensic science Document examination Questioned documents Ink Dating Relative age Absolute age Ball point pen Roller ball pen Gel ink pen

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Closed and open systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Static and dynamic proﬁles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Relative and absolute age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Mass invariance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Mitchell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Soderman and O’Connel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. 1959, 1960, 1963—Kikuchi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Sen and Ghosh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ball point pen ink dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Composition of ball point pen inks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Aging ink evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Methods of ink age evaluation based on the evolution of resins over time. 4.3.1. 1980—Cantu´ and Brunelle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. 1987—Cantu´ and Prough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. 1987, 1989—Brunelle, Breedlove, Midkiff and Brunelle, Lee . . . . . 4.3.4. 1990—Isaacs and Clayton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5. 1993, 1994—Aginsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.6. 1995—Brunelle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.7. 2005–2006 Kirsch, Weyermann, Koehler, Spengler . . . . . . . . . . . .

* Corresponding author. Tel.: +34 94 601 2686; fax: +34 94 601 3500. E-mail address: qmeg@telefonica.net (M. Ezcurra). 0379-0738/$ – see front matter ß 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.forsciint.2009.11.013

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2 3 3 3 3 4 4 4 4 4 4 5 5 5 5 7 7 8 8 8 9 9

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4.4.

Methods of ink age evaluation based on the study of volatile compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1. 1982—Stewart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2. 1985—Humecki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.3. 1988—Cantu´ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4. 1993, 1994, 1997 Aginsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5. 2000—Brazeau and Gaudreau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.6. 2002—Brazeau and Gaudreau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.7. 2004—Locicirio, Dujourdy, Mazzella, Margot, Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.8. 2005—Bu¨gler, Buchner and Dallmayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.9. 2007—Weyermann, Kirsch, Costa Vera, Spengler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.10. 2008—Weyermann, Spengler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Methods of ink aging evaluation based on the variations observed in the dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. 1993, 1995—Aginsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. 2001—Lyter, McKeonwn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. 2001—Grim, Siegel, Allison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4. 2001, 2002—Andrasko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5. 2005—Andrasko, Kunicki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.6. 2005—Siegel, Allison, Mohr, Dunn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.7. 2006—Weyermann, Kirsch, Costa-Vera, Spengler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gel ink dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Composition of gel ink pens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Gel ink dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Study of the degradation of blue gel ink dyes by IP-HPLC and electrospray sequential ionization–mass spectrometry (ESI-MS/MS) 5.2.2. Dating black ink strokes of roller ball and gel by GC and UV–vis spectrophotometry [56] . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3. Classiﬁcation and dating of black gel inks by Ion-Pairing High-Performance Liquid Chromatography (IP-HPLC) . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 9 10 10 10 11 11 12 12 13 13 13 13 14 14 14 15 15 15 16 16 16 16 17 17 18 19 19

1. Introduction Document dating and, therefore, the time that a document and/ or ink could have been once placed on the paper, is one of the most difficult and hardest problems to solve in forensic science. This is mainly due to the great variety of inks that exist on the market, the complexity of chemical processes that inks undergo from the time they are entered on the paper when they begin their aging process and the amount of external factors that can influence this aging process (environmental factors: light, humidity, temperature; in short, storage conditions of the document) [1]. In spite of the complexity of the issue, important advances have been made with the single objective of determining for how long the ink has been deposited on the paper, which would lead to establishing the date on which the document was produced. The modern writing instruments, Table 1 – those we can find nowadays in any store of any country and, therefore, those that are most frequently used in questioned documents – are divided into two fundamental groups [2]: 1. Ball point pens: Containing oil-based inks and whose colorants are dyes. 2. Non ball point pens: Containing water-based inks and whose colorants are dyes as well as pigments (one, the other or both). In this second group fountain pens, roller ball pens, as well as markers and gel ink instruments are included. Table 1 Relevant introduction dates into the market in the field of ‘‘modern’’ writing instruments. Year

Event

1945 1950 1955 1963 1967 1970 1984

Ball point pen. Glycols as inks’ solvents. Copper Phthalocianyne as a new dye in inks. Felt tip pens. Roller ball pens. Highlighters. Gel ink pens.

If a study of the ink-aging processes is intended, the elements involved and the physical–chemical processes that these undergo after depositing the ink on the paper, should also be known. In a general and simple manner, it is possible to say that the inks of manual writing instruments are composed of a colorant or mixture of colorants, and a carrier or vehicle with one or several solvents and one or several resins [3]. – Colorants are divided into dyes (soluble in the vehicle and used in viscous and fluid inks) and pigments (dispersed in the vehicle and used, in certain cases, in fluid inks in addition to dyes). – The vehicle contains a solvent or mixture of them (fast drying organic solvents, water). – One or several resins that contribute to the properties of the inks, such as the viscosity or adhesion of the ink to the paper. – Other components are also added in a smaller proportion in order to modify the rheological properties of inks. These additives are usually kept secret by the manufacturing industry. – In addition to this basic composition a chemical marker system which is no longer in use because of its high cost, was implemented in the 70 s by the Office of Alcohol, Tobacco and Firearms of the American Treasury Department. – Manufacturers included a tag (chemical marker as rare earth organometallic compounds and traces of optical whiteners) [4–6] which did not vary over time, and another tag that varied yearly. The identification of one or both of these markers can lead forensic scientists to define the earliest possible date of the studied document. On the other hand, this labeling system would undoubtedly imply the need for knowing which manufacturer used which chemical marker. The aim of this work is to carry out a chronological review of the different analytical methodologies used for ink dating. The review of papers and technical contributions on the professional meetings begins with the marketing of the first ball point pen back in 1945 and goes on until nowadays.

M. Ezcurra et al. / Forensic Science International 197 (2010) 1–20

2. Basic concepts

colorants, evaporation of solvents and hardening-polymerization of the resins.

2.1. Closed and open systems To begin dealing with this subject it is necessary to define the following terms: closed system, refers to the ink inside the writing instrument reservoir, and open system, refers to the ink already entered on the paper and subjected to environmental conditions. From the first studies on ink aging, it was assumed that these do not undergo considerable variations within the writing media reservoir (closed system). The first study done with the purpose of determining if this hypothesis was correct or if the ink degraded inside the instruments reservoir before making contact with paper, dates back to 2002 [7], and, making use of Laser Desorption–Mass Spectrometry (LDMS). Authors concluded that most of the older instrument’s inks studied had not aged within the chambers, as had been assumed until then. Nevertheless, the analysis of some of the ink samples did suggest some aging even within the cartridge. In 2005, Andrasko and Kunicki [8] ran a new study on aging using inks inside the ball point pen chambers. These authors found that there was no indication of aging in terms of changes in the composition of dyes within the chamber of a regularly used ball point pen, but detected considerable aging in inks near the tip in ball point pen chambers that had not been used to write for several years. In this case they detected evaporation of the volatile compounds (specifically phenoxyethanol), as well as the degradation of the dye mixture. These facts were observed in the first three centimeters of writing, with the exception of BIC brand ball point pens for which it was observed during the first 50 centimeters. 2.2. Static and dynamic profiles The static and dynamic profiles are established for open systems, that is, inks that age outside the ball point pen reservoir, on a paper. The static profile [2], defined as the analytical profile of an ink that includes the stable properties of the same, that is, those properties that do not undergo variations over time. Static profiles of inks are usually constituted by a registry of their optical properties and their thin-layer chromatogram which allows establishing the existence of the different colorants that compose the ink. The more detailed the profile is, the better degree of discrimination between the inks shall be. The static approach allows determining: 1. Whether an ink displays differences or not regarding its composition with another ink by comparing their static profiles. 2. The exclusion or the identification of a source for an ink by matching its profile within the patterns profiles gathered in an ink library. The larger an ink collection is, the more reliable the match will be. 3. The date of first introduction of a given ink. If researchers have access to a large data base of the various inks, their components, tags and year of manufacture (Ink Library [9]), they can delimit the period of time in identifying an ink. The border of this period would always be the first year of introduction of the ink communicated by the manufacturers. As a result of this, detecting anachronisms, fraud can be detected. Another concept to define is the dynamic profile [2], an analytical profile of the ink that considers the processes and changes that occur in the different components of the ink, once it has been entered on the paper, hence beginning its contact with air, light and relative humidity. When an ink is entered on a paper the following physical–chemical processes begin: degradation of

1. Degradation of colorants: Some of the most typically used colorants in the ink manufacturing industry for writing instruments decompose gradually. If photo unstable compounds exist in the inks composition, such as the methyl violet family, whenever the ink on the paper is exposed to light it will decompose, while sometimes even appreciating, at first sight, a loss of color. In the methyl violet family of dyes, the more methyl groups the compound contains the more intense the color is. Crystal violet CV loses a methyl becoming methyl violet, which at the same time loses another methyl group to give rise to tetramethyl-p-rosaniline (TPR) (Fig. 1). This decomposition even takes place in the dark, ought to the oxidative action of the oxygen in the air. No variations in the solubility of the colorants are observed with the passage of time. 2. Evaporation of solvents: The volatile compounds (solvents) of the ink diminish over time. Most volatile components of the ink will evaporate in the first minutes, just after depositing the ink on the paper. This initial loss will be of up to 90%, then the amount of evaporated components decreases and, after a period of time, which for ball point pens could be between 1 and 2 years, the amount of solvents present stabilizes. The time elapsed until the volatile compounds stabilize is dependent on the ink formula and its storage conditions. 3. Hardening-polymerization of the resins: The resins present in inks begin to harden as soon as the ink is entered on the paper. The solidification or hardening of resins is a complex physical– chemical process that includes polymerization, a decrease of intermolecular distances, crossed bonds, etc. When resins harden their solubility diminishes but, what is more important, they trap the colorants and the volatile components in such a way that, the longer the ink has been on the paper, the more difficult it is to extract. This process has a limit, as it has been observed that the hardening of resins stabilizes in an interval of between 8 months and 2 years. In order to date an ink it will be necessary, therefore, to ascertain a relation of these physical–chemical processes described with measurable and reproducible parameters. The establishment of the variation of these parameters over time will provide the information needed for dating a given ink. The dynamic approach allows distinguishing between inks that only differ on their age. 2.3. Relative and absolute age Another two terms that are used in dating studies of ink within the frame of the dynamic profile are the relative and absolute age. [16] Relative age: refers to establishing which of two inks with the same formula placed on the same paper has been entered prior to the other. This concept is used in the dynamic approach where, as it is said above, two inks that only differ on their age can be compared. On the other hand the relative age concept involves sine qua non conditions: ink samples being compared must have the same

Fig. 1. Desmethylation processes of methyl violet family.

M. Ezcurra et al. / Forensic Science International 197 (2010) 1–20

formula, must be placed on the same paper and have to be kept under the same storage conditions. The amount of the ink in all the samples must be the same in all cases. Absolute age: This is a practical concept introduced operationally by Aginsky [10] to estimate the age of the ink itself without requiring the identification of the ink formula, nor a pattern comparison. His proposal was to determine the rate at which an ink ages by heating a sample of the questioned ink and comparing this with another sample of the same questioned ink that has not been heated. Heating the ink sample induces an artificial aging of the ink. 2.4. Mass invariance [16] Since dating an ink always requires carrying out a comparative study, the amount of ink taken from the paper and then tested should be the same, for all samples. Hence, the results cannot be influenced by the amount of ink sampled. But, even in the unlikely event of having, the same quality stroke (same thickness, same pressure, same width, etc.), the amount of ink collected could be different if the sampling would be repeated. Therefore, a methodology that ensures the measurements are independent of the ink amount sampled (mass invariance) is needed. One solution to this problem is taking ratios. If two measurements depend linearly on the amount sampled, their ratio is independent on the amount sampled. 3. Background The study of the dating of current inks has its embryo and origin in the studies performed with iron-gall inks. The studies of the dating of Iron Gallotanate inks, consider three different kinds of test: ion migration test, color change test and solubility test. Several investigations will be exposed. 3.1. Mitchell Mitchell [11] proposes the extraction of iron-gall ink from the paper when it is deposited with a reagent for its ulterior study. Experimental procedure: Among the different reagents tested as hydrochloric acid (HCl), bromine water, hypochlorous acid and hydrogen peroxide, the author proposes a 5% solution of oxalic acid (C2O4H2) to apply a drop to one stroke of ink and study the amount of the diffusion, if any, of the blue pigment. It must be taken into account the speed of the reaction and the amount of diffusion. Findings: When an iron-gall ink is recently deposited on a paper, it rapidly reacts with oxalic acid. If this ink has been placed on the paper for a year or so, it reacts slowly, and if it has been on the paper for 6 or 10 years, it does not react with oxalic acid for a long time. 3.2. Soderman and O’Connel Soderman and O’Connel [12] referred for the first time about the accelerated aging of an ink for the determination of its age. That concept developed by Van Ledden Hulseboch from Holland, appears in their book ‘‘Modern Criminal Investigation’’. Likewise, they compare the ink that has not been aged with that which has; this comparative process being the one that will subsequently be used in present ink dating.

Experimental procedure: In their book, they propose covering the whole document with a metallic sheet except for one letter. Expose this letter to UV radiation for a quarter of an hour at a distance of 5 inches (12.7 cm) and later study the ink exposed to radiation and that covered by the metal. Findings: If it results that the non-exposed dissolves in water with greater ease than the one exposed to radiation we can say that the entry is recent. In case both equally dissolve, the test would be non-conclusive. It is true that they work with iron-gall inks, but it is equally true that the concept of accelerated aging by exposure to UV radiation and the comparison of this exposed ink with the ink itself, without accelerated aging, are introduced. 3.3. 1959, 1960, 1963—Kikuchi Works by a pioneer woman in this ﬁeld, Yukie Kikuchi [13–15] deserve a signiﬁcant mention in this section. She examined the dispersion of an iron-gall ink in a spot test with oxalic acid, as Mitchel’s works, considering that the greater time it takes into dissolution, the older the ink is, based in the fact that ink solubility decreases with age. The differences from Mitchell’s work are mainly: 1. The concentration of oxalic acid reagent solution is between 0.01% and 0.025% instead of 5% of Mitchell used. 2. Quantitative analyses: she measures the time to reach the beginning of dissolution instead of Mitchell that gave a non measurable result. 3. She introduces the measurement of the paper as a blank and takes into account the error ranges margins. 4. Aging curves: She plots dissolution time (s) vs. elapsed time (months). Therefore, the time for dissolution to start is a function of the ink’s age.

Experimental procedure: Drop an oxalic acid solution on the ink stroke and timed until the ink started to dissolve. Then measure the dissolution rate under similar conditions. Findings: The relations between dissolution rate and time the writings had been deposited on the paper (period of time) were divided into four different categories with the following results: (1) for a period of time of few days, a very fast dissolution. (2) For a period of time of 6 months, fast dissolution but with small resistance. (3) For a period of time of 5–6 years, rapid decrease in dissolution during the first few months and after that more gradually dissolution. (4) For a period of time over 6–7 years. The error range margin for this work was 4 months in the first and second period and about 3 years for the third period. Subsequently, and as Cantu´ [16,17] exposes from his private conversations with Kikuchi, this author was one of the pioneers first to develop an analysis for the dating of ball point inks. She expanded the application of her technique to ball point pen inks, measuring discoloration of the ink by adding a drop of diluted hydrochloric acid at one point of the ink stroke. 3.4. Sen and Ghosh Sen and Ghosh [18] measured the changes in iron-base ink strokes from a period of 28 years by thin layer chromatography (TLC) examination of the blue dye and iron content. They

M. Ezcurra et al. / Forensic Science International 197 (2010) 1–20

Table 2 Experimental methods developed by different authors and their contribution to the actual approaches in dating inks. Year

Author

Experimental Methods

Analytical measurement

1920 1935

Mitchell Soderman and O’Conne

Qualitative

1959 1971

Kikuchi Sen and Ghosh

Extraction with 5% Oxalic acid solution. Acceleration of the age of an ink by UV and comparison with itself. Extraction with 0,01% Oxalic acid solution. TLC examination of inks.

introduced the idea of a ratio to achieve the mass invariance in the measurements. Experimental procedure: They extracted the inks with methanol and then spotted on the TLC plate. After that, they developed the plates using a solvent system consisting of n-butanol– acetic acid–water (45:10:45). Blue spots with the same Rf values in all the chromatograms were found. The area of those blue spots, the main colored compound of the inks, was scanned by means of a photodensitometer. After the separation of dyestuffs, the paper of strokes was ignited at a low temperature to burn off the carbon, the ash was dissolved in HCl, a solution of ammonium thiocyanate and of amyl alcohol was added and the result of this was spotted on a TLC plate and evaluated photodensitometrically. The invariance of mass was achieved by doing the ratio between the first and the second measurements. Findings: They found that the main dyestuff, deep blue spot, of the iron-base inks shows a linear decrease against the time for at least 28 years and this behavior is sufficient to specify the age of the ink. And they also found another characteristic, as an oxidation product of a dye component, which appears at about 3 months and disappears when the inks were around 9 years old. This fact can allow establishing the age of the ink. The underlying ideas of these works that are still present in the nowadays researches are: (1) changes of the extent of extraction with time, (2) comparison of one ink with itself when this has been submitted into an accelerating age process, (3) achievement of mass independence by doing ratios. In Table 2, a summary of different experimental methods detailed above for dating ink is collected. 4. Ball point pen ink dating 4.1. Composition of ball point pen inks Most of the studies that have been done in the field of ink dating have been carried out on ball point pens. The inks of these instruments are viscous and are insoluble in water. These inks are comprised of colorants dissolved in one or several solvents and resins to which other components can be incorporated as additives, in order to modify the properties of inks, such as viscosity adjusters, elasticity modifiers, corrosion inhibitors or lubricants for the sphere of the ball. Resins are natural or synthetic substances of high molecular weight that are initially liquid and that little by little dry and harden. Among those currently used in ballpoint inks, Brunelle et al. [19] included alkyd resins, polyester resins, colophony resin, phenolic resins . . . and, later, Weyermann [20] pointed out chloride and polyvinyl acetate, oleylamine ethoxylate, phthalic acid ester, hydrogenated acetophenone, condensed formaldehyde. The solvents used at the beginning of these inks were oleine, castor oil, and mineral oil. In the 1950s glycols as solvents were introduced. The most widely used today in this ink group are

Contribution to the actual approaches Accelerated aging ink

Quantitative Ratio for mass invariance

phenoxyethanol, phenoxyethoxyethanol, dipropylene glycol, phthalic anhydride, oleic acid, benzyl alcohol, 2-pyrrolidone, butylene glycol, among others [19,20]. The colorants used in writing inks can be the so-called dyes, or colorants soluble in the vehicle and pigments or insoluble colorants. In ballpoint pen inks, soluble dyes that dissolve in vehicle are used, among which stand out Victory Blue (VB); rhodamine B and 6G; the Methyl Violet group (pararosanilines with four, five or six methyl group) made up by Crystal Violet (CV), Methyl Violet (MV), and tetramethyl-pararosanilines (TPR); and the copper phthalocyanines introduced in the ink industry in 1954 (Fig. 2). The phatolocyanines shown in Fig. 2 are pigments. The copper phthalocianines dyes are produced by introducing solubilizing groups such as one or more sulphonic acid functions in CPC structure, e.g. CI solvents blue 38, amine salt of CPC used in ball point pen inks, CI 48, amine salt of CPC, used in flexographic inks, CI direct blue 199, CPC derivative, used in water based inks, and CI direct blue 86, CPC derivative also used in water based inks. 4.2. Aging ink evaluation The date on which an ink is entered on paper could be calculated if it were possible to track the behavior of one of its components over time. From the revised bibliography three big lines of investigation can be carried out, taking into account the following aspects: 1. The polymerization and hardness of resins: researches based on the extent, simplicity and amount of ink extracted with a solvent. In this section the factor being considered is the rate of dryness and hardening of resins, since the drier and harder they are, more complexity will be observed in the extraction. 2. The loss of the solvents over time: The investigations that have studied the behavior of ink volatile compounds (IVC) with respect to time. 3. The degradation of dyes: The investigations in which degradation of current dyes in ballpoint inks has been monitored. The processes suffered by the ink in the three cases are interrelated but, in spite of this fact, and knowing the groups are not watertight compartments, because they are overlap among them, the work have been divided following those criteria. 4.3. Methods of ink age evaluation based on the evolution of resins over time In this section the changes of extraction efficiencies over time are studied. Mainly the extraction of dyes but also the extraction of other non volatile and colorless components of inks is studied. The efficiency of the extraction can be characterized by two concepts [21]: 1. The rate of the extraction: that is the speed the ink is extracted over time. They are two different parameters that characterize the rate of extraction:

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Fig. 2. Structure of different colorants used in ballpoint inks.

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1.1. The R-ratio: the amount of ink extracted after a time, t1, as a fraction of the amount of ink extracted after a longer period of time, tf. Usually tf is the time after which no more ink can be extracted from the paper. It is expected than R decreases with age. 1.2. The Lth extraction time, tL: The time it takes to extract L. When L is the amount of ink extracted from the paper. L varies between 0 and 1. It is expected that tL increases with age. 2. The extent of the extraction: the amount of ink that can be extracted. 2.1 The percent of extraction, P: The percent of an ink that can be extracted just before no more ink can be extracted from the paper. It is expected that P decreases with age. 4.3.1. 1980—Cantu´ and Brunelle Kikutchi [15] was the ﬁrst to measure the dissolution of a ballpoint pen ink by doing a spot analysis on one point of the ink stroke with diluted hydrochloric acid in order to determine the ease with which it was extracted. Based on these experiments, in 1980 Cantu´ and Brunelle [16] present investigations on the relative age of inks which they had been developing since 1968 at the Bureau of Alcohol, Tobacco, Firearms and Explosives. The extent of dye extraction with different solvents was studied. They no longer did it like Kikutchi on the same sheet of paper, but they introduced the innovation of taking samples from the document. They measured extractability, in outline, such as the optical result of color density, introducing the use of spectroscopic techniques for measuring ample samples and densitometry for small samples. The authors made a differentiation between the factors that inﬂuence the extractability of the ink (E) dividing them in two main groups: (1) The ink’s own factors (formula of the ink, type of paper on which it is deposited, amount of ink removed for analysis) and (2) Parameters of the extraction (solvent mixture used in the extraction, volume of the mixture, extraction time, etc.). E ¼ EðF i ; P e Þ where E is the extractability of the ink, Fi the factors of the ink and Pe are the parameters of extraction. Experimental procedure: They used the mixture water:ethanol (1:1), as extractant for the same ink entered at different moments on the paper. With the purpose of quantifying ‘‘the extent of extraction’’ densitometer was used for small samples. Findings: They observed that the change of color for the oldest is weaker and also they noticed that there was a smaller degree of extraction, whereas for most recent a fast change of color was perceived, as well as a greater percentage of extraction. They also observed that the drying process of ballpoint inks occurs over a long period of time, 10 years had been studied. The conclusion they arrived at and enunciate as a postulate is: ‘‘if there are two identical inks on a same support one older than the other, the extractability of the oldest is smaller and slower than the one of the most recent’’. Once arrived at this point the ‘‘time’’ factor is introduced in the following experimental procedure: Experimental procedure: A plate of thin-layer chromatography with stains obtained from the extraction at different extraction times is spotted. Each sample will be more intense than the previous one, since with more time it will be able to extract more amount of ink. If the density of color of these points is moderate and they are put in an axis of coordinates

based on time, continuous curve extractabilities will be obtained. Findings: It is evident that if different amounts from the same ink are taken, they obtained different values, reason why it was necessary to look for a parameter independent of the amount of ink sampled. At the same time, to achieve this, it was necessary to divide all the extractabilities at the different times between the extractability at the time ‘‘t’’. This will give a parameter or ratio that zero and one will vary between, and it will be independent of the mass. This ratio will vary with the age of the ink. 4.3.2. 1987—Cantu´ and Prough In 1987 Cantu´ and Prough [22] developed and described indepth the Solvent Extraction Technique to measure the relative age of an ink. Before developing the method they put the limitations that the compared inks must have the same formula and must appear on the same paper, or, in its defect, equal papers (of equal manufacture) on different sheets, but with the same storage conditions. The solvent extraction technique is supported on the measurement of the efficiency of the extraction based on one hand, on the premise that the longer an ink has been deposited on the paper the drier it will be and, therefore, the more difficult it will be to extract it or, what is the same, the efficiency of extraction will be smaller. The opposite, that is to say, the fresher the ink the easier it is extracted is also assumed as true. This approach is a proposal to make the extraction concept more quantitative by using analytical methods. The ‘‘effectiveness’’ of the extraction is going to be measured so much by the rate as by the extent of the extraction, that is to say, how quickly and how much ink is extracted. 4.3.2.1. The rate of extraction. When an ink is extracted, the concentration and color of the ink in the solvent being used as an extractant increase with time of extraction (t). The representation of the variation of concentration over time constitutes an extraction curve, E(T,t), with T being the age of the ink on paper and t the extraction time. That is, for each T (for each age of the ink) it will be able to obtain a curve of dependent concentration of the extraction time. It is evident that this curve will become asymptotic for t = 1, because in a determined moment the degree of extraction of the ink will be the maximum with that solvent. The extraction curve can be achieved by measuring the absorbance of the colored solutions. If the absorbance value of the solution at its maxima absorption wavelength is measured, a curve of extraction based on the time, t, will be obtained. In order to measure this absorbance one aliquot of the solvent at a given moment is applied on a TLC plate and the intensity of these results is measured by densitometry. According to Beer’s law, each value of absorbance of an extraction curve is proportional to the concentration of the ink, which, as well, depends on the amount of sample ink. In order to avoid mass dependence a mass invariant extraction rate curve, X(T,t), can thus constructed from an extraction curve, E(T,t) normalizing it by its asymptotic value E(T,1). That is dividing each of its absorbance values by the asymptote. 4.3.2.2. The extent of extraction. In the case of trying to compare the extent of the purpose of determining the relative age of the ink, it must be independent of the mass. A way to achieve this is the procedure of solvent sequential extraction technique, in which, after the first extraction with a weak solvent (weak solvent being that which has poor extraction capacity of the ink, that is, it extracts little ink or it extracts it slowly) one second extraction with a

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strong solvent (all extracting solvent) is made. If both volumes of extraction are the same then the absorption of the ﬁrst extraction divided by the sum of this absorption plus the absorption of the second extraction becomes the percentage of extraction of the ink for a time t given in the ﬁrst solvent P(T,t). PðT; tÞ ¼

EðT; tÞ 100 EðT; tÞ þ E ðT; 1Þ

where E(T,t) is the degree of extraction of the ink in the first solvent (weak solvent) after t minutes of extraction and E*(T,1) is the degree of extraction of all the ink that was left remainder in the residuum after the first extraction that is extracted with the second ‘‘strong’’ solvent. P(T,t) represents the degree of extraction independent of the mass. On the other hand, an extraction curve is the summation of all the individual extraction curves for each of the dyes: X EðT; tÞ ¼ Ei ðT; tÞ That is, when an aliquot is placed on a thin-layer chromatography plate originating from an extraction at a specific time, a densitometric value will be obtained. If development of the chromatogram is made, the dyes will separate thereby obtaining different densitometric values for each one of the dyes. Experimental procedure: On the experimental part, the authors propose several methods, however they incline toward thinlayer chromatography densitometry: first, it is necessary to take a series of samples of the referenced paper (so that an average can soon be done) using a biopsy needle or a scalpel. These samples are placed in conical-shaped vials. At time zero t0,10 ml of solvent are added. Later, to obtain points of an extraction curve, it is necessary to remove aliquot parts from the solution of extraction at preselected times (t1 = 5 min, t2 = 10 min, . . ., tn = 30 min, being tn the time on which the extraction is almost complete) and measuring their color. This measurement is indirectly taking by spotting each aliquot on a TLC plate (as, for example, a Merck silica gel without fluorescence indicator) and taking a densitometric measurement of the wavelength indirectly from its highest absorbance (for blue and black inks, the 580 nm value is the most suitable). At this point a plate with different spots will be able to be seen, each one of which has a higher intensity of color than the previous. Findings: The solvent extraction technique distinguishes ink of the same formula written at different times on the same paper. Ink of different age differ more in the extent of extraction, as percent of extraction, than in their rate of extraction. The choice of the solvent is one of the key elements. The strength of the solvent is guided by the ability to discriminate age units, that is, days, weeks, months, years, etc. In this type of methodology the necessity of a statistical treatment is evident. 4.3.3. 1987, 1989—Brunelle, Breedlove, Midkiff and Brunelle, Lee In the same year, 1987, Brunelle et al. [23] presents a work on the relative dating of ballpoint inks, using the Single-Solvent Extraction Technique. This procedure implies ink extraction with weak solvents, spotting the extracted sample on a thin-layer chromatography plate and measuring the amount of extracted ink densitometrically. The difference between this method and the Cantu´’s Ratio method is that the amount of ink extracted is measuring directly without using ratios [24]. The age of the ink is compared with inks of well-known date, therefore, the limitation to the method is that it is dependent of the

mass and it requires identical amounts of the questioned ink and knowledge of the ink age. With the purpose of surpassing this disadvantage Brunelle and Lee [25] develop, in 1989, another method for dating ball point pens, but this time it is independent of the mass. This reviewed procedure is known as Dye Ratio Technique. The method was developed, as well, for two inks do not classified as ball point pen inks. They assumed that: 1. Ink components become less soluble in organic solvents as the ink ages. 2. The subtle fading or degradation of the dyes is, also, a factor of aging. Experimental procedure: The technique consists of extracting the ink either with a weak (n-butanol) or a strong solvent (pyridine). The extracted ink is spotted on a thin-layer chromatography plate and the dyes are separated using a solvent mixture of ethyl acetate:ethanol:water (70:35:30). After TLC development, the relative concentration of dyes was measured scanning the dyes by a densitometer. All the possible ratios were calculated. Findings: (1) Ratios of ink dyes separated by TLC vary with the age of the ink. (2) Some of the inks will continue aging for over 5 years. (3) For some inks, it was possible to estimate the age of ink up to 5 years after it was written. (4) Different paper causes different shaped ink aging curves. 4.3.4. 1990—Isaacs and Clayton In 1990 Isaacs and Clayton [26] presented a work to assess the relative aging of ball point pen ink strokes made by the same pens over a period of several months by extracting the ink from the paper with polar solvents. Their approach was an attempt to reduce the influence of the manipulative skill of the examiners in the Brunelle and Cantu´ methods. They used a diode array UV/vis spectrometer and they obtained extraction curves. Experimental procedure: Inks were deposited regularly on the paper at intervals of two to three days for a period of 4 months. Then, they extracted the paper microdots and placed them at the inlet of the HPLC of the flow cell whence solvents could be pumped through the paper directly into the cell. After that, measured by a spectrophotometer, the extraction characteristics of the individual dye components of the ink. Findings: Fresh ink marks tended to be more completely extracted than older marks, nevertheless satisfactory results about reliable indications of aging were not found. 4.3.5. 1993, 1994—Aginsky In 1993, Aginsky [27,28], proposed four different ideas for dating ball point pen inks. At least in one of these proposals, resins are clearly involved. He outlined a method of thin-layer chromatography to determine the changes with age of resins and other nonvolatile, colorless compounds in ball point inks. These changes were detected by observing the results of the thinlayer chromatograms under UV light and can be evaluated using scanning densitometry. This is illustrated with the analysis of two components that a blue Parker ball point pen has, as well as two components of the Russian-made Soyuz blue-violet ball point pen which are fractions of the resin phenoloformaldehyde and the verification that their proportion equilibrates close to 3 years after the ink has been entered on the paper.

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Experimental procedure: Parker blue and Soyud blue-violet ball point inks were placed on the paper for a period of 6 years. Three steps were carried out for this purpose: First step: Extract the samples of the known ink (having the same formula of the questioned one) by a solvent and applied onto a HPTLC plate with ﬂuorescent indicator. Develop the plate by an able solvent to separate colorless components of the ink examined and to prevent overlapping the zones of the colorless components and the components of the paper. Observe the resulting chromatogram under UV illumination to detect the zones of colorless non-volatile components. Correlations between the content of two colorless components found and the age of ink were established. Second step: repeat the same procedure for the questioned ink. Third step: calculate the age of the questioned entry by comparing the corresponding data obtained for the questioned and the known entries. Findings: For the Parker ink: The ratio substance A/substance B gradually increases with the age, being minimum for the fresh ink and maximum for the 6 year old entry. For the Soyuz ink: two different colorless components (C, D) were seen on the chromatogram. The relative proportions of those components, fractions of phenolformaldehyde resins, increases with the age, but it equilibrates after about a year and a half since the ink has been deposited on the paper. 4.3.6. 1995—Brunelle Brunelle [29] in 1995 described a new method that compares two ink samples taken from the same entry with no need of patterns; one of them was heated to induce aging and the other one remain unheated. In this case the four dyes from the family of methyl violet are taken into account. The analyzed ink was a Formulab 587 black ball point ink. Experimental procedure: Two samples of the ink were taken. One of them was treated heating the sample at 100 8C for 20 min, the other one remained untreated. Both of them were extracted ﬁrst by n-butanol for 12 min, secondly by pyridine for 15 min. The TLC plate was developed, and only the four central bands of the TLC plate were tracked. The extent of extraction decreases with the time being the overall decrease 31% for the unheated sample and 27% for the heated sample. If the four bands of methyl violet family are taken into account the overall 31% can be transformed into 3% + 11% + 11% + 6%, and the 27% into 3% + 10% + 9% + 5% (the percentages of each dye are placed in the same position in the sum). Findings: they obtained a normalized extraction curve for each band along the TLC plate. As it is already said the extents of extraction in a weak solvent are more sensitive to aging than the rates of extraction.

4.3.7. 2005–2006 Kirsch, Weyermann, Koehler, Spengler After reviewing the diverse specialized scientific publications on the subject, very few publications are found regarding ink dating based on the evolution of resins over time, and specifically, on the evolution of the rate and extent of the extraction of the dyes. The latest researches about resins deal with their polymerization. Resins, ought to their strong molecular character, have been less investigated so far. However, Kirsch et al. demonstrated that direct identification and quantification of the molecular components and their aging products by MALDI and/or ESI–MS could be succesfully used to identify specific batches and to determine their date of production, as well as to detect the decomposition products by thermal and photochemical aging processes. In addition to MALDI- and ESI–MS, High Resolution Fourier Transform Ion Cyclotron (FTICR) has been used in order to avoid complex MS spectra of resins [30–32]. In Table 3, a summary of the different methods described above are collected. 4.4. Methods of ink age evaluation based on the study of volatile compounds All the methods described below involve the loss of volatile components of an ink after it has been deposited on the paper. Nevertheless, in many cases the role of the hardness of the resins is important as well. With ink solvent, there are always two different processes that take place at the same time: one is the volatilization of the IVCs and the other is the hardening of the resins. Thus, an evaluation of the amount of a volatile component in t time implies both processes. In this sense, the option chosen in this work has been to include all the methods that involve ink volatile components in this group, taking into account they are the pioneers’ ones. 4.4.1. 1982—Stewart In the study of volatile compounds with the purpose of dating an ink entry on paper, Stewart [33] presented a pioneering study in 1982. He found that the proportion of the ink volatile compounds (IVC) decreases over time since the ink is placed on the paper, by monitoring IVC with a gas chromatography-flame ionization detector (GC/FID). The procedure used is the following: 1. Identify the ink formula through thin-layer chromatography. 2. Obtain this ink from the manufacturer or from the ink library. Find the percentage of volatile compounds contained in the fresh ink, using GC-FID. 3. Place same-formulation ink samples on a sheet of paper on different dates and storing them in standard file conditions. 4. Remove small samples from the previous sheet with a biopsy needle and deposit them in sealed micro-vials. 5. Add from 10 to 15 ml of methanol through the cover of the micro-vial and place it in an ice bath for 5 min to slow down the

Table 3 Chronology of the different methods of dating inks by changes in the extractability of the ink dyes and colorless non volatile components (resins). Year

Author

Method

1980 1987 1987 1988 1990 1993 1995

Cantu´ and Brunelle Cantu´ and Prough Brunelle Brunelle Isaacs & Clayton Aginsky Brunelle

2005

Kirsch, Weyermann, Koehler, Spengler

Different extent and extraction rate Solvent Extraction Method Single Solvent Extraction Technique Dye Ratio Technique Solvent extraction/Spectrophotometry TLC for colorless, non-volatile compounds of inks, resolution under UV light Four methyl violet dyes Extractability Resins as new criteria for authenticating questioned documents and for dating of ball point entries

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Fig. 3. Structure of 2-phenoxyethanol.

6. 7. 8.

propagation of methanol on its walls. An aliquot of between 510 ml is extracted to be injected into the gas chromatograph. Detect and quantify the micro-amounts of the volatile compounds by a GC/FID. Obtain chromatograms and identify the peaks using the data obtained from the ink manufacturer. Plot an aging curve. This curve is achieved by ﬁnding two sufﬁciently resolute peaks to make a ratio. The ratio of Peak A/ Peak B is tabulated versus to the age of the sample in days. This gives an aging curve for that particular ink formulation. Analyze the questioned sample in the same way, and, obtain the ratio of the area of both peaks. The previously calculated age curve is used to determine the age of the questioned sample. This age would be absolute if, and only if, the storage conditions of the questioned ink and that which is known were identical.

Nowhere throughout the cited article, are the volatile compounds corresponding to the analyzed peaks A and B specified, however, in the bibliography below, so much LaPorte [39], as Gaudreau-Brazeau [37] indicate phenoxyethanol (PE) (Fig. 3) as one of the volatile compounds to which Stewart refers. This method has two clear limitations: first that the formula of the problem ink needs to be identified and obtain information on its volatile compounds through the industry; second, the importance of storage conditions of both inks. Known and questioned inks cannot differ if a comparison is intended. 4.4.2. 1985—Humecki Three years later in 1985, Humecki [34] introduced Fourier Transformed Infrared Spectroscopy (FTIR) for the study of the behavior of ball point inks over time, measuring the decrease of the –OH band, what seems to be related with the loss of solvents as the ink aging. This work consisted of taking a specific ink formula and sample on a paper for a period of 22 years. Samples were dissolved in an ethanol: pyridine mixture (50:50), or just in pyridine. The extractions were measured in a spectrometer (Digilab FTS-20C) equipped with a triglycine sulphate (TGS) detector. Previously, a spectrum of the salt of the spectrometer window and the paper on which the ink had been deposited were performed as the blank verifying that, at least in this case, the paper did not interfere since the spectrum was essentially the same. Comparing the IR spectra of the ink samples of different years, he observed that the O-H band located on the 3 mm decreased as an older sample was analyzed. An aging curve was constructed plotting the absorption ratio OH/CH (O–H band over the 3 mm and C–H band over the 3.4 mm) against to the age of the ink in years. That became asymptotic over the 10 years. It is necessary to emphasize that the O–H band, which probably represents some volatile compounds, decreases more rapidly in the first years, and that later its diminution slows down. Other ratio was also made taking into account the carbonyl group band (C5 5O, over 5.8 mm) instead the OH band. Contrary to what happens with OH band, the carbonyl band increases over time, which would indicate an increase of some oxidized substance of unknown identity. The measures obtained for this ratio are worse than for the previous one. The studies by Humecki brought about an advance as far as evaluation techniques, as well as demonstrated the decrease of

solvents and increase of oxidation of an ink over time. As for determining factors, just to say that for its application, the same way Humecki did, it is necessary to identify the questioned ink, and possess an ink library in which this ink is stored and entered on a paper for a period of time 0 to 10 years in order to obtain the age curve of the ink with the ratio OH/CH bands versus the time in years. 4.4.3. 1988—Cantu´ In 1988, Cantu´ [21] takes a qualitative step in the writing ink dating field introducing his work the ‘‘Comments on the Accelerated Aging of ink’’ for the comparison of an ink to itself. In this sense he also establishes the aging parameters to produce an age curve with the aim to determine if two curves (obtained under normal or at elevated temperatures conditions) can be related; so that, one curve can predict the other. The great advance of this contribution is that it will not be necessary to know either the formula of the questioned ink nor comparative patterns which are not always available. In this study, aging parameter that decrease monotonically with age were considered by Cantu´, who developed various approaches to check several major theoretical hypothesis of ink aging using the percentage of extraction of a fluorescent rhodamine dye in a particular ink as aging property. He proposes that the heating of this ink at 100 8C for 4 min would be equivalent to 3 months of natural aging at 20 8C of this ink. This concept developed by Cantu´, ‘‘accelerated aging of ink’’, can be applied to any parameter that decreases monotonically with age to estimate the age of an ink if there are not standards. Specifically, the accelerated aging of ink has been widely used to measure the loss of volatile compounds. According Aginsky and other, the ink volatile compounds (IVC) of ball point inks level off between 2 and 3 years. If a questioned ink is younger there will be differences between the measurement of the volatile compounds of the non-heated ink and the measurements of the heated ink. If, on the contrary, the ink was older than 2 or 3 years, there would be no difference in the measurements of the volatile compounds of both inks (heated or unheated) because of the stabilization of the IVC. 4.4.4. 1993, 1994, 1997 Aginsky In 1993, Aginsky [27] developed two methods based on volatile components; the first one was a combination of gas chromatography (GC) and spectrophotometric methods for determining the mass ratio for ‘‘volatile compounds/dyes’’ of inks that decreases with their own age. The second was the use of GC to determine the reach of extraction of the volatile compounds of the inks that decreases when the ink ages on paper. In these two approaches is necessary to know the formula of questioned ink and to obtain it from the manufacturers. The first approach was based on measuring the amount of all available volatile components and of all dyes from the ink deposited on the paper, and, on determining all relevant ratios (volatile/volatile, volatile/dye). Experimental procedure: First of all, the questioned ink formula was indentified using TLC and GC or GC/MS. And information about this ink is obtained from the manufacturers. Questioned ink, as well as known age ink (at different ages) was removed from the paper. All the volatile components were separated by GC obtaining their mass (m) by measuring the area of their peaks. An internal standard was used. The ink was extracted into a strong solvent as pyridine in order to obtain all the dyes. The absorbance (A) measured at the absorption maximum of a dye was recorded.

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If there were two different volatile components, X and Y, the following ratios can be calculated: Ratio 1 = mx/my Ratio 2 = mx/A Ratio 3 = my/A Findings: The age of the questioned ink can be evaluated as a mean value of the three results obtained above. The second approach was the sequential solvent extraction approach. It included the sequential extraction procedure with weak and strong solvent. Weak solvent extracts volatile components of an ink with the extent of extraction depending on the ink age. Strong solvent completely extracts the volatile components of the ink (with the same formula) fresh and old. Experimental procedure: Volatile components were extracted into a weak solvent and analyzed the extraction sample by GC; the mass of the volatile component is measured M1. Dry the sample and extract the remaining components into a strong solvent and measure the mass M2. With those measurements calculate the percent of extractions as the percent of the mass of the ink volatile components (%M) extracted in the weak solvent relative to its total amount contained in the analyzed sample. %M = [M1/(M1 + M2)] 100. The %M is plotted versus the age, obtaining a percent extraction aging curve. Findings: Using a percent extraction aging curves the age of the Q entry analyzed can be determined. Aginsky [28] in 1994 develops another different procedure for ball point ink dating. This approach combines GC, to determine the degree of extraction of an IVC, with the accelerated aging technique. This procedure is very effective when both entries being compared have been written in different moments with ball point pens of the same ink formulation that includes ingredients such as phenoxyethanol or phenoxyethoxyethanol and resins capable of polymerizing when the ink ages on the paper. The great advantage that these methods contribute is the nonuse of dated patterns, that is, establishing the ink formula is not required and they do not require a reference sample of that same ink, instead the ink is compared to itself. Experimental procedure: the procedure is the same than in the previous approach. However, in this case, another sample is taken from the ink entry and heated moderately, at 80 8C for 5 min, and analyzed as the same way. The percentage extraction value is calculated for the heated sample too. Both of them, % of extraction for heated (%Mt) and unheated (%M) sample, are compared. Findings: If the difference between the percent of extraction for heated and unheated sample is: a. less than 10%, it must be chosen another weak solvent. b. approximately 10% or larger it can be concluded the ink is fresh. c. if %M is >70% and the difference is >10%, it can be concluded the ink analyzed is less than 6 months. The method demonstrated to be useful in the cases that the given entry or signature was made after the time the investigation began. Subsequently, Aginsky [10], in 1997, compared the Brunelle method (4.3.6.) that analyzes dyes with his own method described above that analyzes the IVC, with the purpose of evaluating which of the two methods is better. For this, seven different ball point pens with blue and black inks, some fresh and some old, were used. In this work Aginsky [60] reached the

conclusion that the studies of dyes are not as satisfactory as the studies of the volatile compounds due mainly to three sources of error: The existence of dyes on the surface of the stroke that are easily extracted with a weak solvent independently of the age of the ink. The mass dependency in the method developed by Brunelle, instead of that in the Aginsky’s method, the obtained ratio eliminates the error source due to the mass dependency. The error associated to densitometry measures taken of the spots on the thin-layer chromatography plate because the scan centers do not agree with the chromatographic spot centers. 4.4.5. 2000—Brazeau and Gaudreau In the year 2000, a new study is presented by the Canadians Brazeau, Gaudreau [35], published in 2007 [44], which shows that the volatile compounds of ball point inks can be quantified by direct analysis on paper, implementing the Solid Phase Microextraction (SPME) technique prior to GC–MS. A home-made sampling cell allows the non-destructive analysis of volatile compounds, using SPME. that does not require the use of solvents to extract analytes, it is used as the technique to monitor the evaporation of the IVC as the ink ages on a document. Andrasko [36] also reported in 2003 a SPME extraction of ink with the purpose of dating. 4.4.6. 2002—Brazeau and Gaudreau Brazeau and Gaudreau [37] present the approach used by the Canada Customs and Revenue Agency to determine the approximate age of inks. This method is called Solvent Loss Ratio Method (SLRM) and is included among the dynamic methods for ink dating. As its own name indicates it measures the evaporation of solvents in the ink to achieve an approximation of the age of the ink. This method is based on publications by Aginsky (1996) [38] and can be applied to determine if an ink that contains phenoxyethanol (PE) as a solvent has been entered on the paper in a period previous to a year since the analysis is performed. Phenoxyethanol was chosen as the most appropriate solvent after analyzing a sample of 63 ball point pens and discovering that phenoxyethanol is one of the most common solvents in all the samples analyzed. As in the Aginsky method, ink reference library is not required. The basis of the method is that phenoxyethanol contained in an ink evaporates at great speed in the first 6–8 months from the application of the ink on the paper. The rate of evaporation levels off in a period that goes from 6 to 18 months. Last, the evaporation of phenoxyethanol stops being significant after a period of approximately 2 years. This dynamic process is, precisely, the one used in this method for measuring the approximate age of the ink. Experimental procedure: The method consists of extracting two sets of ink samples (10 discs 1 mm Ø with 15 ml acetonitrile containing an internal standard as cresol). One of them will age artificially by heating the set at 70 8C for 120 min, whereas the other will remain in the conditions in which it has been extracted. The amount of phenoxyethanol of each set of samples is determined using a gas chromatograph coupled to mass spectrometry (GC–MS), (measurements are taken and an average is made). Findings: The amount of PE that an ‘‘old’’ ink loses will be less than that lost by a ‘‘fresh’’ ink; for the simple reason that since it is older the loss of solvent velocity has decreased. On the other hand, if both sets of samples are compared from the other it is obtained, if in the 120 min (Dt) the set of

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in solvent content was smaller than the error of quantiﬁcation.

Fig. 4. The amount of volatile components that evaporates during a natural or accelerated aging process.

non-heated ink samples goes from having a ta age to having a (ta + Dt) age and an amount of phenoxyethanol [PE]a to [PE]a+Dt; the same heated ink will have changed from having a ta age to a t1 age and an amount of phenoxyethanol [PE]a to [PE]1, Fig. 4. The solvent loss ratio (SLR) can be calculated the following way: R(%) = [(unheated ink heated ink)/unheated ink] 100. The age of the ink entered on the paper will therefore depend on %R, since this value will give the loss of solvent. A clear marker of the %R for each phase of the solvent evaporation, including the experimental error, should be established. The last point in which the curve %R can be 50 is established at 150 days; which allows establishing that for values greater or equal to 50 the ink will have been entered on the paper in the 150 days preceding the completion of the analysis. The last point in which the curve %R can be 25 is established at 300 days. This allows establishing that for values greater than 25, the ink has been entered on the paper in the 300 days prior to carrying out the analysis and, finally, that after 300 days the %R values are less than 25. In 2004 LaPorte et al. [39] attempt to determine the frequency with which phenoxyethanol is found in ink formulae. For this, the authors analyze the inks of 633 ball point pens using GC/MS. Phenoxyethanol was identified in 85% of black inks and 83% of blue inks. 4.4.7. 2004—Locicirio, Dujourdy, Mazzella, Margot, Lock Lociciro et al. [40] extracted ink from the paper by a solvent and then analyzing this ink by gas chromatography–mass spectrometry (GC–MS). They determined the solvent content in the sample and related it to the amount of an unidentified ink compound, which was observed to be stable in time. Experimental Procedure: one cm of the ink stroke were extracted from the paper with a scalpel and then cut into smaller pieces. PE was extracted from ink samples and submited to a derivatization process using a mixture of chloroform/pyridine/MSTFA (5:5:1) for its subsequent quantification by GC– MS. 2 ml of the extracts were injected into a GC, two unknown substances were chosen as the stable compounds. These compounds were quantified and used to calculate the evaporating compound-to-stable compound ratios. Findings: No correlation was found between the above mentioned ratios and the age of the inks. So, they concluded, ink dating is impossible using this approach, as the decrease

4.4.8. 2005—Bu¨gler, Buchner and Dallmayer In 2005, Bu¨gler, Buchner and Dallmayer [41,42] describe the application of thermal desorption followed by GC–MS analysis for dating ball point pen inks. The method uses a thermal desorption technique in two stages of the ink sample on paper which is explained below. The proportion of the amount of volatile compounds found with a desorption at a low temperature, as opposed to the amount of the volatile compounds found at a high temperature, is established as the determination of the ink age of the stroke being studied. This approach proposes a method independent of the amount of ink sampled and that avoids any contamination caused by the sample treatment. These authors maintain that the methods applied until now, in which a ratio is made between the amounts of phenoxyethanol found in two samples of the same ink, one without treating and another heated, may have significant error due to possible variations in the two samples removed from the same ink. Their method only involves one sample which is then heated at two different temperatures. With respect to how the ink behaved on paper, based, in the first place, on the amount of the PE that ball point inks initially contain between 30 and 140 ng/mm, they verified that when depositing the ink on paper 95% of PE was lost in the 3 first days, that after this, the PE decreases insignificantly but permanently and after that the amount of PE, trapped in the matrix ink resin/paper, remains constant and can be identified in insignificant amounts even in 50year old samples. The PE quantification was performance using an internal and/or external standard to make accurate measurements in the described procedures. The limit of quantitation (LOQ = mean of blank measurements + 10 standard deviation of blank measurements) was 1 ng. And, the limit of detection (LOD = mean of blank measurements + 3 standard deviation of blank measurements) was 0.4 ng. Experimental Procedure: Three steps: (1) A sample of ink on paper is heated at a T1 = 70 8C for 20 min. The evaporated solvent is collected and quantified (M1). (2) The same sample is heated at T2 = 200 8C for a period of 5 min. The evaporated solvent is quantified (M2). (3) The ratio V = M1/(M1 + M2) 100 is calculated. Findings: The ratio ‘‘V’’ between the amounts of PE obtained at low temperature as opposed to the total amount of PE obtained in both steps is a direct evaluation measurement/calculation of the age of the ball point ink. It is mass independent but it depends on the type of the paper. They found three different kinds of inks depending on its behavior: 38.5% of the inks have not a detectable amount of solvent (PE) evaporated at 70 8C, 25% of the inks are fast aging inks, that means their V decreases below 10% within 2 weeks and staying on the same level for the following 20 months, 36.5% of the inks are slowly aging inks, their V ratios are high for fresh samples, but decrease within the test period values below 10%. The method is applicable to slowly aging ball point pen inks with an age of up to 1.5 years. From the study, general rules were deduced to determine the age of an unknown ink sample, of which we do not have a knownage curve. If V is greater than 20%, the ink that is being investigated has an age of less than 30 months. The values between 15% and 10% indicate that the ink is between 15 and 9 months, respectively. Since there are numerous ball point inks that give V values below 5% even at the age of 1 month, if a V value of 5% or less is obtained, it

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Table 4 Chronology of ink aging evaluation methods based on the study of volatile compounds of inks. Year

Author

Method

1982 1985 1988 1993, 2000

Stewart Humecki Cantu´ Aginsky Brazeau, Gaudreau

2002 2003 2004 2004 2005 2007 2008

Brazeau, Gaudreau Andrasko La Porte et al. Locicirio, Mazzella, Dujourdy, Lock, Margot Bu¨gler, Buchner, Dallmayer Weyermann Weyermann, Spengler

GC/FID Infrared Spectroscopy - FTIR Accelerated ink aging to compare one ink to itself GC/spectrophotometric methods ratio volatile compounds/dyes GC decrease of volatile compounds Solid Phase Microextraction + GC/MS GC/MS + accelerated aging Solid Phase Microextraction + GC/MS PE incidence in ink formulas Quantiﬁcation of PE by GC/MS Thermal Desorption in two phases + GC/MS GC/MS variation of PE over time in laboratory conditions. Modelling of natural aging curves based on artiﬁcial aging curves

is not possible to conclude that the ink is old. In fact, a practical rule for real cases is that if V gives less than 10%, the test is inconclusive because the method is not reliable in these cases. On the other hand, the authors concluded that the proposed method depends on the type of paper with the standard offices paper (80 g/m2) resulting in the highest values V. 4.4.9. 2007—Weyermann, Kirsch, Costa Vera, Spengler Weyermann et al. [43] develop a method in which splitless gas chromatography-mass spectroscopy in selected ion mode for the quantitative analysis of solvents after liquid extraction with dichloromethane (DCM) containing 1,3-benzodioxole-5-methanol as internal standard (IS). Also the drying mechanism of ball point pen ink on paper was quantitatively characterized by measuring how the solvent of an ink stroke disappears over time; both an evaporation and diffusion process are considered. Experimental procedure: Extraction of solvents from the entries was made with DCM containing IS in a ultra-sonic bath during 10 min. The extraction sample was injected to the GC/MS for its detection and quantification in the TIC mode. 15 particular ions were selected and monitoring in the SIM mode. Quantification was performed by calculation the relative peak area (RPA) as follows: RPA ¼ Asi =AIS where Asi is the solvent peak area and AIS the peak of internal standard Findings: under laboratory storage conditions, it is possible the differentiation between fresh ink (<2 weeks on the paper) and older inks. In real cases phenoxyethanol can migrate from one sheet of paper to another when they are stored successively, as in a book or a notebook, therefore, more parameters have to be studied.

4.4.10. 2008—Weyermann, Spengler In many of commented approaches, questioned documents or their inks have been exposed to high temperature (as well as to light) to accelerate their aging process in order to simulate an artificial aging and reproduce their aging curves. However, in a natural aging process, a document might be exposed to a variety of different conditions such as air flow, humidity, light, heat, . . ., completely different from those used in the simulation in the laboratory. The modeling of natural aging of dyes and solvents from ball point inks, proved to be very complex, because of the initial ink composition, the paper substrate and the storage conditions. These factors must be taken into account in any attempt to compare artificial aging to natural aging. According to Weyermann and Spengler [1], no accelerated aging model can be

standardized for all inks since these are stored under different conditions and on different papers. Despite of this fact, a mathematical transformation of artiﬁcial aging curves into modeled natural aging curves was developed by Weyermann and Spengler [1] with a speciﬁc ink composition on a certain type of paper substrate stored in controlled conditions. This mathematical model could provide a good simulation, especially for solvents. Reproducing environmental conditions prove to be too complex in most cases. In Table 4, an analytical methods list reported based on the study of volatile compounds of inks are given. 4.5. Methods of ink aging evaluation based on the variations observed in the dyes All the methods describe below involve the evolution of the colored components of the ball point pen inks over time. 4.5.1. 1993, 1995—Aginsky In 1993 Aginsky [27] develops a method of microspectrophotometric determination of the color change velocity of inks as a result of the reaction with strong organic bases such as benzylamine or piperidine. This method follows work by Tamara Saphronenko [27] who used aqueous spot tests to distinguish the age of fountain pen inks, work that is based on what Mitchell [11] had done. The same year, Aginsky [45] at the meeting of the International Association of Forensic Sciences in Dusseldorf presents the results of his study on the aging of CV (Crystal Violet) and MV (Methyl violet), two of the dyes that are normally used in blue, violet and black writing inks. These dyes are not stable and decompose with light, but if they are not exposed to light and are kept in dark folders they will undergo no changes. However, Aginsky demonstrated that these dyes, even in the dark, undergo an oxidative process with the oxygen in the air, forming diphenylmethane derivatives (Michler’s ketone) and Phenol (Fig. 5). In any case, there was no correlation between the concentration of Michler’s ketone of inks and their age.

Fig. 5. Diphenylmethane derivatives Michler’s Ketone and Phenol formed as products of the oxidation of Crystal Violet family dyes.

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In 1995, Aginsky [46] publishes a non-destructive method based on microspectrophotometry to consider the relative age of ballpoint inks denominated the Proportion of Dyes Method. The method consists of determining the proportions of stable to unstable dyes from the superficial layer of the questioned ink stroke and from the whole ink film by comparing the reflectance values measured with natural light (specular reflectance) and with polarized light (diffused reflectance) respectively. Examinations using polarized light involves using a cross polarizer to eliminate the shine/brightness from the surface of the ink. The older ink is the one that has less proportion of unstable to stable dyes on the top layer of the ink. That is to say, the idea underlying this method consists on when an ink ages the greater changes take place on the superficial layers of inks that are in contact with the environment. At the surface, the unstable dye undergoes more change than stable dye (thus, their ratio goes down with age). Experimental Procedure: It was considered that the majority of blue, violet and black ball point pen inks contain two dyes, one very stable, as is phthalocyanine copper (D1) and another, very little stable, as is MV or CV (D2). For each ink stroke, eight points were taken that had similar thickness, similar superficial characteristics and homogeneity in the distribution of the ink. These points were located under the microscope with polarized light (with the polarizer and analyzer crossed) and recorded the reflectance spectrum with polarized light (‘‘dif’’ series, diffused reflectance) and with nonpolarized natural light (‘‘spec’’ series, specular reflactance). For each series, two wavelengths l1 and l2 were chosen for D1 (phthalocyanine copper: l1dif = 685 nm, l1spec = 670 nm) and for D2 (methyl violet: l2dif = 615 nm and l2spec = 670 nm). Afterwards, for each point analyzed, the value of the ratio, Zi, is found. This is the ratio of the relatively unstable dye to the stable dye on the surface layer of the ink. It is given by: Zi ¼

ðZ dif Þi ðZ spec Þi

where (Zdif)i = {R(l1dif)i C}/R(l2dif)i and (Zspec)i = {R(l1spec)i K}/ R(l2spec)i ‘‘i’’ is the index of the point; and C and K are specific coefficients using an iterative technique. Say that for ink line examined, the average or median of Z1 to Z8 is computed along with the dispersion (e.g. error bar) and that the method gives rather large error bars. Findings: The aging parameter (specular reflectance ratio/ diffuse reflectance ratio) gradually decreased with age during a 6 year-period. It is important to observe that no signal to reach a point of leveling off was appraised in a period of 6 years.

In this case it is very important to consider that the changes that non-stable dyes (by successive loss of methyl groups) suffer are caused by light, not by temperature and that documents kept in the dark will undergo less dye degradation than those exposed to light. Therefore, storage conditions are a very important variable in this approach. This proposal is a non-destructive method. Nevertheless the technique must be performed efﬁciently in order to decrease the distributional error in the reﬂectance measured areas of ink lines due to inherently irregular distribution of ink per area. 4.5.2. 2001—Lyter, McKeonwn At the Annual Meeting of the American Academy of Forensic Sciences that took place in Seattle, Washington, in 2001, Lyter and

McKeonwn [47] developed a new GC method using Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) technique for the Dating of Writing Ink. Measurements of the possible chemical changes that ink undergoes when it ages were recorded by TOFSIMS technique. They examined natural and artificially aged writings of a single ink and found that ‘‘the method used could distinguish writings of different dates through the presence of different proportions of dye ions present in inks’’. 4.5.3. 2001—Grim, Siegel, Allison In 2001, Grim et al. [48,49], published a study about the cationic dye Methyl Violet 2B, and the anionic dye, Solvent Black. They studied those two dyes by laser desorption–mass spectrometry (LD/MS) to provide molecular information and, also, their degradation products information. When the ink on the paper suffers an accelerated aging using UV irradiation, dye degradation products were formed and those products were detected using LD/ MS. They measured ratios of the dyes molecules and the degradation products and those ratios reflect the age of the ink. 4.5.4. 2001, 2002—Andrasko In 2001, and the following year 2002, two studies by Andrasko [50,51] were published, both about the changes in the composition of ball point inks, first, of inks stored in different lighting conditions and, second, inks aged in the dark. The changes reviewed are limited to dyes which compose the inks. In these studies HPLC with Diode Array Detection (DAD) at 540 nm was employed to monitored changes in the chemical composition of the dyes. The first makes reference to the changes revealed in dyes that compose ball point inks exposed to daylight and artificial fluorescent lights. The tracked dyes, Crystal Violet, CV, Methyl Violet, MV and Tetra para-Rosaniline, TPR, are usually found as cations in the studied inks at pH values in which the work is done. The sample treatment consists on remove single written asterisk from the paper. The ink material was extracted with 0.2 ml of methanol for 30 min at room temperature followed by heating the vial content at boiling point for 1–2 min. After that the extract was evaporated to dryness by stream of nitrogen and the dry residue was dissolved in methanol, during the whole extraction procedure the ink and the extract were kept in darkness or protected from the exposure to intense light. On the other hand, these three dyes have poor resistance to light and so much CV as MV decompose in daylight. This decomposition implies a successive loss of methyl groups that are replaced by hydrogens, that is, CV decomposes in MV and MV in TPR, whereas TPR decomposes in other similar substances due to the gradual loss of the methyl groups. The changes of composition of CV, MV and TPR are illustrated in ternary diagrams. These three compounds are related among each other in such a way that the sum of the area of the three peaks detected at 540 nm is 100%, without considering the presence and concentration of other compounds. In Fig. 6 a ternary diagram of six different inks put under different light conditions is presented. In it, the arrows mark the initial composition of the fresh inks, without exposing to light. The H2, H0 and B6 inks were exposed to daylight in the laboratory, that is, the inks were kept in the laboratory and samples were taken for analysis in intervals of ten days. The V1 and GA inks were exposed to fluorescent light at a distance of about 5–10 cm. The samples for analysis were taken in two-hour intervals. The points ^ and ^ represent the composition of the inks H0 and H2 exposed to fluorescent light for 4 h. The changes of the L7 ink are due to normal aging of the ink, the sampling has been carried out in 1 year intervals, for a period of 3 years. For the H2 ink, we can gather that four hours exposure to fluorescent light ages more than ten days exposed to environmen-

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Fig. 6. Ternary diagram of composition changes of CV, MV, and TPR and the different light conditions presented by Andrasko in his mentioned paper.

tal light within the laboratory (which initially has a percentage of 60% CV; 36% MV and 4% TPR) and almost twenty days exposure to the environmental light of the laboratory for the H0 ink (which initially presents percentages of 49% CV, 40% MV and 11% TPR). The conclusions of Andrasko’s study are that it is not always easy to state that two ink entries from different documents are different at any time, a small exposure to daylight causes the optical (color, infrared luminescence) and chemical properties to vary with changes like the one previously tracked CV ! MV ! TPR. The changes can be seen in the tertiary diagrams. These changes can determine the age for same inks entered on the same paper. In his second study, published in the 2002, Andrasko [51] studied the chemical changes suffered by compounds kept in the dark, concluding that they were similar to those obtained in inks exposed to light or heat, but much slower. As in the previous case, ink aging was monitored through ternary diagrams that combined the dyes CV, MV, TPR and Victoria Blue (VB), proposing this system of ternary diagrams to conclude the relative age of two inks of the same composition that are entered on the same paper. As a consequence the proposed method should be applied mainly for ink entries in dairies and similar documents were more than two inks samples may be found in chronological order. 4.5.5. 2005—Andrasko, Kunicki In 2005, Andrasko and Kunicki [8] ran a study on ink aging in ball point pen chambers, particularly near the end of the instrument, finding that there was no indication of aging in terms of changes in the composition of dyes within a regularly used ball point pen, but detecting what was sometimes considerable aging in inks near the tip of the chambers not used to write for several years. In this case, PE evaporation was detected as well as the aging of the cationic dye mixture. This was only detected in the first three centimeters of writing, except for a BIC ball point pen which was observed for the first 50 cm. 4.5.6. 2005—Siegel, Allison, Mohr, Dunn In 2005, Siegel et al. [52] published a study on the use of LDI/MS to not only explain the structures of dyes used in ink manufacturing but also to follow up on the chemical variations that these present over time. The dyes were artificially aged by Using UV or incandescent light and then analyzed by LD/MS to characterization. 4.5.7. 2006—Weyermann, Kirsch, Costa-Vera, Spengler In 2006 Celine Weyermann et al. [53] did a study on aging processes, concretely on the loss of colour of ball point ink dyes on

paper. More specifically, the degradation processes of MV, and EV, were studied using laser desorption ionization (LDI) in comparison with matrix assisted laser desorption ionization (MALDI), and Mass Spectrometry (MS) directly on paper. The influence of the sample preparation technique was evaluated by comparing MALDI-MS spectra of extracted ball point strokes in the solvents. The possible application of these methods to forensic document examination was also evaluated. MALDI differs from LDI in the use of a matrix mixed with an analyte before the analysis that absorbs light at a given laser wavelength, which allows the compounds, that do not absorb laser light to be desorbed and ionized without much fragmentation. Generally, the addition of the matrix improves the sensitivity of LDI-MS. The matrix protects the analyte and helps in the ionization and desorption process. 2,5-Dihydroxybenzoic acid powder (DHB) was used and prepared at a concentration of 10 mg/unit in a solution of H2O:EtOH (3:2). The degradation of ball point ink dyes was studied under laboratory conditions influenced by different factors such as light, light wavelength, heat and humidity. Later, strokes from the same ball point pen were allowed to age naturally in the dark or under the influence of light for a year and then analyzed. The results showed that the degradation of dyes is directly influenced by light. Humidity also increases the degradation, which can be explained by the alkaline nature of the paper (which has a thin calcium carbonate layer for bleaching purposes). The influence of heat in the degradation process was very weak. Likewise, it was observed that the dyes did not suffer great degradation after a year of storage in the dark. For the realization of this study, pure MV and Ethyl Violet, EV, (purchased to Fluka and Sigma–Aldrich companies, respectively) and also ink strokes made with a BIC medium blue ball point pen were used. Being submitted to artificial aging and natural aging (the ball point pen inks: it was written every month for a year, one test sample was kept in the dark and the other in daylight—in winter and summer). The sample treatments consist on heating the samples at 100 8C in an oven to exposure the sample to light; a Xenon light pressure lamp was used in a wavelength range 250–1000 nm with high fluence. The sample preparation was carried out as follows: Dissolved references substances in methanol and analyzed 0.5 ml The ball point entries were extracted from about 2 cm strokes in ethanol, TFE, phenoxyethanol and BIC mix (ethoxyethoxyehtanol:dipropylen glycol 1:2) during 10 min at 60 8C. MALDI/MS for pure MV and EV compounds are characterized by the presence of the molecular ions M+ = 372.2 u and M+ = 456.3 u, respectively; it was observed that EV and MV degradation under the influence of light was characterized by a loss of CnH2n groups. MV presents six degradation products (D = 14 u) and EV another six (D = 28 u), Table 5. The degradation is quantified by the relative peak area of (RPA) Ai 100% RPAi ¼ Atot where Ai is the area of an ion signal for a m/z = i and Atot is the total area of all the signals. Table 5 Characteristic ions (m/z) obtained for MV and EV from mass spectra. Dye

m/z (u)

MV EV

358.1 428.2

344.1 400.2

330.1 372.2

316.1 344.1

302.0 316.1

288.0 288.0

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Table 6 Chronology of ink dating based on the degradation suffered by its dyes. Year

Author

1993 1993 1995 2001 2001 2001 2005 2006 2006

Aginsky Aginsky Aginsky Lyter, McKeonwn Grim, Siegel, Allison Andrasko Andrasko, Kunicki Siegel, Allison, Mohr, Dunn Weyermann, Kirsch, Costa-Vera, Spengler

With this definition the RPAi age curves are constructed based on time and these fit in an exponential function as follows: y ¼ y0 þ Aeð x=zÞ where X is the time, y is the RPA value, y0, A, z are constants. Among the conclusions to this study are the following: 1. The extraction of the analyte with a determined solvent can induce undesirable effects. 2. A laser’s high density energy has as a consequence greater fragmentation of the molecular ion. Therefore, a density of energy near the detection threshold of the ion has been used. 3. MALDI and LDI-MS contribute valuable information about the degradation of inks. 4. The age curves used are a tool that gives a lot of information on the process of degradation process of dyes, with reproducible results and a very small error margin. 5. The degradation of dyes depends significantly on the storage conditions of the sample. The degradation of dyes is caused mainly by the absorption of light with wavelengths in the UV and at the dyes’ maximum absorption. High humidity conditions in the presence of light increase the degradation of dyes. (a dry environment for storage is advisable) Heat is a weak influence in the degradation of dyes, but can mainly be considered especially for exposures of over 100 8C. The strokes kept in the dark at room temperature did not show any degradation after a year. But, it is necessary to know the storage conditions to give an accurate interpretation through the evaluation of dyes of an ink for forensic purposes. 6. After developing this study, a priori condition is postulated: the initial composition of the ink must be known for the interpretation of a MS.

Micro-spectrophotometry/change of color with strong organic bases. Degradation of CV and MV, appearance of Michler’s ketone and phenol. Micro-spectrophotometry/Dye proportion method. TOF-SIMS LD/MS HPLC/tri-dimensional diagrams CV ! MV ! TPR HPLC LDI/MS LDI-MS, MALDI-MS. MV and EV Study

In 2005, Mazzela and Buzzini [59] established that among blue gel inks, based on pigments, the following two were mostly detected: Blue 15 (C.I. 74160) Phthalocyanine blue, and Violet (23 C.I. 51319) Carbazole violet (Fig. 7). 5.2. Gel ink dating The only researches carry out about dating gel inks appeared in 2006. The publications about this subject are the followings: 1. Study of the degradation of blue gel ink dyes by IP-HPLC and electrospray sequential ionization–mass spectrometry ESI-MS/ MS [55]. 2. Dating black ink strokes of roller ball and gel by GC and UV–vis spectrophotometry [56]. 3. Classiﬁcation and dating of black gel inks by Ion-Pairing HighPerformance Liquid Chromatography (IP-HPLC) [54].

5.2.1. Study of the degradation of blue gel ink dyes by IP-HPLC and electrospray sequential ionization–mass spectrometry (ESI-MS/MS) Yi-Zi Liu et al., in 2006 published a study on the degradation of blue gel ink dyes by IP-HPLC and electrospray sequential ionization mass spectrometry ESI-MS/MS [54]. In the ﬁrst part of this research IP-HPLC with UV detection was used to analyze the blue gel pen inks and their photo-degradation products after an aging process. Experimental procedure I: 47 blue gel pens were collected from markets. The aging samples were carried out exposing ink samples to UV light at 254 nm and to a ﬂuorescent tube from about 10 cm distance. Naturally aging samples were sotred in

In Table 6, analytical methods of ink dating described above based on the dyes degradation studies are collected. 5. Gel ink dating 5.1. Composition of gel ink pens Gel ink pens became popular writing instruments all over the word due to its smooth writing characteristics since 1984, when they were first manufactured. Until now, there have been developed several methods with the aim of analyzing and classifying them. In this sense, Mazzela, Khanmy-Vital [57] examined and classified various gel inks in 2003 using filtered light examination, Raman spectrometry and SEM. On the other hand, Wilson et al. [58] developed a systematic determination of black gels using optical and chemical techniques, i.e. microscopy, vis, NIR reflectance, NIR luminescence, TLC, spot tests and GC/MS.

Fig. 7. Main pigments in gel inks.

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natural conditions at room temperature and preserving from sunlight. For each sample, 5 cm ink line was cut out and extracted with 0.5 ml TBA/acetonitrile (1:1) during 10 h at room temperature. The chromatographic conditions were optimized by selecting suitable ion pairing reagent for achieving a satisfactory separation of dyes, its concentration and pH value. A series of volatile and non volatile ion pair reagents alkyl ammonium salt with different alkyl chain were studied such us TEA, TBA, TBABr, and DHA. TBA acetate was chosen as the ion pairing reagent to perform IP-HPLC analysis at pH 7.0. The UV detector was set at 580 nm. A previous solubility test with methanol of 47 gel inks was performed with the purpose of separating two groups, those inks based on dyes, 27, and those based on pigments, 20. These latter could not be extracted from the paper for a subsequent HPLC analysis; while the first 27 were separated by IP-HPLC, efficiently separating all the dyes. An ink was taken at random, G7 and its aging process followed; on the one hand under fluorescent light and on the other in natural aging conditions during 9 months. Findings. Two peaks were initially observed; peak 1 with a retention time of 6.2 min and peak 2 with a retention time of 8.3 min. After putting the sample under fluorescent light it was observed that two new peaks appeared; 3 and 4, with retention times of 3.9 and 4.9 min respectively, which indicated that new components had formed. The two new peaks become more prominent with the age; whereas, weaker peaks appear around these after 36 h under fluorescent light. After 100 h these small peaks become more prominent while the intensity of peak 1 decreases with time. For inks recorded in natural conditions two main peaks were observed (that concur with the aforementioned with retention time of 6.2 min and 8.3 min), but there was practically no change in these peaks after 9 months of storage with the exception that the relative intensity of peak 1 slowly decreases, while peak 2 slowly increases. This phenomenon illustrates that the component of peak 1 decomposes in natural aging conditions. These results are not the same ones than those shown by Andrasko [8,50,51]. In the second part UPLC/MS/MS is used to identify the dye and its degradation products. Ammonium Carbonate was used as ion pairing reagent because TBA produced interferences. It was inferred that the molecular ion of molecular weight 749 was ‘‘Acid Blue 9’’ (brilliant blue FCF, food blue 2, etc.) (Fig. 8) Several degradation processes of this dye and also several degradation compounds of this main dye are proposed. 5.2.2. Dating black ink strokes of roller ball and gel by GC and UV–vis spectrophotometry [56] This study on the dating of water-based inks (so much the roller as gel) was presented by Yuanyan Xu, Jinghan Wang, Licuan Yao who used GC and UV–vis spectrometry [56].

Fig. 8. Acid Blue 9 Structure.

In this study, 6 German and Japanese roller and gel inks are analyzed. Their storage conditions are a drawer. Two centimeters samples are taken and analyzed with GC and UV-vis. Each sample is extracted and then placed into a vial with 1 ml of methanol that contains ethyl benzoate during 20 min. After which 2 ml of each sample were analyzed with GC. The residual solution was extracted with 0.9 ml of FMF during 20 min to maximize the total amount of dye. Then UV–vis spectra were registered. For each sample, absorption measurements for the maximum absorption of the dye present in the ink are read. This absorption is taken into account with the purpose of eliminating the influence of the thickness of the ink in the stroke. The aging parameter is defined as follows: Ratio ¼ Apeak ðsolventÞ=Apeak ðexternal comparative matterÞ Apeak ink The aging curve is established, drawing the ratios versus the age on a graph. Through these curves, the age of a questioned document can be calculated. If the ink samples have two or more solvents, two or more age curves will be obtained. In order assess the absolute age of the ink, that is to say, in order to make an evaluation of the ink without the necessity of an ink reference for comparison other percentage D are established, and the D values related to age are established. The ink strokes are heated up to 60 8C in an oven for 1 h, and D is defined as: D% ¼ ½ðR RT Þ=R 100% where R is the value of the ink ratio without heating and RT is the value of the ratio of the heated ink. Depending on the D values, ink ages can be established on the following way: 30% D% < 80% the inks are fresh. 0% < D% < 30% the inks are between 10 and 90 days. D% = 0 the ink is old having over 90 days. In no case, throughout the whole article, is reference made to the solvent(s) being analyzed. 5.2.3. Classification and dating of black gel inks by Ion-Pairing HighPerformance Liquid Chromatography (IP-HPLC) This study is made on the basis of tracking on the colorants of a sample set of 93 black gel ink instruments. Running a previous solubility study with methanol, they are divided in two groups: those that have an ink based on dyes, a total of 50, and those that have an ink based on pigments, a total of 43 (this does not mean that inks based on dyes do not contain some pigment). The dye based gel ink group is normally acid dyes or direct dyes that are ionic compounds and more easily extracted from the paper than pigments. In this study, IP-HPLC with UV detection at 580 nm, which is a powerful method to separate ionic compounds, is used to analyze ionic dyes of gel inks, assuming that the study of dyes gives more useful information to assess the age of the ink than the study of pigments. Experimental procedure: Five centimeters ink line was cut out from the paper and the ink entries were extracted by addition of 0.5 ml TBABr 40 mM:acetonitrile (1:1) for 12 h at room temperature. A series of solvents and mixtures of solvents with different polarities was examined to extract the dyes. A mixture of a buffer solution at pH 7.0 of TBABr and acetonitrile 1:1 as eluent was employed. Findings: It was discovered that all inks have 2 or more dyes, and they were again divided in three groups depending on the retention times and the number of dyes present.

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Fifty percent (50%) of analyzed inks belonged to the first of the groups which displayed peaks at 8.8 min; 10.8 min; 13.1 min; 20% of analyzed inks belonged to a second group that displayed peaks at 6.2, 8.8, 10.8, 13.1 min; and 16% of the inks composed a third group that displayed peaks at 7.0, 15.3 min. There are a percentage of inks, which could be included in a fourth group that has more than two chromatographic peaks but whose retention times are different between themselves. Within each group the different ink formulas present the same dyes but with different percentages of each (which is translated in the areas of the peaks). At no point throughout the study is mention made about any of the peaks having been identified. The aging for each of the groups has been tracked separately by taking, at random, one ink from each group (no. 32 for the first group, no. 1 for the second group, and 21 for the third group). Group 1: Peaks 1 and 2 decrease with time whereas group 3 increases gradually, not originating new peaks in the chromatograms, and so much the samples aged with ultraviolet light as those that underwent natural aging. Group 2: The dyes of this group decompose easily. Peaks 5 and 6 are more prominent after storing for four weeks in environmental conditions, peak 4 obviously decreases. It is deduced that compounds 5 and 6 are originated by degradation of component 4 based on the fact that no new peaks appear after the degradation of peaks 1 and 3. On the contrary, the weight of peaks 5 and 6 decreases with increasing age under UV light. The relative changes in the weight of the peaks reflect the decomposition speed of dyes and the increase of the relative weight of the peaks indicates that the decomposition speed of the relevant components is lower than those of other compounds. When the aging has been achieved naturally it is observed that the changes in the composition were significant with time. Peaks 5 and 6 that have been discussed above, and that appeared after a week of storage, disappeared after 13 months in natural aging conditions. The tendencies of the changes in the composition were slightly different from the samples aged under light. The relative weight of Peak 3 increases with time in natural aging, and indicates that the decomposition speed of this component is smaller than any of the other components in relation to its degradation in artificial aging by light. Component 4 decomposes very quickly the first days whereas the speed of decomposition decreases later. Component 4 can be more sensible to air, with the most superficial layers decomposing and the deepest layers presenting little influence. Peaks 2 and 3 are the same as the previous group and their changes with time can contribute information on the age of the ink. The changes in the relative weights of peaks 4, 5 and 6 can contribute the key to differentiate inks from the second group of those of the first. Group 3: Ink 21 was selected as representative of the group. The chromatogram displayed two main peaks; 1 and 2 and two other smaller peaks, 3 and 4. With time in the samples exposed to fluorescent light, peaks 1 and 2 decrease, whereas 3 and 4 increase. The results are not consistent with those of the samples aged with UV light. For inks aged under natural conditions, in addition to the aforementioned four peaks, appear peaks 5 and 6 close to peak 4 and 1, respectively. Peaks 3 and 4 behave as those samples exposed to fluorescent light, increasing, whereas peaks 1 and 2 decrease with time. Peaks 5 and 6 become more prominent with time. This fact indicates that peaks 5 and 6 are products of the degradation of peaks 1 and 2. The mechanisms of aging of these inks based on the degradation of dyes have been studied. This degradation is reflected in the

chromatograms, establishing differences in the aging mechanisms when it has been performed naturally or artiﬁcially.

6. Conclusion The present revision includes major articles and/or contributions to conferences since the year 1920 through 2008 in the field of the ink dating. From them, we can deduce that ink dating is an extremely complex problem, due to the amount of variables that influence the ink–paper system, and despite the valuable contributions that have been achieved in this field, no solution has yet been found. Prior to 1950, separation techniques were foreign to the field of ink analysis because the methods themselves were in fledgling stages of development. Document examiners relied upon filter photography, alternate light sources, and chemical spot tests to differentiate ink samples. Non destructive methods such as IR and diffuse reflectance IR, microspectrophotometry, visible and IRL remain important, valuable tools for the document examiner. In fact, TLC is one of the most popular methods because its use and its ability to quickly generate qualitative information unavailable through non-destructive spectroscopy. TLC it is not the most indicative technique neither to study non coloured components of inks nor to make quantifications; so, much research has been developed to explore other applications of instrumental techniques, especially GC, for the analysis of VCs in ink dating procedures. From the point of view of analytical chemistry, accuracy and repetitivity play an important role. Accuracy of the aging techniques due to aging curve is a decreasing exponential curve and therefore, accuracy decreases dramatically as the aging process levels off. And repetitivity measurements must be conducted when determining relative age of ink to show how reproducible or reliable the measurement is. It is now widely accepted that the dating methods based on artificial aging and sequential extraction of dyes are not reliable [1,60–63]. Nowadays the main method used by a variety of laboratories in USA and in Europe to date inks is comparing an ink to itself, exposing it to artificial aging and comparing the concentration of volatile compounds of the aged ink with those from that same ink without aging, concretely, phenoxyethanol for ball point inks. PE has been the most studied compound using the most sophisticated instrumental techniques, mainly: - Thermal desorption and SPME for the separation step; - GC/FID or GC/MS for the identification and quantification. This method has its limitations, however, due to: 1. Volatile compounds stabilize after a period of approximately 2 years; if the ink is older it cannot be concluded. 2. PE, can migrate through the paper and from one sheet of paper to another when they are stored successively. 3. The latest research carried out at our laboratories [64]. allow us to spot several error sources in the methods mentioned above which are still unstudied such as the different kinetics in the loss of PE, mass invariance, importance of the extraction times, cross contamination, etc. These error sources can lead to mistaken conclusions in many cases. In our experience those methods are suitable only in a very few number of cases. It is important to emphasize that most investigations are centered on the ink of the glycol based ball point pen, and that the only studies known for non-ball point inks are those elaborated for gel inks.

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About gel ink dating poor knowledge has been carried out. The compositional changes of different classes of gel inks studied have a tight relationship with the aging of time, although the mechanism of their compositional changes were complicated and not understood yet. Nevertheless, some difference has been observed in the aging process of the gel ink entries on paper depending on conditions, which are useful to identify whether a suspicious document was artificially treated or not. A third way is the study of the polymerization of resins. There are very few number of research in this field, the most important and recent are the ones done by Kirsch [30–32]. However, this is an unexplored way which could give satisfactory results unless its research is very complex. It is expected that the use of non-destructive, greater sensitivity and resolving power techniques will further improve the chemists’ ability to resolve the studies in the forensic ink dating field. Final recommendations for future works can be summarized as following: 1. Need for inter-laboratory validations of the methods based on the loss of volatile components. According to our latest studies the Retention Time Lock mode in GC–MS [64] can be used as a really useful tool for the inter-laboratory validations. 2. The further study of the kinetics of the resins with time and its use in ink dating. New investigations about resins are more promising for age determination over a longer time range. Acknowledgements We really much appreciate the invaluable collaboration given by Ph. D. Anthony Cantu´ in the revision of the full article, and also for his really interesting suggestions. Authors also thank the University of Basque Country/EHU for financial support. Finally, we also thank very much Ms Ruth Wolff for her final English revision. References [1] C. Weyermann, B. Spengler, The potential of artificial aging for modelling of natural aging processes of ballpoint ink, Forensic Science International 180 (2008) 23–31. [2] Cantu´, A., Agu¨ı´ Palomo, A.L. Ana´lisis Forense de Tintas. Curso de Avances en Criminalı´stica y Gene´tica Forense. Instituto de Medicina Legal de Valencia 2006. [3] R. Brunelle, W.R. Reed, Forensic Examination of Ink and Paper, C.C. Thomas Publisher Springfield, 1984, pp. 9–42. [4] Ames Laboratory, ERDA, Iowa State University; Development of an Ink Tagging Program. Ames, IA, USA, March 1973. [5] Brunelle, R., Cantu´, A. New Developments in the Dating of Inks. Interpol Meeting 4th International Forensic Science Symposium, Saint Cloud, France, July 1975. [6] Brunelle, R., Cantu´, A., Lyter III A.H., Current Status of Ink Analysis, 1978. [7] D. Grim, Does ink age inside of a print cartridge? Journal Forensic Sciences 47 (November (6)) (2002) 1294–1297. [8] J. Andrasko, R. Kunicki, Inhomogeneity, Aging of ballpoint pen inks inside of pen cartridges, Journal of Forensic Sciences 50 (May (3)) (2005) 542–547. [9] J. Hargett, The International Ink Library, Scientific Police R.I.P.C. (July–August) (1990) 33–34. [10] V. Aginsky, Current Methods for Dating Inks—Which is the Best? in: 49th Annual Meeting American Academy of Forensic Sciences, New York, 1997. [11] C. Mitchell, Examination of the Age of Ink in Writing, The Analyst XLV (July (532)) (1920) 247–258. [12] H. Soderman, J. O’Connel, Modern Criminal Investigation, 1st ed., Literary Digest Books, Funk & Wagnalls Company, New York/London, 1935, p. 408. [13] Y. Kikuchi, Estimation of Age of Blue Black Ink writing (I), Japanese Police Science Laboratory Report 12 (3) (1959) 379–386. [14] Y. Kikuchi, Studies on the Age of Iron-Gallotannate Ink Writing (II) the Chromatic Study of Ink Stain, Journal of Criminology 26 (2) (1960) 39–59. [15] Y. Kikuchi, Estimation of Age of Blue Black Ink Writing (III), Japanese Police Science Report 16 (1) (1963) 83–86. [16] Cantu´, A., Brunelle, R. the Relative Aging of Ink. Technical Communication in the 1980 Annual Meeting of the American Society of Questioned Document Examiners. [17] A. Cantu´, A Sketch of Analytical Methods for Document Dating. Part II. The Dynamic Approach Determining Age Dependent Analytical Profiles, International Journal of Forensic Document Examiners 2 (July/September (3)) (1996) 192–208.

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