By Chris Gilles
Bioanalytical Techniques
Simultaneous ESI/APCI Measurement with a Dual Ion Source
Every day, researchers synthesize and test large amounts of compounds in the chemistry laboratories of pharmaceutical and chemical manufacturing companies. In these labs, many LCMS instruments run simultaneously. Although software improvements can enhance the efficiency of analytical throughput, software alone can not improve the analysis.
T
he question concerning researchers is whether the LCMS really detects synthesized compounds. Addressing this issue requires the application of new technology linked to the LCMS hardware. In LCMS measurement, the polarity of the target compounds determines the selection of the ionization mode. For example, electrospray ionization (ESI) generally is selected for analyzing high-polarity compounds typically found in drugs and pesticides. Atmospheric pressure chemical ionization (APCI) is selected for compounds having lower polarity. (Figure 1.) When analysts know the properties of a substance, selecting the appropriate ionization mode is relatively easy. However, for synthesis researchers who handle a wide variety of compounds, the time required for selecting the best ionization mode for each compound can have a negative impact on research and development efficiency be-
cause it may be difficult to predict the best ionization method for a variety of samples. Additionally, because each ionization approach can produce a variety of background and adduct ions, the question remains as to whether the synthesized compound is actually detected. For many analysts, the primary goal is to achieve detection of the target compounds as quickly as possible. However, acquiring data using both ESI and APCI ionization modes generally requires the use of an ion source (probe) specific to each of the modes. For a researcher looking for any way possible to shorten analysis time, installing a second probe and re-running an analysis seems like a waste of time.
A Single Analysis for ESI and APCI Acquisition For the researcher looking for ways to shorten the time required for analysis and
Figure 1: Selection of Ionization Mode with Respect to Analyte Polarity and Molecular Weight
method development, a technique combining two different ionization modes would significantly improve laboratory efficiency and confidence in results. How might sample analysis be conducted using both ESI and APCI ionization concurrently without severely compromising either technique? Due to the substantial differences in the ionization mechanisms of ESI and APCI, achieving this goal requires significant development. Ionization is produced in ESI by applying a high voltage to the capillary through which a stream of ionized analyte molecules exits in nebulized droplet form. In APCI, reaction ions are formed by the electrical discharge around a high voltage corona needle. These ions react in the gas phase with neutral analyte molecules to produce ionization. Because ESI and APCI probes are each designed to optimize their respective technique, a new technology would be required to integrate both of these probes to achieve simultaneous measurement of ESI and APCI data. The DUIS-2020 is a dual ion source (DUIS) consisting of an integrated probe for analysis using both ESI and APCI techniques. The ionization flow using the DUIS is as follows: a) Sample (including mobile phase) is introduced into the probe. b) In the ESI probe, the liquid is nebulized into a fine spray. c) As a drying gas is used to evaporate the nebulized spray, ionization by APCI is produced by the corona needle. d) The sample ionized by the ESI and APCI techniques passes through the desolvation line to enter the vacuum region. ESI and APCI ionization are executed concurrently and continuously with the DUIS without relying on switching between modes. Software control of the individual continued on page 20
18 BIOTECHNOLOGY FOCUS MAY 2011
