Gas phase ion chemistry: what do we know about reactions and ion formation?

Abstract

It is certainly the case that numerous sensor techniques operate based onmeasuring principles which are far less complex than those associated with ion mobility spectrometry (IMS). However, the additional complexity of IMS provides a higher level of quantitative and qualitative information. The ion mobility measurements provide characteristic spectra for investigated samples and do not merely provide a sum signal. The results are available within seconds and contain a high level of information density with excellent detection limits for numerous chemical substances. The formation of ions from neutral sample molecules is the first and possibly the most significant event during ion mobility measurements. The ionization of a sample occurs in air or nitrogen at ambient pressure. This atmospheric pressure chemical ionization (APCI) provides both the sensitivity and selectivity achieved by IMS. The most common APCI source used in IMS analyzers today is a radioactive ionization source, typically 100 or 370 MBq of Nickel (Ni) or Tritium (H). Unfortunately, these ion sources with radioactive isotopes require special use permits and licensing procedures. Moreover, the freedom to transport such analyzers is limited by certain legal constraints and semi-annual leak tests represent an additional cost and burden of ownership. For these reasons, nonradioactive ion sources have been explored and developed as alternatives to traditional radioactive sources. These developments led to the discovery of a variety of ion sources which can be used for gas sensing with IMS. More than 10 different ionization principles are described in recent review articles and monographs for the measurement of gaseous compounds [H. Borsdorf, T. Mayer, M. Zarejousheghani, G.A. Eiceman, Appl. Spectrosc. Rev. 46 (2011) 472–521; G.A. Eiceman, Z. Karpas, H.H. Hill, Ion Mobility Spectrometry, Third Edition, Taylor & Francis Group, 2013]. Each of these ion sources provides different ionization pathways and ionization reactions. Additionally, the gas phase reactions during and after ionization are considerably affected by experimental parameters (e.g., humidity, temperature, pressure). In particular, differences in temperature and humidity can result in the formation of different reactant ions and product ions. Furthermore, the sample matrix and accompanying substances can affect the gas phase reactions in IMS. All of these parameters not only influence the nature of ions formed during gas phase ion chemistry, but also affect their relative abundance depending on the experimental conditions and sample matrix. As can be seen, gas phase ion chemistry is quite complex and the prediction of possible reaction pathways requires the consideration of numerous parameters. As we move away from radioactive ionization sources and develop new ionization techniques, the study of the ionization chemistry is critical both in maintaining the ability to detect analytes in existing IMS applications as well as the development of methods to detect emerging threats. While IMS was solely used as a gas-phase analyzer in preceding decades, further developments, especially in electrospray ionization and its derivatives, now permit it to be routinely used for the analysis of liquid samples. These ion sources use high electric fields on a solution as a means to ionize polar non-volatile substances using different mechanisms than those in APCI techniques. However, ion chemistry does not stop in front of the shutter grid. Reactions can also occur as ions travel through the * Helko Borsdorf helko.borsdorf@ufz.de