ABSTRACT
The chemistry of ionization of constituents in a sample links the observed mobility spectrum to the composition of the sample introduced in the ion mobility spectrometry (IMS) analyzer. This is arguably the most essential step in an IMS measurement because the mobility characterization has little practical analytical value if ions are not formed from a sample or if they are not representative of the sample. In addition to this initial step of ion formation, the subsequent behavior of ions, including the lifetime of the ions and further chemical reactions, in the supporting atmosphere of the drift tube will affect the mobility spectrum. This ionization step in IMS occurs through gas-phase ion-molecule reactions, commonly in air and at ambient pressure. Historically, this has been both the strength of IMS as an analytical method and a practical complication in interpreting the analytical response. The ionization processes occurring in the source region of a mobility spectrometer have been variously referred to as atmospheric pressure ionization (API), atmospheric pressure chemical ionization (APCI), or gas-phase ion-molecule reactions at ambient pressure; the terms are used interchangeably in this chapter.
