ABSTRACT
Having summarized the foundations of ion mobility separations (Chapter 1), we now focus on the high-field phenomena that provide the basis for or are encountered in differential ion mobility spectrometry (IMS). Much of the relevant theory, now often rediscovered in the FAIMS context, was established in 1970-1980s by Mason and collaborators, with a singular contribution of Larry A. Viehland (presently at Chatham University, Pittsburgh). Unlike a generally polished edifice of gas-phase ion transport at low electric fields (Chapter 1), understanding the high-field phenomena is work in progress. The simplest behaviors (those of atomic ions in atomic gases, 2.2, or their mixtures, 2.4) are now modeled with certainty approaching that of low-field IMS, others are rationalized qualitatively though not quantified accurately or at all (e.g., those of polyatomic ions in polyatomic gases), and for many (such as the effects of clustering, 2.3, inelastic collisions, 2.5, and collisional and dipole alignment of ions, 2.6 and 2.7) the exploration has just begun. The interplay of these phenomena controlling high-field mobilities of polyatomic ions creates both complexity that presents an outstanding physical challenge and richness lying at the cornerstone of impressive flexibility of differential IMS.
