ABSTRACT
The narrative now shifts from the science of high-field ion transport to its exploitation in differential IMS and, specifically, FAIMS. This chapter starts from the basics of separation utilizing asymmetric waveforms (3.1.1) with their profiles optimized without constraints (3.1.2), for practical solutions based on harmonic oscillations (3.1.3), and globally with variable waveform amplitude (3.1.4). We compare the performance of various waveform classes (3.1.5) and look at the optimum waveforms for global (3.1.6) and targeted (3.1.7) analyses in realistic regimes. Then we probe the limitations on differential IMS paradigm imposed by translational inertia (3.2.1), consider the options for dispersive (3.2.2) and filtering (3.2.3) FAIMS modes, and describe the methods for extraction of mobility (field) functions from FAIMS data and their validation employing DT IMS (3.2.4). We move on to FAIMS separation parameters, reviewing their nomenclature (3.3.1), classification of ions by their trends (3.3.2), their dependence on the ion properties (3.3.3) and gas temperature (3.3.4), and their behavior in the pendular regime (3.3.5). Next are separations in heteromolecular media, including gas mixtures (3.4.1), vapors in general (3.4.2), vapors exchanging with the ion (3.4.3), and vapors solvating the ion (3.4.4). Finally, we discuss ion transformations in FAIMS induced by field heating of ions, addressing their overall effect on FAIMS operation (3.5.1), their relationship to the field intensity as characterized using FAIMS=DT IMS systems (3.5.2) and FAIMS alone (3.5.3), the approaches to their suppression (3.5.4), and their value for improving FAIMS specificity (3.5.5). In this chapter, the features of FAIMS separations are illustrated by model experiments, with real analytical applications being the subject of a future companion volume. The performance metrics of actual FAIMS systems and their control by instrumental and operational parameters are covered in Chapter 4, and the use of FAIMS for guidance and trapping (rather than separation) of ions is relegated to 5.1.
