PNNL

Abstract

Differential mobility spectrometry or field asymmetric waveform ion mobility spectrometry (FAIMS) has emerged as a major tool for separation and identification of gas-phase ions, particularly in conjunction with mass-spectrometry. In FAIMS, ions are filtered by the difference between mobilities in gases (K) at high and low electric field intensity (E) using asymmetric waveforms. An infinite number of waveform profiles is possible and maximizing the performance within engineering constraints is a major issue in FAIMS technology development. Earlier optimizations assumed the non-constant component of mobility to scale as E2, producing the same result for all ions. Here we show that the optimum profiles for either rectangular or sinusoidal-based waveforms are controlled by the full series expansion of K(E) that always includes terms beyond the 1st proportional to E2. For many ion/gas pairs, the first two terms have different signs and the optimum profiles at sufficiently high E in FAIMS may substantially differ from those found previously, improving the resolving power by up to 2 - 2.5 times. This situation arises for some ions in all FAIMS systems, but becomes more common with recent miniaturized units that employ higher E. With realistic K(E) dependences, the maximum waveform amplitude is not necessarily best and reducing it by up to 20 - 30% is beneficial in some cases. Present findings are particularly relevant to targeted analyses where separation depends on the spread between K(E) functions of specific ions.