Optimum waveforms for differential ion mobility spectrometry (FAIMS)

Differential mobility spectrometry or field asymmetric waveform ion mobility spectrometry (FAIMS) is a new tool for separation and identification of gas-phase ions, particularly ill conjunction with mass spectrometry. Ill FAIMS, ions are filtered by the difference between mobilities ill gases (K) at high and low electric field intensity (L) Using asymmetric waveforms. An infinite number of possible waveform profiles make maximizing the performance within engineering constraints a major issue for FAIMS technology refinement. Earlier optimizations assumed the non-constant component of mobility to scale as E-2, producing the same result for all ions. Here we show that the optimum profiles are defined by the full series expansion of K(E) that includes terms beyond the first that is proportional to L. For many ion/gas pairs, the first two terms have different signs, and the optimum profiles at sufficiently high L in FAIMS may differ substantially from those previously reported, improving the resolving power by LIP to 2.2 times. This situation arises for some ions in all FAIMS systems, but becomes more common ill recent miniaturized devices that employ higher E. With realistic K(E) dependences, the maximum waveform amplitude is not necessarily optimum, and reducing it by Lip to similar to 20%, to 30% is beneficial in some cases. The present findings are particularly relevant to targeted analyses where separation depends oil the difference between K(M) functions for specific ions.