Pushing the Frontier of High-Definition Ion Mobility Spectrometry Using FAIMS

Differential ion mobility spectrometry (FAIMS) separates ions in gases based on the difference between their mobilities in strong and weak electric fields, captured directly employing a periodic waveform with dissimilar profiles in opposite polarities. As that difference is not tightly correlated with the ion size or mass, FAIMS separations are generally quite orthogonal to both conventional IMS (based on the absolute ion mobility that reflects the physical ion size) and mass spectrometry (based on mass). Until a few years ago, that advantage was largely offset by poor FAIMS resolving power (∼10-20), an order of magnitude below that achieved with conventional (drift-tube) IMS. This article summarizes the major recent technical developments that have raised FAIMS resolving power up to ∼500. These include use of higher and more stable voltages provided by new waveform generators, novel buffer gas compositions comprising high helium or hydrogen fractions, and extended filtering times up to ∼1 s. These advances have enabled previously unthinkable analyses such as broad baseline separations of peptide sequence inversions, localization variants (post-translationally modified peptides with differing PTM attachment sites) even for the larger "middle-down" peptides and smallest PTMs, and lipid regioisomers.