FAIMS separates ions by their differential mobility, meaning the difference in how an ion responds to the high-field and low-field portions of the applied waveform. Ions with different mobility changes behave differently while traveling through the carrier gas. Adjusting the compensation voltage counterbalances this behavior for a selected ion, allowing that species to pass selectively.
The compensation voltage selects which ion has the appropriate net mobility for transmission through the FAIMS device. Because ions respond differently to the alternating field strengths, changing this voltage changes the ion population that passes onward. Scanning or setting the compensation voltage therefore provides a way to separate ion signals before mass-spectrometric detection.
The waveform creates alternating periods of high and low electric-field strength rather than applying one constant field. This exposes ions to unequal mobility responses during each part of the cycle. Those differences generate the separation that the compensation voltage can resolve, making the field pattern central to FAIMS selectivity and its ability to reduce spectral overlap.
FAIMS adds a rapid gas-phase separation based on differential mobility, whereas mass spectrometry provides ion detection and liquid chromatography offers a separate separation dimension. Because these approaches distinguish ions using different principles, combining them can improve ion identification and reduce overlap in complex chemical samples. FAIMS therefore functions as an orthogonal complement rather than a replacement.
A typical workflow sends ions through a carrier gas while an asymmetric electric waveform and compensation voltage control which ions are transmitted. The selected ions then enter the mass spectrometer for detection. Researchers can vary the compensation voltage to examine different transmitted ion populations, producing separated signals that support interpretation of complex mixtures.
FAIMS is particularly useful when a sample contains many ions whose signals may overlap during analysis. Its separation can improve identification of pharmaceuticals, metabolites, proteins, and reaction products before mass-spectrometric detection. The added mobility-based dimension is valuable for complex mixtures because it helps distinguish chemical components that might otherwise produce crowded or ambiguous spectra.