Elimination of the helium requirement in high-field asymmetric waveform ion mobility spectrometry (FAIMS): beneficial effects of decreasing the analyzer gap width on peptide analysis

Cylindrical geometry high-field asymmetric waveform ion mobility spectrometry (FAIMS) focuses and separates gas-phase ions at atmospheric pressure and room (or elevated) temperature. Addition of helium to a nitrogen-based separation medium offers significant advantages for FAIMS including improved resolution, selectivity and sensitivity. Aside from gas composition, ion transmission through FAIMS is governed by electric field strength (E/N) that is determined by the applied voltage, the analyzer gap width, atmospheric pressure and electrode temperature. In this study, the analyzer width of a cylindrical FAIMS device is varied from 2.5 to 1.25 mm to achieve average electric field strengths as high as 187.5 Townsend (Td). At these electric fields, the performance of FAIMS in an N(2) environment is dramatically improved over a commercial system that uses an analyzer width of 2.5 mm in 1:1 N(2) /He. At fields of 162 Td using electrodes at room temperature, the average effective temperature for the M+2H](2+) ion of angiotensin II reaches 365 K. This has a dramatic impact on the curtain gas flow rate, resulting in lower optimum flows and reduced turbulence in the ion inlet. The use of narrow analyzer widths in a N(2) carrier gas offers previously unattainable baseline resolution of the M+2H](2+) and M+3H](3+) ions of angiotensin II. Comparisons of absolute ion current with FAIMS to conventional electrospray ionization (ESI) are as high as 77% with FAIMS versus standard ESI-MS.