Electrospray ionization high-field asymmetric waveform ion mobility spectrometry-mass spectrometry

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is a new technique that separates gas-phase ions at atmospheric pressure (760 Torr) and room temperature. A FAIMS instrument acts as an ion filter and can be set to continuously transmit one type of ion. Despite the stringent requirement for a flow of clean, dry gas in the FAIMS analyzer region, a method of coupling electrospray to FAIMS has been developed. The identity of the electrospray ions separated by FAIMS was determined using mass spectrometry (FAIMS-MS). The theory of FAIMS is discussed, and electrospray FAIMS-MS spectra of several compounds in modes P1, P2, N1, and N2 are presented. Ions appearing in P1 and N1 modes tend to have mobilities that increase as a function of increasing electric field strength, whereas ions appearing in P2 and N2 modes tend to have mobilities that decrease. In general, low-mass ions are focused in P1 and N1 modes, whereas larger ions (e.g., proteins) are focused in P2 and N2 modes. Short-chain peptides, (Gly)(n) where n = 1-6, are shown to cross over from P1 mode into P2 mode as the chain length increases. The removal of the low-mass solvent cluster ions, combined with a reduction of the background noise in electrospray FAIMS-MS, results in an improved signal-to-noise ratio for mass spectra of larger ions (e.g., cyctochrome c) when compared with conventional electrospray-MS. Preliminary results also suggest that various charge states of cytochrome c can be distinguished by FAIMS, implying that the ion mobility of these species at high electric field strength is sensitive to the structure of the protein ion. The linearity of response of electrospray FAIMS-MS was investigated using leucine enkephalin and shows the calibration curve to be linear for ～3 orders of magnitude.