High field asymmetric waveform ion mobility spectrometry-mass spectrometry: an investigation of leucine enkephalin ions produced by electrospray ionization.

High field asymmetric waveform ion mobility spectrometry (FAIMS) provides atmospheric pressure, room temperature, low-resolution separation of gas-phase ions. The FAIMS analyzer acts as an ion filter that can continuously transmit one type of ion, independent of m/z. The combination of FAIMS with electrospray ionization and mass spectrometry (ESI-FAIMS-MS) is a powerful technique and is used in this study to investigate the cluster ions of leucine enkephalin (YGGFL). Separation by FAIMS of leucine enkephalin ions having the same m/z (m/z 556.5), [M + H](+) and [2M + 2H](2+), was observed. In addition, four complex ions of leucine enkephalin, [2M + H](+), [4M + 2H](2+), [6M + 3H](3+), and [8M + 4H](4+), all having m/z 1112, were shown to be separated in FAIMS. Fragmentation of ions as the result of harsh conditions within the mass spectrometer interface (FAIMS-MS) was shown to provide similar information to that obtained from MS/MS experiments in conventional ESI-MS.