Mirror Switching for High-Resolution Ion Isolation in an Electrostatic Linear Ion Trap.

Ion isolation was achieved via selective pulsing of the entrance and exit ion mirrors in an electrostatic linear ion trap mass spectrometer (ELIT). Mirror switching has been described previously as a method for capturing injected ions in ELIT devices. After ion trapping, mirror switching can be used as a method for ion isolation of successively narrower ranges of mass-to-charge (/) ratio. By taking advantage of the spatial separation of ions in an ELIT device, pulsing of the entrance and/or exit mirrors can release unwanted ions while continuing to store ions of interest. Furthermore, mirror switching can be repeated multiple times to isolate ions of very similar / values with minimal loss of the stored ions, as is demonstrated by the isolation of protonated l-glutamine and l-lysine (Δ / = 0.0364) from a mixture of the two amino acid ions and the isobaric mixture of [PC P-18:0/22:6] and [PC 19:0/19:0] (Δ / = 0.0575). As isolation is accomplished due to the spatial/temporal separation of ion packets within the ELIT, multiple reflection-time-of-flight (MR-TOF) mass spectra are shown to demonstrate separation in the ELIT at the time of isolation. An isolation resolution of greater than 35 000 fwhm is demonstrated here using a 5.25 in. ELIT. This resolution corresponds to the fwhm resolution necessary to reduce contaminant overlap of an equally abundant adjacent ion to 1% or less of the isolated ion intensity.