Parallel valet parking: selective ion escape over an AC barrier during ion/ion reactions in a quadrupole linear ion trap

Ion storage in a quadrupole linear ion trap (QLIT) in the x- and y-dimensions relies on a quadrupolar electric field that oscillates at a radio-frequency. In the z-dimension, trapping can take place either via a fixed DC electrostatic potential, via oscillatory AC voltages, or via a combination of the two applied to plates situated at the ends of the QLIT. With a combination of an AC barrier with an attractive DC potential, it is possible to effect storage in the z-dimension that is mass-to-charge (<i>m/z</i>) dependent. In this work, the <i>m/z</i>-dependent release of ions from a QLIT to an adjacent time-of-flight (TOF) mass spectrometer is demonstrated. An application of this phenomenon is demonstrated with gas-phase ion/ion proton transfer reactions in which high <i>m/z</i> ions generated via charge-state reduction are released to a TOF mass analyzer before a significant degree of neutralization can take place. The transfer of a product ion from a QLIT to an adjacent mass analyzer is referred to as 'valet parking'. Valet parking has been demonstrated previously using ion/molecule proton transfer reactions with the transfer of ions within a narrow band of <i>m/z</i> values. The selective release of ions that exceed a threshold <i>m/z</i> value allows for the simultaneous valet parking of ions derived from mixtures of analytes. This is referred to a 'parallel valet parking', which is demonstrated here with a three-protein mixture of cytochrome <i>c</i>, ubiquitin, and myoglobin.