Duty Cycle Improvement for a Quadrupole—Time-of-Flight Mass Spectrometer and its Use for Precursor Ion Scans

Abstract

Normally, quadrupole–time-of-flight (QqTOF) instruments benefit from a simultaneous recording of all masses, and their sensitivity is higher than that of a triple quadrupole when a full scan is recorded. This is not the case when only a single type of ion has to be monitored, as in precursor ion scanning or multiple-reaction monitoring (MRM) mode. This is because ion losses in TOF are higher than in a quadrupole. A significant part of these losses is due to the low duty cycle of an orthogonal TOF instrument, typically 5 to 30%. Trapping ions in the collision cell and then gating them out in short bursts synchronized with TOF extraction can restore the duty cycle at the expense of mass range. The mechanism of ion trapping, gating and bunching is studied under various conditions. By varying trapping and gating parameters, different modes of operation are achievable: from a 100% duty cycle restoration for a narrow mass range to <50% duty cycle for a wider mass range. The former mode is best for a single-fragment precursor ion scan, whereas the latter is better when a full product-ion spectrum needs to be recorded, or when a multiple-fragment precursor ion scan is performed. Several examples of precursor ion scans obtained with peptides and lipids are demonstrated. Sensitivity improvements range from ×15 for the immonium fragment ion ( m/z 86) to ×5 for simultaneous recording of lipid fragments with m/z = 184 and 264.