Losses of ions during forward and reverse scans in a quadrupole ion trap mass spectrometer and how to reduce them

The higher order fields present in the quadrupole ion trap may have beneficial effects such as increases in mass resolution in the mass-selective instability or resonance ejection modes of operation, but may also result in losses of ions due to nonlinear resonances. In this work, the reduction in ion intensities observed in the mass spectra of polyethylene glycol (PEG 1000) has been utilized to monitor the ion losses resulting from these higher order fields during the rf voltage scans in both the forward and reverse directions. Extensive ion losses were observed in reverse rf voltage scans at q _z=0.64 (a _z=0), which corresponds to octopole resonance at β _z=1/2. The losses depended upon rf voltage scan rate and ion mass being greater for lower scan rates and lower masses. For ions of m/z 877, losses of up to 60% of the stored ions were observed at low scan rates (<1×10⁴ Da/s), but were minimal at higher scan rates. Thus, it is possible to avoid such losses during reverse scans by scanning the region q _z=0.64 at rates in excess of 4×10⁴ Da/s. In forward rf voltage scans, ion storage was considerably more reliable, with significant losses observed only at very high scan rates near the region q _z=0.78 (hexapole resonance at β _z=2/3).