Systematic factors affecting high mass-resolution and accurate mass assignment in a quadrupole ion trap

Abstract

The separate and combined influences of a phase-locked relationship between the drive (r.f.) and excitation (a.c.) frequencies, and the noise level on the drive voltage, on the ejection of ions from a quadrupole ion trap have been investigated at each of two scanning rates, 10 and 5000 u s −1. The half-width of an individual mass peak, which has been shown previously to vary directly with the mass scanning rate, is dependent also on the a.c.-r.f. phase relationship, and can be minimized. However, it is not practical to resolve an ion-intensity signal beyond that point where the peak halfwidth becomes significantly less than the uncertainty in its position. It has been observed that the position of mass peaks with half-widths of 1 or 2 mu can vary by as much as 0.25% as the a.c.-r.f. phase difference is varied over one complete cycle. Furthermore, the noise level present on the amplitude of the r.f. drive of the ion trap used in this study (primarily 60 and 120 Hz) can cause an uncertainty in peak positions of as much as 0.1 %. With the a.c.-r.f. phase difference locked at a favorable value and the scan function triggered by the phase of the line voltage, it is possible to achieve mass-peak stability to within 5 mu for m/z 264 corresponding to a resolution of 50 000. The results of a simulation study of peak position as a function of the a.c.-r.f. phase difference at a mass-scanning rate of 5000 u s −1 are in good agreement with experimental results. Simulated trajectories were used also to determine the effect of the a.c.-r.f. phase relationship on the phase-at-ejection of the a.c. and r.f. potentials.