Mass shifts due to ion/ion interactions in a quadrupole ion‐trap mass spectrometer

Abstract

AbstractMass spectra recorded using a Paul ion trap, operated in the mass‐selective instability scan mode, give peak positions which are shown to depend on ion abundance. The isolated molecular ions of benzene, anisole, cresol, and n‐butylbenzene are ejected during the mass selective instability scan at times which display a linear dependence on ion abundance for low populations of trapped ions. Remarkably, the degree to which the peaks are shifted is also compound dependent. Mass shifts are also influenced by the presence of other ions in the trap and this effect is amplified as the mass difference between the analyte and matrix ions decreases. Mass shifts are also affected by the helium pressure which controls the ion density. Extrapolation to zero ion abundance and to zero helium pressure is suggested as a method of eliminating these effects and hence of improving the mass measurement accuracy attainable using the ion trap.