Mass shifts and local space charge effects observed in the quadrupole ion trap at higher resolution

Abstract

Mass measurements made using the quadrupole ion trap mass spectrometer under high resolution conditions display non-random errors which are attributed to local and general space charge conditions. Each ion in the trap experiences a degree of charge interaction which depends on (i) the total number of charges in the trap, (ii) the number of ions of the same m/z value, and (iii) the abundances and mass differences of neighboring ions. The net field formed by the sum of these ion/ion interactions produces delayed ion ejection. Mass assignments are observed to shift on simply increasing the ionization time, so increasing the total number of trapped ions. The effect of the presence of other ions on mass shifts is amplified as the mass difference between the two populations of ions decreases. As ions of a particular m/z value are isolated by gradual elimination of neighboring ions of lower and, more particularly, higher mass, the observed mass of the ion of interest shifts to lower mass due to the reduction of space charge. The different environments experienced by calibrant and analyte ions constitute a source of error in mass calibration methods which employ external mass standards. Current calibration procedures which involve the construction of a calibration line based on the observed masses of CsI cluster ions are in error because these ions have much higher abundances than the analyte ions. However, extrapolation of experimental data to the point of zero space (i.e. zero ion abundance) eliminates the differences in the charge environments experienced by each ion. Clear trends are observed, even for high resolution experiments on peptide mixtures, between the number and masses of ions in the trap and the observed ejection times of ions of different masses.