Chemical mass shifts in ion trap mass spectrometry: Experiments and simulations

Experimental data and simulations of trapped-ion motion are used to characterize the phenomenon of compound-dependent mass shifts in a quadrupole ion trap mass spectrometer. The ratio of axial to radial dimensions of the ion trap and the nature and pressure of the bath gas are identified as experimental variables which influence the chemical mass shift. Systematic changes in chemical shifts occur with changes in the chemical structure of the ion, for example between members of a homologous series of alkylbenzene molecular ions. Simulations, performed using a new version of the program ITSIM, indicate that the mass shift is the result of two interacting factors: (i) delayed ion ejection from the trap during the mass analysis scan due to field imperfections associated with the end-cap electrode apertures and (ii) the compound-dependent modification of this delay by collisions with the bath gas. Both elastic collisions and inelastic collisions, including those which lead to dissociation, appear to contribute to the shortening of the delay and hence to affect the magnitude of the chemical mass shifts.