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

Experimental data and simulations of trapped-ion motion are used to charact erize 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 experime ntal 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 molecul ar ions. Simulations, performed using a new version of the program ITSIM, i ndicate that the mass shift is the result of two interacting factors: (i) d elayed ion ejection from the trap during the mass analysis scan due to fiel d imperfections associated with the end-cap electrode apertures and (ii) th e compound-dependent modification of this delay by collisions with the bath gas. Both elastic collisions and inelastic collisions, including those whi ch lead to dissociation, appear to contribute to the shortening of the dela y and hence to affect the magnitude of the chemical mass shifts.