MECHANISM FOR AN ION ACCUMULATION PROCESS IN EXTERNAL SOURCE FOURIER-TRANSFORM MASS SPECTROMETERS

An ion accumulation effect has been observed in the trapped ion cell of a Fourier transform mass spectrometer during the injection of externally generated ions. The accumulation is unexpected because, in the absence of deceleration in the cell, ions with kinetic energy greater than the trapping potential should pass through the cell or collide with the rear trap plate. Previous suggestions that the trapping mechanism involves collisional damping or radial magnetic field inhomogeneities could not be demonstrated under the conditions employed in this work. We propose, instead, an alternate mechanism for ion accumulation that involves charge exchange between injected ions and neutral analyte molecules present in the analyzer. It is shown that the observed accumulation rates are consistent with known rate constants for charge exchange and are directly proportional to the density of neutral analyte molecules in the analyzer. The energetics of charge reactions are shown to account for the presence of apparent dissociation effects and for the absence of an expected production ion in the spectrum of one analyte. © 1990.