Ion dissociation reactions induced in a high-pressure quadrupole collision cell.

This work concerns a new high-pressure quadrupole collision cell, designed for triple-quadrupole mass spectrometers. This new collision cell operates at pressures up to 10 mTorr, an order of magnitude higher than conventional cells of this type. Previous investigations have concentrated upon the significant increases in transmission efficiency and in resolving power for fragment ions which result from the use of this new design. The present work reports an investigation into the nature of the dissociation reactions which can be induced by collisions in this high-pressure cell. Charge-site-remote fragmentations of a simple precursor ion were chosen as a test case, and were found to be observable at laboratory collision energies lower by a factor of 4-5 than those found previously to be necessary when using conventional low-pressure quadrupole collision cells. It was also shown that the charge-site-remote reactions were accompanied by the mixed-site-fragmentation reactions described by Tuinman and Cook (J. Am. Soc. Mass Spectrom. Vol. 1, p. 85 (1989)). Ionization of collision gas was observed in the case of xenon. Efforts to observe charge-site-remote fragmentations of peptide ions were marginally successful. Highly basic peptides, which have been problematic for sequencing by low-energy tandem mass spectrometry, did not yield useful fragment-ion spectra in the new cell. The fragmentation behaviour of protonated Leu-enkephalin, for which fragmentation pathways have been thoroughly studied previously, suggested that the observed spectra reflected integration of the fragmentation kinetics over a considerably longer time, thus involving many more reaction steps. These combined observations are considered in terms of a qualitative model based on a rapid decrease of ion kinetic energy during passage through the cell, with much longer residence times than for conventional quadrupole cells.