TANDEM MASS-SPECTROMETRY OF POLY(ETHYLENE GLYCOL) PROTON-ATTACHMENT AND DEUTERON-ATTACHMENT IONS

A detailed study has been carried out of the fast atom bombardment tandem mass spectrometry (MS-MS) behavior of proton- and deuteron-attachment ions from several ethylene glycol polymers (PEGs). Both cyclic (crown ether) and linear (hydroxyl- and methyl-terminated) oligomers were studied. Collisional activation was carried out in the "collision octapole" of a BEoQ hybrid mass spectrometer at a translational energy of 50 eV, with air as collision gas. Interestingly, it was found in several experiments that considerable H/D exchange occurs during the formation of hydroxyl-terminated carbonium product ions from [M + D]+ precursors. This H/D mixing cannot be explained by simple, charge site-initiated C-O bond cleavages. A more complex decomposition pathway is proposed that appears to be consistent with the MS-MS data. The key intermediate is a hydroxyl-terminated (-OD) carbonium ion that can be formed from either cyclic or linear PEG [M + D]+ precursors. It is proposed that reversible H/D exchange occurs between the terminal hydroxyl group (-OD) and the carbonium center (-CH2+) of this intermediate. This H/D exchange and the subsequent elimination of (neutral) ethylene oxide groups is shown to be more prominent at higher collision gas pressures.