Fragmentation in chemical ionization mass spectrometry and the proton affinity of the departing neutral

Chemical ionization mass spectra of six 5,6-dihydro-2-methyl-1,4-oxathiins, and some of the sulfoxides and sulfones derived therefrom, have been determined employing hydrogen, methane and isobutane as reagent gases. The major fragmentation reaction of the protonated molecule, [R′COX·H]+, involves loss of the neutral HX molecule. For the sulfides and sulfones, with X ranging from OH to N(CH3)C6H5, it is observed that the importance of this fragmentation is inversely correlated with the proton affinity of the departing HX molecule in both the H2 and CH4 chemical ionization. For the sulfoxides no consistent correlation is observed and this is attributed to the interference of competing and/or consecutive fragmentation reactions. In the isobutane chemical ionization mass spectra only the protonated molecule is observed for most of the compounds studied.