ENERGY-RESOLVED MASS-SPECTROMETRY OF DIETHYL ALKYL PHOSPHONATES WITH AN ATMOSPHERIC-PRESSURE IONIZATION TANDEM MASS-SPECTROMETER

Energy resolved mass spectrometry (ERMS) was used to characterize the dissociation processes of four diethyl alkyl phosphonates where the alkyl R group attached to the central phosphorus atom ranged from methyl to n-butyl. An atmospheric pressure ionization triple quadrupole mass spectrometer was used in the analyses. Dissociation of the MH(+) and further dissociation of the resulting progeny ions was induced between the atmospheric-vacuum sampling orifice and the first ion focusing RF-only quadrupole, Q0, instead of the central Q2 quadrupole. The major dissociation pathways of the four phosphonates could be characterized by two sequential McLafferty rearrangements yielding an alkyl phosphonic acid. This acid further dissociated losing H2O followed by ROH to produce PO+, or the same process occurred but in the reverse order, i.e. first losing ROH followed by H2O to yield PO+. The latter process was the energetically favored process. The alkyl group attached to the phosphorus atom of the protonated alkyl phosphonic acid was observed to have a significant influence on the stability of the protonated alkyl phosphonic acid. As the R group increased in size with a concomitant increase in electron-donating power, the alkyl phosphonic acid was less prone to dissociation than other alkyl phosphonic acids which possessed smaller R groups. A significant difference was also noted in the alkyl phosphonic acid dissociation processes; methyl phosphonic acid dissociated predominantly via H2O loss unlike the other acids which favored the loss of ROH over H2O.