Characterization of O-alkyl Alkylphosphonic Acids by High-Energy Collision Induced Dissociation in Negative Mode Electrospray Ionization Tandem Mass Spectrometry

Gas chromatography (GC) coupled to mass spectrometry (MS) has for a long time been the predominating method for the qualitative analysis of O-alkyl alkylphosphonic acids. Prior analysis, trimethylsilylation, is used for esterification in order to increase the volatility prior to GC. To minimize the need for sample preparation, methods based on liquid chromatography-tandem mass spectrometry (LC/MS/MS) with electrospray ionization (ESI) in the positive ion mode have been developed. MS and MS/MS-spectra from these methods give diagnostic fragments from the alkylphosphonic acid, which together with the molecular ion gives information about the size of the O-alkyl group. However, information about branching of the O-alkyl group cannot be found, and identification is limited to the number of carbons in the O-alkyl chain only. In the present study, high energy collision induced fragmentation in negative mode ESI-MS/MS showed to produce improved structural information compared to GC/MS (EI) analysis. The product spectra show fragments form homolytic cleavages in the O-alkyl chain (charge-remote fragmentation), which makes elucidation of the structure of the O-alkyl chain possible and thus provides full identification of isomeric phosphonic acids.