Determination of organophosphorus acids by liquid chromatography positive electrospray ionization tandem mass spectrometry after chemical derivatization

Negative electrospray ionization tandem mass spectrometry (ESI-MS/MS) is the first choice for detecting and identifying organophosphorus acids such as alkyl methylphosphonic acids, dialkyl phosphates and dialkyl thiophosphates. However, chemical noise in the lower mass regions, ion suppression and poor chromatographic retention of small, highly polar acids render the identification of such acids difficult, especially in complex matrices. To address this difficulty, we evaluated charge- reversal derivatizations of several polar organophosphorus acids with eight commercially- available, mostly pyridine-based reagents, possessing high proton-affinity amines. A simple and generic derivatization procedure of such acids in the presence of potassium carbonate and 18-crown-6 in acetonitrile was developed. After derivatization, the samples were analysed by LC-ESI(+)-MS/MS, and a fragmentation study was carried out. For all derivatizing agents, the resulting acid-derivatives were highly retained by LC and well responded in the ESI⿿MS/MS in the positive-ion mode. However, for several reagents, structural information losses were observed in MS/MS. Interestingly, each derivatizing agent exhibited a different MS/MS behaviour towards the analytes. Some derivatizing agents underwent considerable collision-induced dissociation exclusively at the amine tag portion, while others dissociated at the analyte portion and at the amine portion, depending on the alkyl side chain linked to the oxygen. Only the 3- (bromoacetyl)pyridine derivatives were highly specific and provided rich informative MS/MS spectra. The MS/MS identifications were based on characteristic neutral losses, diagnostic ions for the organophosphorus skeleton, and amine tag-characteristic product ions. This derivatization is beneficial in the identification of target and unknown organophosphorus acids.