Alkylsilyl derivatization of glyphosate and aminomethylphosphonic acid followed by gas chromatography mass spectrometry

Abstract

Prior to any chromatographic analysis of N-(phosphonomethyl)glycine, glyphosate (GLYP) and its principal metabolite aminomethylphosphonic acid (AMPA) – being particularly polar herbicides – first, derivatization needs to be the performed.In this paper, for GLYP and AMPA for the time being not applied, alkylsilyl derivatization speciation is described to quantitate them with N-methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA) reagent, resulting in fully labeled trimethylsilyl (TMS) derivatives, followed by gas chromatography mass spectrometry (GC-MS). MSTFA triggers the so far not recognized reactivity of these species: GLYP's equilibrium led to five active proton containing species, ready to form GLYP.5TMS (m/z 529), while AMPA show up as AMPA.4TMS (m/z 453). This recognition opens a new aspect of analyzing GLYP and AMPA. The equilibrium phenomenon of GLYP, proving the existence of the five active proton containing moiety, was confirmed by density functional theory (DFT) computation. Optimum analytical conditions both for GLYP selectively and for GLYP and AMPA simultaneously were highlighted. Impact of the solvents applied (pyridine, PYR, acetonitrile, ACN and ethyl acetate, ETAC) resulted in different distribution of GLYP.5TMS and GLYP.4TMS. In favor of GLYP alone, MSTFA/PYR proved to be the reagent of choice – while for the simultaneous quantification of GLYP and AMPA – the MSTFA/ACN composition had to be selected. The commonly used labeling with N-tert.-butyldimethylsilyl-N-methyltrifluoroacetamide (MTBSTFA), based on self experiences, was documented and critically evaluated. LOQ values, expressed as ng/injection values proved to be for GLYP.4TMS+GLYP.5TMS: 1.41ng (4.82%), AMPA.4TMS: 0.45ng (3.4%). Practical utility of TMS derivatization was shown by the simultaneous GLYP and AMPA analysis of four herbicide market samples, characterized with relative standard deviation percentages (RSD%) of measurements: varying between 6.1 RSD% and 2.43 RSD%. All four herbicides contain AMPA impurity as follows: GlialkaR Star, 3.70g/L (4.57 RSD%); FozátR, 2.68g/L (4.16 RSD%); Medallon P, 1.56g/L (4.19 RSD%) and Total Spray, 0.17g/L (3.39 RSD%), respectively.