Dual element (15N/14N, 13C/12C) isotope analysis of glyphosate and AMPA by derivatization-gas chromatography isotope ratio mass spectrometry (GC/IRMS) combined with LC/IRMS

To assess sources and degradation of the herbicide glyphosate [N-(phosphonomethyl) glycine] and its metabolite AMPA (aminomethylphosphonic acid), concentration measurements are often inconclusive and even C-13/C-12 analysis alone may give limited information. To advance isotope ratio analysis of an additional element, we present compound-specific N-15/N-14 analysis of glyphosate and AMPA by a two step derivatization in combination with gas chromatography/isotope ratio mass spectrometry (GC/IRMS). The N-H group was derivatized with isopropyl chloroformate (iso-PCF), and remaining acidic groups were subsequently methylated with trimethylsilyldiazomethane (TMSD). Iso-PCF treatment at pH < 10 gave too low N-15/N-14 ratios indicating an incomplete derivatization; in contrast, too high N-15/N-14 ratios at pH > 10 indicated decomposition of the derivative. At pH 10, and with an excess of iso-PCF by 10-24, greatest yields and accurate N-15/N-14 ratios were obtained (deviation from elemental analyzer-IRMS: -0.2 +/- 0.9 % for glyphosate; -0.4 +/- 0.7 % for AMPA). Limits for accurate delta N-15 analysis of glyphosate and AMPA were 150 and 250 ng injected, respectively. A combination of delta N-15 and delta C-13 analysis by liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) (1) enabled an improved distinction of commercial glyphosate products and (2) showed that glyphosate isotope values during degradation by MnO2 clearly fell outside the commercial product range. This highlights the potential of combined carbon and nitrogen isotopes analysis to trace sources and degradation of glyphosate.