AMPA-¹⁵N - Synthesis and application as standard compound in traceable degradation studies of glyphosate

Stable isotope labeling of pollutants is a valuable tool to investigate their environmental transport and degradation. For the globally most frequently used herbicide glyphosate, such studies have, so far, been hampered by the absence of an analytical standard for its labeled metabolite AMPA-¹⁵N, which is formed during the degradation of all commercially available glyphosate isotopologues. Without such a standard, detection and quantitation of AMPA-¹⁵N, e.g. with LC-MS/MS, is not possible. Therefore, a synthetic pathway to AMPA-¹⁵N from benzamide-¹⁵N via the hemiaminal was developed. AMPA-¹⁵N was obtained in sufficient yield and purity to be used as a standard compound for LC-MS/MS analysis. Suitable MS-detection settings as well as a calibration using the internal standard (IS) approach were established for Fmoc-derivatized AMPA-¹⁵N. The use of different AMPA isotopologues as IS was complicated by the parallel formation of [M+H]⁺ and [M]^+• AMPA-Fmoc precursor ions in ESI-positive mode, causing signal interferences between analyte and IS. We recommend the use of either AMPA-¹³C-¹⁵N, AMPA-¹³C-¹⁵N-D₂ or a glyphosate isotopologue as IS, as they do not affect the linearity of the calibration curve. As a proof of concept, the developed analysis procedure for AMPA-¹⁵N was used to refine the results from a field lysimeter experiment investigating leaching and degradation of glyphosate-2-¹³C-¹⁵N. The newly enabled quantitation of AMPA-¹⁵N in soil extracts showed that similar amounts (0.05 - 0.22 mg·kg^-1) of the parent herbicide glyphosate and its primary metabolite AMPA persisted in the topsoil over the study period of one year, while vertical transport through the soil column did not occur for either of the compounds. The herein developed analysis concepts will facilitate future design and execution of experiments on the environmental fate of the herbicide glyphosate.