Re-evaluation of glyphosate determination in water by liquid chromatography coupled to electrospray tandem mass spectrometry

An analytical method based on on-line solid-phase extraction–liquid chromatography coupled to electrospray tandem mass spectrometry (SPE–LC–ESI–MS/MS) for the determination of glyphosate and aminomethylphosphonic acid (AMPA) residues has been applied to the analysis of water samples within a Round Robin Study. The method had been previously validated in a variety of water samples and it fulfilled all the parameters of precision, accuracy, sensitivity and unequivocal confirmation. The results within the study that we participated were highly satisfactory in all cases with the only exception of glyphosate in groundwater samples, where surprisingly recoveries around 15% were obtained despite the use of isotope-labeled glyphosate as internal standard (I.S.). A slight modification has been introduced in the method, simply consisting of the acidification of sample with hydrochloric acid (HCl) to pH 1. Then, the sample is neutralized and immediately derivatized with 9-fluorenylmethylchloroformate (FMOC) before LC–MS/MS determination. Round Robin Study samples were reanalyzed using this approach, and the recoveries increased up to 98%. A possible explanation might be the slow kinetic interaction between glyphosate and some components of the matrix. These components might act as chelating agents, making glyphosate unavailable for derivatization and therefore for analysis. Several water samples collected at the Mediterranean area of Spain, and previously analyzed and being found to contain glyphosate, were also reanalyzed using this approach, obtaining higher concentrations (between 2 and 14 times) in most of cases.