Determination of Aliphatic Organo-Phosphorus Compounds in Environmental Samples by Integrated Pulsed Amperometric Detection-Ion Chromatography

Glyphosate (N-(phosphonomethyl) glycine) is globally the most commonly used herbicidal active ingredient. Public concerns have elicited environmental agencies to monitor the persistence and occurrence of glyphosate. Recent environmental assessments quantify its annual mean concentration well within the parts per trillion range for US natural water sources. The environmental metabolites of glyphosate, aminomethyl phosphonic acid (AMPA), and the herbicidal active ingredient glufosinate are also frequently analyzed alongside glyphosate due to their similar chemical structures. However, their determination is labor intensive and not amenable to typical analytical methods due to the polyprotic and photo-inactive properties of these compounds. These compounds are typically analyzed by derivatization-based single residue methods (SRMs), where a few analytes undergo quantitative analysis by Liquid Chromatography-Tandem Mass Spectrometry method only (LC-MS/MS). The United States Geological Survey (USGS) has developed a method for glyphosate determination using online SPE-LC-MS/MS. Yet, studies utilizing similar LC-MS/MS methods have suggested that derivatizing steps are susceptible to salt-associated matrix effects. Ion Chromatography-Integrated Amperometric Detection (IC-IPAD) offers simple, direct analysis of aliphatic organo-phosphorus compound without derivatization steps. In this study, an offline SPE-IC-IPAD method was developed and validated for the determination of glyphosate and other aliphatic organophosphorus compounds in environmental water samples. The linear range was found to be 3-750 µg/L (R squared = 0.9973, 0.9998, 0.9983) and the limits of detection (LOD) were found to be 0.950, 0.402, and 0.252 µg/L for glyphosate, glufosinate, and AMPA in reagent water, respectively. The offline solid phase extraction (SPE) method provided excellent recovery values (104-131%) by standard addition of spiked glyphosate in Horsepen Bayou. Although our SPE-IC-PAD method was not able to detect glyphosate, glufosinate, and AMPA in several surface water samples collected within the Houston area, a satisfactory recovery value of 96.8% was achieved for certified reference material containing glyphosate. Further, our study on optimal sample storge conditions suggested that glyphosate is stable in acidified surface water for 36 days. Our study recommends acidified or refrigerated conditions following sample collection intended for glyphosate determination. Optimal method development parameters and development strategies were also discussed with the intention to further lower LOD values and improve precision in natural water matrices.