Enhancing determination of organophosphate species in high inorganic phosphate matrices: application to nerve agent degradation products

Abstract

Developing a reliable and robust trace level method for organophosphates in the presence of relatively high inorganic phosphate is an area of interest for further method development. Since the advent of collision/reaction cell technology, inductively coupled plasma mass spectrometry (ICPMS) has been used with a variety of sample types for elemental phosphorus detection at m/z = 31. However, with many samples inorganic phosphate may be orders of magnitude higher than organophosphates, presenting a major interference to quantification. Therefore, removal of inorganic phosphate to levels low enough to minimize the interference would prove beneficial for more effective organophosphate analyses. In this study, applicable to most organophosphate containing samples, the illustration is to nerve agent degradation products as they might contaminate food and environmental systems after their initial formation. Calcium chloride was chosen as a coagulant mix and ammonium hydroxide was chosen to adjust the pH in order to remove inorganic phosphate at the sample preparation step. High performance liquid chromatography (HPLC) coupled with ICPMS was utilized for analysis. The results show that inorganic phosphate, at concentrations as high as 10 000 µg mL⁻¹, can be sufficiently removed by precipitation with calcium at pH > 9 while the organophosphorus chemical warfare agent degradation products, CWADPs, remain intact. Applications to apple juice and cola drink indicate that this method is suitable for more complex matrices containing relatively high levels of inorganic phosphate.