Ion-pair liquid–liquid–liquid microextraction of nerve agent degradation products followed by capillary electrophoresis with contactless conductivity detection

Abstract

The four nerve agent degradation products methylphosphonic acid (MPA), ethyl methylphosphonic acid (EMPA), isopropyl methylphosphonic acid (IMPA) and cyclohexyl methylphosphonic acid (CMPA) have been successfully extracted from aqueous sample solution by ion-pair liquid–liquid–liquid microextraction. In this procedure, the target analytes in the sample solution were converted into their ion-pair complexes with tri- n-butyl amine and then extracted by an organic solvent (1-octanol) layer on top of the sample solution. Simultaneously, the analytes were back-extracted into a drop of an aqueous acceptor solution which was suspended in the organic phase at a microsyringe needle tip. The factors influential to extraction: type of organic solvent, type of ion-pair reagent and its concentration, pH values of sample solution and acceptor aqueous phase, stirring rate and extraction time were investigated in detail. After extraction, the drop of the acceptor solution was withdrawn into the syringe and injected into a capillary electrophoresis system for analysis. Using contactless conductivity detection, direct quantification of these compounds is possible. Moreover, large-volume sample injection was employed for further preconcentration. Improvements in the limits of detection between 2.5 and 4 orders of magnitude could be achieved and concentrations at the ng/mL level can be determined. This newly established approach was successfully applied to a spiked river water sample.