DETERMINATION OF ACIDIC PESTICIDES IN WATER BY A BENCHTOP ELECTROSPRAY LIQUID-CHROMATOGRAPHY MASS-SPECTROMETER

A very sensitive and specific analytical procedure for determining 20 acidic pesticides in aqueous environmental samples using pneumatically assisted electrospray (ES) LC/MS is presented. This procedure involve s passing 1- and 4-L river water and drinking water samples, respectiv ely, through a 1-g graphitized carbon black (GCB) extraction cartridge . By exploiting the presence of positively charged active centers on t he GCB surface, isolation of the acidic pesticides from base/neutral s pecies was made possible by differential elution. Recoveries of the an alytes were higher than 85%, irrespective of the aqueous matrix in whi ch they were dissolved. Adoption of the ion pair technique by addition of 0.1 mmol/L K2HPO4 and 0.2 mmol/L tetrabutylammonium fluoride to th e mobile phase allowed analytes to be analyzed as pre-formed ions, A c onventional 4.6-mm-i.d. reversed-phase LC C-18 column operating with a 1 mL/min now of the mobile phase was used for chromatographing the an alytes. A now of 30 mu L/min of the column effluent was diverted to th e ES source, while the rest of the mobile phase was delivered to a UV detector set at 220-nm wavelength, The effects of the nature of the io n pair forming agent and its counterion as well as its concentration i n the mobile phase on the response of the ES/MS detector were investig ated. The effects of varying the skimmer cone voltage on the productio n of diagnostic fragments and the response of the MS detector were als o evaluated. For the analytes considered, the response of the mass det ector was linearly related to the amount of the analytes injected betw een 2.5 and 200 ng. A certain variation of the ion signal for the anal ytes considered occurred after several hours of continuous use of the LC/ES/MS instrumentation. For routine use, then, analyte quantitation could be better performed by the UV trace method while entrusting unam biguous identicalness of the analytes to the MS detector. The limits o f sensitivity (signal-to-noise ratio = 3) of the method for the pestic ides considered in drinking water and surface water samples were estim ated to be about 2-6 and 8-25 ng/L, respectively.