Anal. Chem. 1995, 67, zyxwvutsrqpo 1968-1 975 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Determination of Acidic Pesticides in Water by a Benchtop Electrospray Liquid Chromatography Spectrometer Carlo Crescenzi, Antonio Di Corcia,* Stefan0 Marchese, and Roberto Samperi Dipartimento di Chimica, Universita “La Sapienza’; Piazza zyxwvu Aldo Mor0 5, 00185 Roma, Italy zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP A very sensitive and specific analytical procedure for determining 20 acidic pesticides in aqueous environmen- zyxwv tal samples using pneumatically assisted eledrospray (ES) zyxwvu LC/MS is presented. This procedure involves passing 1 - and 4-L river water and zyxwvuts drinking water samples, respec- tively, through a 1-g graphitized carbon black (GCB) extraction cartridge. By exploiting the presence of posi- tively charged active centers on the GCB surface, isolation of the acidic pesticides from baseheutral species was made possible by differential elution. Recoveries of the analytes were higher than 85%, irrespective of the aque- ous matrix in which they were dissolved. Adoption of the ion pair technique by addition of 0.1 mmoVL KzHP04 and 0.2 mmol/L tetrabutylammonium fluoride to the mobile phase allowed analytes to be analyzed as pre- formed ions. A conventional 4.6-mm4.d. reversed-phase LC C-18 column operating with a 1 amin flow of the mobile phase was used for chromatographing the ana- lytes. A flow of 30 zyxwvutsrq pWmin of the column etlouent was diverted to the ES source, while the rest of the mobile phase was delivered to a W detector set at 220-nm wavelength. The effects of the nature of the ion pair forming agent and its counterion as well as its concentra- tion in the mobile phase on the response of the ES/MS detector were investigated. The effects of varying the skimmer cone voltage on the production of diagnostic fragments and the response of the MS detector were also evaluated. For the analytes considered, the response of the mass detector was linearly related to the amount of the analytes injected between 2.5 and 200 ng. A certain variation of the ion signal for the analytes 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 zyxwvuts UV trace method while entrusting unambiguous identification of the analytes to the MS detector. The limits of sensitivity (signal-to-noise ratio = 3) of the method for the pesticides considered in drinking water and surface water samples were estimated to be about 2-6 and 8-25 ng/L, respec- tively. A number of liquid chromatographic methods have been proposed for determining those analytes of environmental interest that are not amenable to analysis by the gas chromatography/ mass spectrometry technique. However, because of the legal implications of many environmental data, coupling LC with MS is a key element for the future of zyxwvuts LC procedures. 1968 Analytical Chemistty, Vol. 67, No. 73, July 7, 1995 Electrospray (ES) ionization has rapidly emerged as a very promising technique for interfacing LC to MS. Similarly to thermospray, ES ionization produces ions at atmospheric pressure, but without the need for high temperatures that could decompose labile compounds. Briefly, the sequence of events in the elec- trospray process leading to generation of gas phase ions involves formation of charged small droplets by an electrical field. The droplets are gradually reduced in size by heat transfer and repeated Coulombic explosion until the radius of curvature of the daughter droplets becomes so small that field-assisted ion “evaporation”competes with further droplet disintegration. The LC/ES/MS system has proven to be a sensitive technique for analyzing compounds of environmental interest that exist as ions in solution.’,* Notably, although the electrospray process generates gas phase ions in a gentle way, structural information on analytes can also be easily achieved by collision-induced fragmentation with a suitable adjustment of the electrical field existing in the intermedi- ate-pressure desolvation chamber located between the ionization source and the mass analyzer region. By suitably exploiting this possibility, Voyksner and Pack3 showed that a single mass analyzer can provide structural information very similar to that obtained by the much more costly tandem MS technique. Recent legislation enacted in many european countries (mem- bers of the European Community, EC) states that pesticides must not exceed the 100 ng/L level in waters intended for human consumption. In order to judge with sufficientconfidence whether a water sample is in compliance with this EC Directive, analytical methods able to detect pesticides at 20-30 ng/L levels are needed. In the recent past, we elaborated a very sensitive and rapid LC/ W method able to analyze more than 120 pesticides in aqueous environmental samples at the ng/L le~e1.~-6 This method involves the use of a 1-g graphitized carbon black (GCB) reversible cartridge for extracting pesticides from water samples. Passing sequentially through the cartridge two suitable solvent systems, isolation of acidic pesticides from nonacidic ones was successfully achieved by differential elution. Class fractionation was made possible because the GCB surface framework is contaminated by (1) Conboy, J. J.; Henion, J. D.; Martin, M. W,; Zweigeinbaum, J. A. Anal. Chem. (2) Popenoe, D. D.; Moms, S. J.: Hom, P. S.; Norwood, IC T. Anal. Chem. 1994, (3) Voykner. R. D.; Pack, T. Rapid Commun. Mass Spectrom. 1991, 5, 263- (4) Di Corcia, A,; Marchetti, M. Anal. Chem. 1991, 63, 580-585. (5) Di Corcia, A; Marchetti, M. Environ. Sci. Technol. 1992, 26, 66-74. (6) Di Corcia, A,; Marcomini, A; Samperi, R.; Stelluto, S. Anal. Chem. 1993, 1990, 62, 800-807. 66, 1620-1629. 268. 65, 907-912. 0003-2700/95/0367-1968$9.00/0 0 1995 American Chemical Society