Simultaneous quantitation of sixteen organochlorine pesticides in drinking waters using automated solid-phase extraction, high-volume injection, high-resolution gas chromatography
Introduction
European legislation (EC Directive 80/778/EEC, Water Act, 1989) requires that many pollutants must be detected when present at very low concentrations in drinking water. This has produced a need for rapid and precise multi-residue analytical techniques for the isolation and detection of these compounds at concentrations of <100 ng l−1 in potable water. In addition to organochlorine pesticides still in use in modern farming practises, stocks of expired organochlorine compound containing chemicals may still exist which present a potential hazard through inadvertent dumping into watercourses. Furthermore, the persistence of organochlorine compounds in the natural environment for many years and their concentration within organisms as they are passed through food webs is well documented 1, 2, 3(and references therein) and warrants routine monitoring in drinking water supplies.
Liquid–liquid extraction (LLE) of pesticides from natural water samples (usually 1–2 l volumes) using organic solvents such as hexane, dichloromethane and petroleum ether are effective [4], but time-consuming and difficult to automate. In addition LLE uses large volumes of costly organic solvents, and complications such as the formation of emulsions can occur [5].
Solid-phase extraction (SPE) is increasingly used for the routine concentration of trace pollutants in natural waters. A variety of solid-phase supports in cartridge or disc form are available for the extraction of pesticides from water 5, 6, 7. The technique is simple and easy to automate [8], and the cartridge may be used as an on-line pre-column in tandem with liquid chromatography instrumentation 9, 10, 11or gas chromatography (GC) 12, 13, 14. Sorbed compounds on solid supports may be thermally desorbed in the injection port of a gas chromatograph and analysed directly [15].
A Zymark Autotrace WorkstationTM facilitates automated extraction, drying and elution of pesticides. Although small elution volumes (<10 ml) are used, it is generally necessary to concentrate the eluate further, using nitrogen aspiration prior to GC analysis to ensure that nanogram detection levels are achieved. Large volume injection (80 μl) eliminates the necessity for a final evaporation step, thus decreasing sample preparation time and eliminating losses through volatilisation. Large volume injection is also a simple way of improving analyte detectability 16, 17. Recent investigations [18]showed that large volume injection may be optimised according to the volatility and thermostability of the analytes by using different injection volumes, injector liner diameters and speed controlled injection.
This paper describes a sensitive, precise, rapid and cost effective method for the identification and quantitation of sixteen organochlorine pesticides in drinking waters.
Section snippets
Preparation of sample
One liter water samples were fortified with 5 ml of MeOH and 25 ml of pH 7 phosphate buffer. All solid chemicals and solvent were of “Analar” and residue analysis grade, respectively. Water was purified with a Milli-Q 18TM (Millipore, Bedford, MA, USA) water purification system.
Preparation of standards
Organochlorine pesticide standards (>95% purity) were supplied by Dr. S Ehrenstorfer (Augsberg, Germany). Individual standard solutions were prepared by dissolving 20 mg of each pesticide in 100 ml of ethyl acetate. A 2
Sample extraction procedure
A Zymark AutotraceTM (Zymark Corporation, Hopkinton, MA, USA) fitted with 3 g octadecyl silica based cartridges (J T Baker, Deventer, Netherlands) was used for fully automated SPE [19]. Six samples were extracted in one batch by the following procedure:
1. Condition cartridge with 2×10 ml of MeOH at a flow-rate of 10 ml min−1.
2. Condition cartridge with 10 ml of deionised water at a flow-rate of 10 ml min−1.
3. Load 1030 ml of spiked water on to SPE cartridge at a flow-rate of 10 ml min−1.
4.
Analysis of pesticide residues using high-volume injection-GC–ECD
The analysis of extracts containing the organochlorine pesticides studied (Table 1) was performed using an AI Cambridge GC94 equipped with a CTC Analytics Liquidsampler A200 SE, optic high-volume injector and ECD using the conditions outlined below: Column: J&W Scientific DB-5 capillary column 30 m×0.25 m×0.25 μm; carrier gas: helium; column temperature: 60°C initially then programmed at 20°C min−1 to 140°C and finally at 4°C min−1 to 280°C. High-volume injector conditions: split mode for 1 min
Results and discussion
The mean correlation coefficients obtained from the calibration curves plotted from eleven batches are shown in Table 1 and indicate the excellent linearity of the method. Calibration curves were forced through the origin. Each batch was run in duplicate, 12 h or more separating a set of two batches.
The chromatogram obtained from an 80-μl injection of an extract from drinking water spiked with the sixteen pesticides at 100 ng l−1 and 20 ng l−1 concentrations (Fig. 1A) is similar to that
Conclusions
The fully automated SPE followed by high-volume injection-GC method described here is a rapid, reproducible and sensitive technique for the analysis of organochlorine pesticides in drinking waters. In this study large volume injection has been shown to be a robust technique that was useful for increasing the detectability of the GC procedure. It eliminated the need for further concentration of the eluate following desorption from the SPE cartridge. It saved preparation time and reduced losses
Acknowledgements
The authors would like to thank Dr. K.A. Quayle and Dr. P.R. Harrigan for correcting the manuscript. The work was financed by Analytical and Environmental Services and Northumbria Water Ltd..
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