The occurrence of synthetic organic chemicals in drinking water supplies has prompted the European Economic Community to establish standards for pesticides. Currently, the limit on total pesticides is 500 ng l−1 with no individual pesticide to exceed 100 ng l−1. The variability of pesticides, both in concentration and composition, indicates that composite rather than grab samples are desirable for analysis of the water quality. Furthermore, the variability in composition makes a broad spectrum analysis preferable to individual compound analysis so that many compounds can be analysed simultaneously.
Although composite broad spectrum samples are desirable from a collection point of view, the occurrence of many pesticides in one sample presents a problem for individual compound quantification and identification. For this reason, gas chromatography (GC) is used to separate the compounds and mass spectrometry (MS) is used to identify and quantify the compounds. Coupled GS-MS avoids interferences which may occur in GC by using a separate method (other than retention time or volume) to identify compounds.
This research compared liquid-liquid extraction (LLE) in the batch and continuous mode. The LLE process is accomplished in a continuous mode by mixing the water and solvent in a coil, allowing the water and solvent to separate in a quiescent decantation cell, then recirculating the solvent by a separate distillation cell while the water goes to waste. Tap water was spiked with pesticides at 10, 50 and 100 ng l−1. Extraction efficiency was compared at each of the three spiking concentrations. The extracts were analysed on GC-MS using a moving needle injection technique to enhance sensitivity. The identification and quantification of compounds were accomplished by MS using the fragmentometry technique which enhance sensitivity of the analysis.
The results indicate that continuous LLE will recover an average of 75% (± 14%) of the pesticides in the 10–100 ng l−1 range recoverable by batch LLE. There was no significant difference in recovery for structurally different pesticides (chlorinated, nitrogenous or phosphoric) as all fell in the 75 ± 14% range. Eighteen pesticides were identified and quantified by the GC-MS in a 25 min analysis.
This demonstrates that the continuous LLE technique can be used to automate collection of composite samples if the reduction in extraction efficiency is taken into account. The continuous LLE can handle much larger volumes of water than the batch LLE. Composite samples can be continuously collected over time, a sampling regime not possible with batch LLE. This technique when coupled with GC-MS can be used to identify and quantify pesticides at the ng l−1 level and can be applied routinely for analysis of pesticides at approximately the same price as individual pesticides analysis.
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