Comparison of various sample handling and analytical procedures for the monitoring of pesticides and metabolites in ground waters

Abstract

Various sample handling techniques such as liquid–liquid extraction off-line and on-line, solid-phase extraction followed by either gas chromatography (GC) with electron-capture, flame photometric or mass spectrometric detection, or liquid chromatography (LC) with diode array detection were applied in the determination of a selected group of insecticides and fungicides in ground water samples at sub-μg/l levels. An evaluation of the advantages and drawbacks in the application of the proposed methodologies for water monitoring studies is discussed. For the selected group of pesticides studied, off-line C₁₈ or polymeric cartridges followed by GC–MS using an ion trap analyzer have been revealed as the more powerful technique. But very polar compounds such as methamidophos or acephate have not been recovered with this procedure. On the contrary, on-line C₁₈ LC–DAD offered a few drawbacks for the trace determination of a large group of pesticides as a consequence of many important interferences in the chromatographic traces. Other techniques evaluated were LC–MS and GC–MS using a quadrupole analyzer, which offered complementary information and were useful for a limited range of analytes. An interlaboratory study was performed using all the methodologies evaluated in this work and the results obtained showed a good agreement between all the applied techniques. The various methodologies were for a ground water pilot survey study in Almeria (Spain). Endosulfan was the most ubiquitous pesticide detected in this area.

Introduction

Monitoring of pesticides groundwater has been a topic of increasing importance over the last few years. In some important agricultural areas in the USA 1, 2and Europe 3, 4, where pesticides have caused contamination in the hydrological system or its vulnerability is high, and where ground water is the primary source of drinking water, several water monitoring programmes have been developed to assess and evaluate pesticide concentration levels. The attention of these programmes has been focused on the most popular classes of pesticides, in terms of amount of production and appliance, i.e., carbamates, phenylureas, triazines and phenoxiacid derivatives. The number of published papers concerning the development and application of multiresidue environmental analysis on pesticides has increased enormously and has resulted in an extensive bibliography, mainly focused on herbicides 1, 2, 5. But a noticeable fact is that until now, there is a lack of information about the presence of insectides and fungicides and their metabolites in natural water as their analytical characteristics are not well-studied compared to herbicides.

In Spain and in a broader sense in many Mediterranean areas, due to the special characteristics of crop production and winter climate, pesticides used and so the target compounds in ground waters are insecticides and fungicides, i.e., organophosphorus pesticides (OPs) and organochlorine pesticides (OCs) [6]. For this reason there is a special interest to develop and to evaluate analytical methods for the analysis of a wide variety of fungicides and insecticides in water samples.

A few points have been carried out to evaluate the protocol of analysis.

One of the main goals in pesticide water analysis is to reach determination limits of about 0.01 μg/l which cover all the requirements of the European Union (EU) Drinking Water Directives as well as the US National Pesticide Survey. Common and well-established multiresidue methods for ground water sample pretreatment are based on the use of either a liquid–liquid extraction (LLE) 6, 7or solid-phase extraction (SPE) 7, 8, 9, 10, 11, 12, 13, 14previous to chromatographic determination. Lately, in SPE the use of mini- extraction columns or extraction discs have gained importance, because of the wide variety of SPE materials developed recently, designed for polar as well as for semipolar compounds. Furthermore, this water sample preparation avoids the use of large amounts of organic wastes and allows an easy automation [15]. Techniques such as supercritical fluid extraction (SFE) or microextraction have also been applied, but to a lesser extent.

The sample preparation step, either off-line or on-line, is followed by gas chromatography (GC) and/or liquid chromatography (LC) separation. These techniques can offer several complementary advantages and the criterion for selecting one of them or both is based on the behaviour of the analyte in the GC or LC column [16]. The final determination can be achieved by coupling a series of selective detectors (electron-capture, nitrogen–phosphorus, UV, fluorescence) 10, 11, 17, 18or more universal detection systems like mass spectrometry (MS) 8, 9, 10, 11, 12, 13. Other developments are based on more complex coupling systems such as SPE–HPLC–GC, SPE–SFE–GC, etc., but we cannot consider these systems easily available for an average routine control laboratory. Undoubtedly MS is the best choice if we take into account the great number of compounds and metabolites to cover in each analysis as well as the high requirements in time and money to perform these analyses.

Normally the use of GC–MS is restricted to a confirmation technique [2]as a consequence of the low limit of detection (LOD) achieved in general with quadrupole analyzers (GC–Q-MS) operating in full scan mode. The use of ion trap analyzers (GC–IT-MS) can overcome this deficiency, making this technique a powerful primary screening tool rather than a secondary confirmation system 19, 20. In ion trap technology, switching from full scan electron impact ionization mode (EI) to chemical ionization (CI) can be achieved in a very easy way providing enough information for the identification and quantitation of pesticides and metabolites rapidly [21]. However, it is well-known that the produced spectra usually indicated a large percentage of EI spectra fragments overlapping the CI spectrum frequently 19, 20.

Hyphenated LC–MS techniques allow the determination of a greater variety of polar compounds compared to GC–MS and can be extended to non-amenable “GC pesticides”. Additional features of LC–MS are that typical GC pesticides can usually be analyzed by this technique and that be more easily coupled to SPE [14]. Nowadays atmospheric pressure chemical ionization (APCI) or electrospray (ESI) are the best options to provide an adequate sensitivity and structural information 22, 23.

A very important item in the development of new pesticide analytical methods is the application to real samples. Even today a great part of new developed methods have only consisted of laboratory made applications. Difficulties related with the presence of metabolites which were produced environmentally to the more polar compounds than parent compounds [13]and the great variety of possible matrix effects can only be evaluated correctly by the analysis of a considerable amount of real samples.

The aim of this work is (i) to present a comparative evaluation of different sample handling by LLE and SPE and the analytical procedures GC–electron-capture detection (ECD), –flame photometric detection (FPD), GC–IT–MS, GC–Q-MS, LC–UV and LC–ESP-MS in their application to the analysis of insecticides and fungicides in ground waters of Almeria (Spain), which is one of the most important areas in crop production of Europe. (ii) The application of the various analytical methodologies to the evaluation of the behaviour of these compounds to leach the ground water of Almerı́a during a pilot monitoring study.