Analytical methods pesticide screens

Most of the analytical methods for the analysis of pesticides in food are based on instrumental approaches based on chromatography coupled to mass spectrometry. However, a great effort of development has been paid to develop rapid screening methods based on biological methods, such as, enzyme linked immunosorbent assays (ELISA). 

Tarbah FA, Mahler H, Temme O, Daldrup T. An analytical method for the rapid screening of organophosphate pesticides in human biological samples and foodstuffs. Forensic Sci Int 2001 121 126-33. 

Nevertheless, due to the increased number of pesticides used worldwide and the variety and eomplexity of food matrices, the use of ultra-fast separations is not enough to develop fast analytical methods for the analysis of pesticide residues in food. Besides, multi-residue screening methods able to analyze not only target but nontarget or even unknown compounds, minimizing the sample manipulation, are dananded. Thus, sample extraction and clean-up treatments must also be optimized when considering... 

Elucidation of how the general principles underlying the concept of validation should be expressed in practice is an evolving process, as exemplified by the ongoing evolution of validation requirements for bioanalytical assays in the pharmaceutical industry (Shah 1992, 2000 FDA 2001 Viswanathan 2007). The complementary principle of fitness for purpose (Section 9.2) applies not only to the assay method but also to the validation process itself. Procedures that are considered to be fit for purpose in validation of an analytical method to be used in drug development, for example, need not necessarily apply to, e.g., methods used to screen pesticide residues in foodstuffs. As noted in Section 9.2, this point of view appears to be consistent with the definition of validation applied to all measurements (ISO 1994) Validation Confirmation by examination and provision of objective evidence that the particular requirements for a specified intended use are fulfilled. Of course, some basic principles are common to all validation schemes. 

Clearly, different compromises must be applied compared with those for analytical methods that target only one or two specific compounds, with respect to analysis time, degree and level of validation criteria including accuracy and precision, recovery, confirmation of identity etc. The broader the class of compounds to be analyzed, the lower the level of validation that is possible, unless it is accepted that large amounts of time and resources are available. Usually broad class multi-residue analyses are designed as semi-quantitative screening methods usually followed by identity confirmation. This section describes some examples of such methods developed for pharmaceutical and pesticide residues in water. 

Enzyme immunoassay kits are now available for qualitative field testing or for laboratory screening and semiquantitative analysis of pesticides, herbicides, polychlorinated biphenyls (PCBs), mononuclear and polynuclear aromatic hydrocarbons, pentachlorophenol, nitroorganics, and many other compounds in aqueous and soil samples. Certain analytes may be quantitatively determined as well, with a degree of accuracy comparable to gas chromatography or high performance liquid chromatography determination. The method is rapid and inexpensive. 

As part of SW-846, the EPA has validated and approved many immunoassay and colorimetric screening methods for a wide range of contaminants, such as petroleum fuels, pesticides, herbicides, PCBs, and explosives. Immunoassay technology uses the property of antibodies to bind to specific classes of environmental pollutants allowing fast and sensitive semiquantitative or qualitative detection. Colorimetric kits are based on the use of chemical reactions that indicate the presence of target analytes by a change in color. Table 3.9 presents a summary of EPA-approved screening methods and their detection capabilities. 

Immunoassays offer much potential for rapid screening and quantitative analysis of pesticides in food and environmental samples. However, despite this potential, the field is still dominated by conventional analytical approaches based upon chromatographic and spectrometric methods. We examine some technical barriers to more widespread adoption and utilization of immunoassays, including method development time, amount of information delivered and inexplicable sources of error. Examples are provided for paraquat in relation to exposure assessment in farmworkers and food residue analyses molinate in relation to low-level detection in surface waters and bentazon in relation to specificity and sensitivity requirements built in to the immunizing antigen. A comparison of enzyme-linked immunosorbent assay (ELISA) results with those obtained from conventional methods will illustrate technical implementation barriers and suggest ways to overcome them. 

The production of SWIFT-WFD RMs had a valuable impact in the evaluation of European laboratories performances using both classical and screening methods for four classes of analytes trace elements, major components, PAHs, pesticides. The produced RMs allowed the evaluation of analytical performances at EU level at different levels of concentration, in different matrices (river water, spring water) with different composition. 

Recently, LC-MS techniques have become increasingly popular as an approach to analytical problems. In keeping with this development, Saint-Marcoux et al described a general unknown screening procedure for serum samples, using SPE and subsequent liquid chromatography-electrospray mass spectrometry (LC-ESIMS). This method allows for the detection and identification of pesticides and also other drugs and toxic compounds. 

High performance thin-layer chromatography (HPTLC) has also been used for the determination of carbamate pesticides.Thus, TLC methods provide increased selectivity through silica derivatization, as well as higher analytical precision and sensitivity with high-performance plates. Butz and Stan reported an HPTLC system with automated multiple development (AMD-HPTLC) to screen water samples for pesticides. The method was applied to the determination of 265 pesticides in drinking water spiked with 100 ng/1 of each analyte. 

All previously revised studies focus on target analyte. Consequently, efficient separation of the investigated analytes from the sample matrix was the main concern during method development, especially when additional structural information was not available. This explains why group-t3q>e anal5 is has received limited attention in the pesticide field, despite the efficiency of this approach for fast screening proposes [43]. 

The evaluation of a number of immunoassay diagnostic kits was undertaken to determine their usefulness in a regulatory analytical laboratory environment in the food, feed and pesticide areas. Four rapid enzyme immunoassay tests for the detection of aflatoxin residues at the 20 ppb level in animal feeds were compared to the official HPLC procedure. In the pesticide area, a commercial pentachlorophenol competitive inhibition assay for residues in water was investigated as to its applicability to poultry and pork liver matrices. In addition, an ELISA screening procedure for the herbicide fusilade was developed. Modifications were incorporated into the rapid immunoband 1-2 Test procedure for the detection of motile Salmonella in various food and animal feed products resulting in quicker analysis than the standard culture method. Also, a comparative evaluation of a Quik-Card Test for sulphamethazine drug residues in pork urine, liver and muscle tissue, is described. 

Screening methods could significantly increase the number of analytical samples examined in contamination incidents when a known pesticide is accidentally, or through misuse, applied to a crop. Availalnlity of a suitable EIA method for aldicarb would have greatly fecilitated analysis during the contamination of watermelons several years ago. The unpredictalnlity of these incidents implies EIA methods for all pestid s would be requited fa- adequate preparation for the next incident however, the cost and resources needed for such an FDA method development effort would be prohibitive. 

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