Pesticide-residue analysis method validation

Komit6 for Levnedsmidler (NMKL)]. The standard presents a universal validation approach for chemical analytical methods in the food sector. This includes methods for the main constituents and also for trace components. Therefore, the NMKL procedure focuses on primary validation parameters, such as specificity, calibration, trueness, precision, LOD or LOQ and does not refer to special requirements of pesticide residue analysis. 

In summary, the procedure of the Nordic Committee describes a comprehensive validation protocol, but it is not specially designed for pesticide residue analysis and has no preferences with regard to single- or inter-laboratory validation. Therefore, if it is applied to pesticide residue methods, some specific validation requirements should be added. The procedure clearly lists all necessary steps of validation and adjusts its recommendations to the degree of previous external validation. 

European Commission (2007) Method validation and quality control procedures for pesticide residues analysis in food and feed. SANCO/2007/3131, available from ec.emopa.eu... 

Immunochemical methods are rapidly gaining acceptance as analytical techniques for pesticide residue analysis. Unlike most quantitative methods for measuring pesticides, they are simple, rapid, precise, cost effective, and adaptable to laboratory or field situations. The technique centers around the development of an antibody for the pesticide or environmental contaminant of interest. The work hinges on the synthesis of a hapten which contains the functional groups necessary for recognition by the antibody. Once this aspect is complete, immunochemical detection methods may take many forms. The enzyme-linked immunosorbent assay (ELISA) is one form that has been found useful in residue applications. This technique will be illustrated by examples from this laboratory, particularly molinate, a thiocarbamate herbicide used in rice culture. Immunoassay development will be traced from hapten synthesis to validation and field testing of the final assay. 

Method Validation and Quality Control Procedures for Pesticide Residues Analysis in Food and Feed, SANCO/10684/2009, Directorate General for Health and Consumers (SANCO), European Commission, Brussels, 2009 (available at http //ec.europa.eu/food/plant/ protection/resources/qualcontrol en.pdf accessed 10/23/10). 

Method Validation and Quality Control Procedures for Pesticide Residue Analysis in Food and Feed (Document SANCO/10684/2009)... 

With the implementation of the original QuEChERS-method in our pesticide residue analysis laboratory in 2002, and the associated validation experiments for numerous pesticides in different representative commodities, it soon became clear that some amendments to the original procedure had to be introduced to improve the recoveries of certain pH-dependent pesticides and to expand the spectrum of commodities amenable to the method. The routine use of the method furthermore raised the need to further improve its selectivity in order to enhance the robustness of determinative analysis. The modifications introduced are subject of this paper. 

In the modern pesticide residues laboratory, analysts are under ever increasing pressure to (1) increase the range of pesticides which can be sought in a single analysis, (2) improve limits of detection, precision and quantitation, (3) increase confidence in the validity of residues data, (4) provide faster methods, (5) reduce the usage of hazardous solvents and (6) reduce the costs of analysis. 

System Suitability System suitability refers to the validation of all components of an analysis system taken as a unit, a "system." For example, the analysis of an environmental water for pesticide residue involves a "method," which includes sampling (must represent the water in question), sample handling (e.g., what container is appropriate), sample preparation (perhaps an extraction process that includes the glassware, technique, timing, etc.), standards preparation (pipets, flasks, technique, etc.), injection technique, the instrument, and data handling (computer hardware and soft-... 

Once a method is established, precision may be determined by suitable replicated experiments. However it is in inter-laboratory trails that the problems with environmental methods often show up. It is accepted that for trace analysis RSD values of tens of percent are likely. In studies conducted in Western Australia on pesticide residues in bovine fat RSD values for dieldrin were 12% and for dia-zonium were 28%. It is typical to see a quarter of the laboratories in such a trial producing values that could be termed outliers. In the previously mentioned study, 5 laboratories out of 26 had z> 3 for aldrin. In a parallel study RSD values for petroleum in dichloromethane and water were 40% and 25%, respectively. The conclusions of these studies was that there was poor comparability because of the different methods used, that accreditation apparently made no difference to the quality of results, and that a lack of understanding of definitions of the quantities to be analysed (for example gasoline range organics ) caused non-method errors. In relation to methods, this is contrary to the conclusion of van Nevel et al. who asserted that the results of the IMEP round of analyses of trace elements in natural and synthetic waters showed no dependence on method [11]. If properly validated methods do yield comparable results, then one conclusion from the range of studies around the world is that many environmental methods are not validated. It may be that validated methods are indeed used, but not for exactly the systems for which they were validated. 

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. 

Dithiocarbamates can be quantitatively converted to carbon disulfide by reaction with tin(II)chloride in aqueous HCl (1 1) in a closed bottle at 80 C. The CS2 gas produced is absorbed into iso-octane and measured by GC-MS. The analysis of DTCs for this application follows the acid-hydrolysis method using SnCl2/HCl (Reynolds, 2006). For method validation of the DTC pesticides, Thiram (99.5% purity) was used as representative DTC compound considering its simple structure (1 mol of Thiram = 2 mol of CS2 = >1 mg of Thiram theoretically generates 0.6333 mg CS2, 1 mL of 100 ppm Thiram in 25 g of grapes = 2.5 ppm of CS2), see Figure 4.38. The residues were estimated by analysis of CS2 as the DTC hydrolysis products by GC-MS. 

This validation typically requires samples with radiolabeled analytes. However, alternative approaches are proposed which involve (i) comparison with extraction of samples using a procedure which has been previously validated rigorously, (ii) comparison with extraction of samples by a very different technique or (iii) analysis of a certified reference material. Generally, this validation should be performed with samples containing analyte incurred by the route by which residues would normally be expected to arise. The simplest option (i) requires fully validated and documented enforcement methods provided by the manufacturer of a pesticide. 

A Joint FAO/IAEA Expert Consultation on Practical Procedures to Validate Method Performance of Analysis of Pesticide and Veterinary Drug Residues, and Trace Organic Contaminants in Food was held in Miskolc,... 

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