Classification of Methods For Regulatory Purposes

Most residue methods used in field applications are qualitative or semiquantitative and are classified as screening methods. Quantitative methods require much more technical expertise, and, therefore, their primary use is in laboratory applications primarily for confirmation purposes. Both screening and confirmation methods can be subclassified into multiresidue methods aiming at the detection of groups of compounds having similar analytical characteristics, and singleresidue methods applicable to only one specific analyte. 

Methods used in the screening phase should be able to detect the presence of as many drug residues as possible at the established for each drug level of interest, with a high sample throughput at low cost. Most microbiological methods 

For reliable identification of a residue, detailed information about the molecular structure of the analyte is essential. The total information about the molecular structure of the analyte is the sum of the information derived from each individual analytical step of tire method. Frequently used selective analytical steps based on chromatography or immunoaffinity, provide more or less general indirect information. For example, solid-phase extraction (SPE) cleanup followed by liquid chromatography/ultraviolet detection (LC/UV) has been suggested for screening and quantification of ivermectin residues in liver, but presumptive positive samples can be confirmed by derivatizing an aliquot of the SPE eluate and reanalyzing the fluorescent derivative of ivermectin in an LC-fluorescence system (17). 

For complete identification, relevant direct information on the molecular structure of the analyte is always more specific and hence more reliable than indirect information. Analytical steps based on molecular spectroscopy all provide direct more or less detailed information on the structure of the analyte. This is particularly true for fourier-transform infrared (FTIR) and mass spectroscopy (MS), where the spectra have a very high information content. 

An alternative interesting classification approach has been proposed within the Codex Committee for Residues of Veterinary Drugs in Foods (20). In this approach, methods are classified according to their performance attributes. This alternative approach defines methods by the level of analytical detail or information provided concerning the amount and nature of the analyte of interest, and identifies three levels. 

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