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For the majority of tests listed in Table 7 the limit of detection is 1 ng of pesticide, which corresponds to 1-10 ppb in a sample solution. RIA and EIA offer comparable sensitivity. The detection limit of RIA is more readily varied because it depends largely on the specific activity of the radioisotope label (17). Use of RIA is restricted to sophisticated laboratories with substantial investment in the equipment. EIA, with ELISA in particular, offers more versatility in analysis than RIA.

There are a number of pesticide detection kits sold commercially. Immuno Systems, Inc. (Biddeford, Maine) sells ELISA kits for triazine (atrazine, simazine, etc) detection as well as one for the cyclodiene insecticides (chlordane, heptachlor, etc). ELISAs for parathion and paraquat are available from Environmental Diagnostics, Inc. (Burlington, N.C.). An assay for pentachlorophenol detection is available from Westinghouse Bio-Analytical Systems (Rockville, Md.). Ohmicron Corp. (Newtown, Pa.) markets pesticide detection kits for aldicarb, atrazine, benomyl, alachlor, captan-captofol, and carbafuran that employ magnetic particle technology.

There are factors that make applications of immunoassays to pesticide analysis complex. It is often difficult to obtain representative samples of agricultural commodities. Commodities are localized temporally and geographically. Local practices can vary leading to complicated research efforts toward assay development. Agricultural commodities are often composed of mixtures of the pesticide and its metabolites at harvest. The high specificity of antibodies used in assays may be disadvantageous because metabolites may not be detected. Also there is fragmentation within the potential pesticide market, potential customers or users could be farmers, consumers, pesticide manufacturers, regulatory agency personnel, food processors, wholesalers, and retailers. The requirement for assay features can vary drastically among these users. Therefore, it is not easy for companies to identify customer needs and project the economic potential of the kits (373).

Officials at regulatory agencies have expressed concern about adopting these qualitative or quantitative immunochemical assays for routine screening of samples. Concern stems first from the need for familiarization and experience with the technology involved in the test systems. Once the testers feel confident that public health protection is not jeopardized by implementing this technology, they must address other concerns. It must be demonstrated for each method, presented in kit form, that there is no appreciable variability from lot to lot production (373).

Many of these immunochemical methods are more sensitive than the confirmatory tests and traditional methods. Therefore, qualitative screening results using IAs cannot be confirmed without effort given to increase the sensitivity of the confirmatory methods (373). As the concerns are addressed, the technology will continue to develop and practical applications of IAs for routine pesticide analysis will occur.

Drug Residues

There is an increasing number of publications that describe the development of ELISAs for drug residue analy sis. One antimicrobial agent, sulfamethazine, has received a great deal of attention in the scientific literature as well as the media. There have been numerous reports of the use, or perhaps the abuse, of sulfamethazine in the livestock industry. It will be used as an example of how ELISAs have been developed for detection of the drug in place of the classical chemical methods.

Sulfonamides, namely sulfamethazine, have been incorporated into animal feeds since the 1950s. They are added as growth promotants and for control of certain diseases (412,413). The drugs are retained in the tissues of the animals eating medicated feeds. Some individuals who ingest the contaminated tissues will experience hypersensitivity allergic reactions. Furthermore, there will be preferential selection of bacterial mutants that are resistant to the drugs, which are also used in treatment of human diseases (414).

The residues are cleared from the tissues if the proper 15 day withdrawal period is followed by the producer (415). If it is not adhered to there will be a good chance for the carcasses to contain violative levels of the drug at the time of slaughter (>0.1 ppm).

Sulfonamide residues can also occur in milk due to a number of reasons, including use of the drugs in mastitis therapy, deliberate feeding, inadvertent feeding, or use of sulfamethazine-containing boluses to prevent infection in cows that have calved. One study (416) showed the 64% of milk sampled in the New York City area, central New Jersey, and eastern Pennsylvania contained one or more antibiotic-antimicrobial residues. Sulfonamide residues appeared in 42% of the samples. Other studies showed similar frequencies of the residues in milk (417,418). The FDA maintains a zero-effect tolerance. However, because approved methods cannot accurately measure extremely low levels of the sulfonamides, the tolerance level is set at 10 ppb.

The concern over the presence of sulfamethazine in meat, namely pork, and dairy products heightened when the drug was found to cause neoplasms in mice and rats (419). The need for a rapid screening method became apparent. Current methods for sulfonamide analysis include gas chromatography (1,3,420,421), gas chromatography-mass spectrophotometric (419), liquid chromatography (1,421-423), colorimetry based on the Bratton-Marshall reaction, and thin-layer chromatography (421,425-427) as well as tandem mass spectroscopy-mass spectroscopy (428,429). These methods are labor intensive. They require extensive sample cleanup or preparation and they often measure at the tolerance level. They do not lend themselves to screening samples in the field. A bacterial receptor assay (430) has been developed for detection of numerous sulfonamides. It can be used for screening samples; however, it lacks the specificity that an antibody-based test provides. The Charm Assay is marketed by Penicillin Assays, Inc. (Maiden, Mass.).

The Food Safety and Inspection Service (FSIS) branch of the USDA began a residue avoidance program (RAP) designed to check animals before they are slaughtered (431). Sulfamethazine has been shown to cause the most violations in swine tissue, approximately 95% of the sulfonamide tissue violations (432). A correlation has been established for the concentrations of sulfamethazine present in the liver and edible tissues as well as how much is present in the urine and serum (432-435). Noninvasive samples can be collected and tested prior to or at the time of slaughter and tested to determine the concentration of the drug in the liver and edible tissues.

The early ELISA developed for detection of sulfamethazine in swine blood (436) required extraction of the drug from the sample and required fairly long times to perform the assay. They were not suitable for use at slaughterhouses. More recent publications describe assays with shorter times and serum or plasma samples can be used directly without extraction (437,438). Assays have also been described for detection of sulfamethazine in milk (439), feed, and tissue (440). A recent paper describes the use of high-performance immunoaffinity chromatography for drug residue analysis (441).

A number of biotechnology companies market inmuno-diagnostics for sulfamethazine detection in dairy products (Neogen Corp., Lansing, Mich.; Idetek, San Bruno, Calif.; and Idexx, Portland, Maine), feed (Neogen Corp., Idetek, and Environmental Diagnostics, Inc., Burlington, N.C.), and urine and tissue (Idetek and Environmental Diagnostics).

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