Although agricultural and industrial practices have drastically changed in recent decades, the threat of harmful residues in foods remains a major concern in the minds of consumers. In addition to pesticide residues, the presence of environmental contaminants such as dioxins, heavy metals and furans are of concern, as well as compounds created in processing, such as acrylamide and leachables from packaging. These concerns are compounded due to the explosion in the use of botanical ingredients that are often imported from countries where agrichemical monitoring is less stringent than in the United States. As a result, industry and regulatory agencies continue to be extremely vigilant in supplying accurate and timely data on residues in food. Because of the huge volume of testing needed to ensure the safety of both domestic and imported commodities, it is essential that fast, accurate and cost-effective methods are available to test samples.
The Food Quality Protection Act (FQPA) made several substantive changes to the manner in which pesticides are regulated. Primary among these are the standards for establishing tolerances and the impact on regulations pertaining to aggregate exposure to pesticides. Determining pesticide-residue levels that may be present in food products is a key part of a risk assessment process that must measure potential exposure from a variety of sources. Regulatory guidelines require that this testing include measuring levels of the compound present in food crops, products derived directly from the crops, and foods derived from the livestock fed treated crops or exposed to environmental sources (e.g., water). As a result of FQPA, the risk-assessment process must consider aggregate exposures from all potential routes of exposure, including dietary, residential and environmental. The data gathered from the studies determine which components of the total toxic residue (TTR) are present, and at what concentration secondary residues could appear. The sheer volume of testing required to monitor human exposure and evolving regulatory requirements have mandated the continuing development and adaptation of analytical methods.
Multiresidue methods
The need for efficient, high-quality testing led to the development of multiresidue screening methods that enable scientists to detect trace concentrations (parts per billion) of many pesticides using a single procedure. In addition to identifying compounds that are expected to be present, the multiresidue screen can alert researchers to the presence of compounds that are not expected or normally encountered in a specific matrix. Modern analytical technology now provides low-cost and high-throughput pesticide methods that can screen for hundreds of pesticides for less than $1 per compound.
Multiresidue screens allow cost-effective and timely samples analysis, produce reliable and verifiable data, and can identify many pesticides in a wide range of matrices at or below tolerance levels. These methods can monitor for parent compounds, metabolites, impurities, alteration products and other pesticide-associated chemicals. FDA’s Pesticide Analytical Manual (PAM) contains protocols and guidelines for these methods, and many have been adopted as official methods by AOAC International. If a pesticide is detected, confirmatory analysis by conducting additional chromatographic or mass spectrometry techniques can provide more comprehensive and sensitive data.
Although the specific parameters and techniques vary, traditional methods for the determination of chemical residues generally involves seven basic steps. Extraction removes the analyte from the matrix, usually by solvent extraction. The cleanup removes coextractives by such operations as column chromatography, liquid-liquid partitioning or volatilization. Some samples may undergo a modification process to convert the analyte to a readily analyzed derivative. The resolution step then separates the analyte from remaining interferences. Using a variety of detectors isolates a response related to the amount of analyte. This response is then measured and compared to that of a standard. A confirmation step provides assurance that the primary method gives correct results by use of a second method
Several multiresidue methods are listed in PAM for regulatory use. All of the methods routinely used are either GC- or HPLC-based. Although each of these employs similar techniques, they differ in the type of product and class of pesticide they are used to detect. The first of the multiresidue screens, the Mills method, was developed in 1959 for the measurement of nonionic organochlorine compounds in nonfatty foods. This method can be used for the determination of more than 150 chemicals, including several organophosphorus pesticides via gas chromatography (GC).
The PAM 302 assay, also known as the “Luke Method,” was originally designed to recover essentially all nonionic pesticides in the organochlorine, organophosphate, organonitrogen and hydrocarbon classes. Because of its ability to measure a wide range of pesticides, this method is used extensively. The Storherr method is applicable to nonfatty foods such as fruits, vegetables and grains for the detection of organophophorus pesticides.
Many of the traditional methods are being replaced by more efficient and accurate methodologies. For example, crises such as the recent incidences of alleged high pesticide levels in soft drinks in India accelerated the development of a new liquid chromatography/mass spectrometry (LC/MS) method that can detect minute levels (0.1 ppb) of pesticides. In addition to providing increased sensitivity, these methods reduce the complexity of the cleanup and extraction processes, thereby greatly enhancing the efficiency of the assay and accuracy of the data. These methods have successfully undergone validation and are being expanded to an increasing range of matrices.
