Research Highlights
Detecting Chemical Agents and Pesticides in Water, Using Enzymatic Test KitsIn the past, people in the United States have largely taken for granted the convenience of potable municipal water. However, the threat of intentional contamination of our water supplies is becoming a concern because of a rise in the number of terrorist acts around the world. As a result, there is much interest in technologies that can be used to detect a contamination event as well as dispel or confirm the credibility of a threat. Such technologies include enzymatic test kits, which are generally designed to be handheld and portable. These kits can detect the presence of chemical agents, carbamate pesticides, and/or organophosphate pesticides through a cholinesterase enzyme reaction. Under contaminant-free conditions, the enzyme reacts with other reagents present in the test kit. However, the presence of contaminants from one or more of the above chemical classes causes the enzyme activity to be inhibited. Inhibition is indicated by a color change. In late 2005 and early 2006, EPA evaluated four enzymatic test kits:
EPA tested each enzymatic test kit to determine the responsiveness to specific toxic compounds as well as to possible interfering compounds and matrices. Because enzymatic test kits are anticipated to serve mainly as screening tools in water monitoring scenarios, this testing produces only qualitative results (i.e., results indicate only the presence or absence of a contaminant). The kits cannot distinguish between contaminants; however, some can indicate the concentration range of contaminants (high, medium, low). Each kit was specifically evaluated for:
Test DesignTable 1 identifies the enzymatic test kits that were tested using various water types fortified (spiked) separately with contaminants and interfering compounds. Table 1. Technologies, Contaminants, and Interfering Compounds
Three types of water samples were tested in these evaluations: performance test (PT), drinking water (DW), and quality control (QC). PT samples were prepared with deionized (DI) water and fortified with the contaminant-only, the interferent-only, or both the contaminant and interferent. Contaminant-only PT samples were tested in a series of concentrations that included the accepted lethal dose concentration and dilutions at approximately 10; 100; 1,000; and 10,000 times less than the lethal dose. DW samples were tested to determine the effects of matrix-specific characteristics (e.g., location, filtering) on the technology being evaluated. DW samples were collected from four geographically diverse municipal sources that varied in source (ground water or surface water), treatment (filtered or unfiltered), and disinfection process (chlorination or chloramination). DW samples were either not spiked or spiked with a contaminant at a single concentration level 10 times more dilute than the lethal dose. All PT and DW samples were analyzed in triplicate. The result of each replicate sample set was reported as a ratio of the number of positive results to the total number of replicates (e.g., 0/3, 1/3). Method blank QC samples consisted of 10% of all samples. The method blanks were not considered in evaluations of the various test parameters. Performance and ResultsThe accuracy of the enzymatic kits was determined by dividing the number of positive responses by the overall number of spiked contaminant-only PT samples. The precision of the technologies was determined by calculating the number of consistent responses for all sample sets. Responses were considered consistent if all three replicates had similar results. A false positive rate was reported as the frequency of positive results out of the total number of unspiked samples. A false negative rate was reported as the frequency of negative results out of the total number of spiked PT (contaminant and interferent) samples and spiked DW samples. Table 2 summarizes the results of the evaluation parameters for each technology. Table 2. Summary of Results
aVX = VX nerve agents, GB = sarin, and GD = soman The following is a summary of the testing of the 20 kit and contaminant combinations:
Note that the total number of matrix and interferent effects did not always equal the total number of false negatives and false positives because some false negatives occurred with contaminant-only PT samples. For more information about enzymatic kits' ease of use by technical and nontechnical operators, and about sample throughput, portability, and cost, visit the NHSRC Web site at www.epa.gov/nhsrc, or view the full report for each technology. Contact: Eric Koglin |