The Methods Section of the EPA Office of Solid Waste (OSW) has stated a need for more rapid, less expensive field screening procedures that do not compromise the accuracy of contaminant evaluation. The OSW screening approach incorporates a single point threshold or multiple individual point discriminators and reports results in semi-quantitative ranges lying above, below or between these points. SDI has validated all of the RaPID Assays using a calibration curve to convert an absorbance response to a numerical value. The performance data provided in the package insert reflects this method of data interpretation. However, an alternate procedure is required by OSW for SW-846 screening applications using this technology. In keeping with the OSW screening method program requirements, those applications of SDI products to projects that require SW-846 methods must follow the procedures described in this bulletin.
USE OF A SITE SAMPLE TO DETERMINE SCREENING THRESHOLD
The most accurate approach to calibration of an immunoassay screening test for 2,4-D contamination is to utilize a known amount of the contaminant itself, taken from the project site, to define the threshold. A site sample with a predetermined SW-846 approved method quantitative result (e.g., Method 8150) is diluted to the appropriate threshold concentration and results of unknowns are then directly compared to the diluted site sample without further unit correction.
CORRECTING CALIBRATOR LEVELS FOR REACTIVITY, EXTRACTION EFFICIENCY AND ANALYTICAL CONFIDENCE
The RaPID Assay Environmental User's Guide describes factors that affect the RaPID Assay system immunoassay results. This information should be considered when interpreting results from a particular remediation project. The 2,4-D RaPID Assay Kit provides standard calibrators composed of the acid form of 2,4-D.