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 giving results in semiquantitative ranges lying 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. Alternate approaches are required by OSW for SW-846 screening applications of the technology. In keeping with the OSW screening method program requirements, those applications of SDI products to projects that require SW-846 methods must be formatted according to these OSW guidelines.
USE OF A SITE SAMPLE TO DETERMINE SCREENING THRESHOLD
The most accurate approach to calibration of an immunoassay screening test for TPH 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 pre-determined SW-846 approved method quantitative result (e.g., Method 8015 or 8440) 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 Total BTEX/TPH RaPID Assay system immunoassay result. This information should be considered when interpreting results from a particular remediation project. The Total BTEX/TPH RaPID Assay Kit provides standard calibrators in terms of a mixture containing equal parts of benzene, toluene, ethylbenzene and the three xylene isomers.