Determining quantitation levels for regulatory purposes

ecently, water regulatory agencies and providers have shown considerable interest in determining the lowest quantifiable level that is suitable for regulatory purposes.1–8 Allowable concentration levels for a contaminant in a particular environmental medium (e.g., soil, air, or water) are often based on health-related risk assessments and are sometimes lower than levels that can be quantitated in a laboratory. For this reason, the lowest quantifiable level frequently becomes the de facto regulatory limit for monitoring and compliance purposes. The lowest quantifiable level has been given many different names. Current terms include the practical quantitation level The authors describe an approach for calculating quantitation levels (QLs) that does not require changes in current laboratory practices. The reliable detection level (RDL), when defined as twice the concentration of the method detection limit (MDL), provides adequate protection against both false-positive and false-negative detection decisions. When analyte concentrations are at the RDL, the corresponding precision in the instrument response is adequate for analytical purposes. Therefore, the RDL (as determined by a single laboratory) is a reasonable lower limit of quantitation for that laboratory. To determine an interlaboratory QL suitable for regulatory purposes, median interlaboratory MDLs were multiplied by a variable factor (usually 4–7, determined from actual laboratory performance data). More than 80 percent of the laboratories surveyed had RDLs less than or equal to the calculated QL, indicating that adequate quantitation was attainable at this level and that the QL should be suitable for regulatory purposes.