WEIGHTED LEAST-SQUARES APPROACH TO CALCULATING LIMITS OF DETECTION AND QUANTIFICATION BY MODELING VARIABILITY AS A FUNCTION OF CONCENTRATION

The limit of detection and limit of quantification are current critica l issues in environmental testing, In most laboratories, limits are cu rrently calculated on the basis of the standard deviation of replicate analyses at a single concentration. However, since the standard devia tion depends on concentration, these single-concentration techniques r esult in limits that are directly dependent on spiking concentration, A more rigorous approach uses a weighted least-squares regression anal ysis of replicates spiked at a series of concentrations-a calibration design, In this work, the use of weighted tolerance intervals is intro duced for estimating detection and quantification limits, In addition, models for estimating the weights used in calculating weighted predic tion intervals and weighted tolerance intervals are presented, Using t his method, detection and quantification limits were calculated for ga s chromatographic analyses of 16 polychlorinated biphenyls. Results sh ow that the approach developed provides improved estimates of analytic al limits and that the single-concentration approaches currently in wi de use are seriously flawed, Future work should reduce the data needed for the calibration design approach so that more rigorous detection a nd quantification limits can be routinely applied.