PRACTICAL QUANTITATION LIMITS

A statistical method for computing practical quantitation limits (PQL) is developed. The PQL is operationally defined as the concentration at which the instrument response signal is 100/alpha times its standard deviation (e.g., for a 10% relative standard deviation (R.S.D.) alpha = 10). The response signal is defined as the ratio of analyte to internal standard peak areas. 95% confidence limits for the PQL are also derived. The PQL is estimated directly from calibration data, and uncertainty in the parameters of the calibration function is incorporated. The non-constant variance problem is dealt with using a variance stabilizing transformation. The method is illustrated using 199 calibration samples for 10 volatile organic priority pollutant compounds. The results of these analyses suggest that estimates of the US Environmental Protection Agency of PQLs are reasonably consistent with the 10% R.S.D. definition.