An inter-laboratory study of detection limits in the analysis of water and wastewater for organo-chlorine pesticides by liquid/liquid extraction and gas chromatography-electron capture detector (USEPA Method 608)

Abstract

An inter-laboratory study was conducted to assess the Kaiser-Currie Model (KCM) for the determination of detection limits. Six laboratories participated in the analysis of samples prepared from distilled water, some containing organo-chlorine pesticides at a concentration of zero and other with a greater than zero concentration. The study consisted of three phases, the first of which was a study to assess the longer term variability of distilled water samples containing no organo-chlorine pesticides prepared by the participating laboratory analysed over a six month period. A second phase consisted of replicates of distilled water samples containing organo-chlorine pesticides prepared at a single concentration greater than zero by the laboratory and were analysed over several days. Finally, a third phase consisted of twelve distilled water samples, eleven containing organo-chlorine pesticides at a concentration of greater than zero and one with a concentration of zero prepared by a third party. Estimated detection limits were determined and then compared to the observed detection limits. Only in a minority of cases, where the distribution of results from samples containing a concentration of zero was normally distributed, did Currie's L _C accurately predict a concentration which corresponded to a 1% false positive rate in distilled water samples with a zero concentration of the study analyte. The USEPA's MDL performed more poorly. In the majority of cases, when any non-zero results were obtained from distilled water samples containing a concentration of zero, they were not normally distributed. Contrary to expectation, false negatives and false positives rarely occurred simultaneously on any given day. The variability between days of analysis and the use of noise reducing techniques proved to be a significant source of the observed non-normal distribution of distilled water samples. Conventional procedures based on the KCM and their underlying analytical and statistical assumptions did not provide useful predictions of laboratory sensitivity in most cases in this study.

Keywords:

• detection limit
• critical level
• drinking water
• wastewater
• regulatory compliance
• gas chromatography
• electron capture detector
• USEPA 608
• organo-chlorine pesticides
• false positive
• false negative

Acknowledgements

The design of the FACDQ inter-laboratory study was conducted by a sub-committee of the FACDQ which included, Zonetta English, Louisville and Jefferson Counties Metropolitan Sewer District, Dr Richard Rediske, Annis Water Resources Institute, Dr Richard Burrows, Test America, Larry LaFleur, National Council for Air and Stream Improvement, Inc. Dick Reding, USEPA Office of Water, Engineering and Analysis Division, Bob Avery, Michigan Department of Environmental Quality, Environmental Laboratory Services. The author would like to offer special thanks to Ken Miller of Computer Science Corporation who was a technical consultant to the USEPA ODW and provided technical support to the FACDQ.