Statistics for Environmental Engineers

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It should be remembered that the various limits of detection (MDL, IDL, etc.) are not unique constants of methodology (see APHA, AWWA, WEF, 1998, Section 1030C). They depend on the statistical definition and how measurement variability at low concentrations is estimated. They also depend on the expertise of the analyst, the quality control procedures used in the laboratory, and the sample matrix measured. Thus, two analysts in the same laboratory, using the same method, can show significant differences in precision and hence their MDLs will differ. From this it follows that published values for a MDL have no application in a specific case except possibly to provide a rough reference datum against which a laboratory or analyst could check a specifically derived MDL. The analyst’s value and the published MDL could show poor agreement because the analyst’s value includes specimen matrix effects and interferences that could substantially shift the MDL.

The EPA Approach to Estimating the MDL

The U.S. EPA defines the MDL as the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte. Similar definitions have been given by Glaser et al. (1981), Hunt and Wilson (1986), the American Chemical Society (1983), Kaiser and Menzes (1968), Kaiser (1970), Holland and McElroy (1986), and Porter et al. (1988).

Measurements are made on a minimum of seven aliquots (n > 7) of a prepared solution that has a concentration near the expected limit of detection. These data are used to calculate the variance of the replicate measurements:

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