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A study of the precision of lead measurements at concentrations near the method limit of detection

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Many problems in environmental quality require measurements on chemicals that are expected to be absent or to exist at very low concentrations. Test specimens delivered to the laboratory for analysis will contain literally “next to nothing” which may lead to some observations being reported as “below the limit of detection.” The limit of detection is a statistical concept that is intended to reflect the magnitude of unavoidable random fluctuations in measurements at these low concentrations.

The purpose of this study was to generate a collection of measurements on samples that have low concentrations, at or near the limit of detection. The results show the magnitude of measurement variability at low concentrations and have some important implications regarding the utility of the limit of detection concept in compliance monitoring.

An interlaboratory study involving eight laboratories, each of which measured lead on 50 test specimens, provides a large collection of measurements on lead at low concentrations, including concentrations below the laboratories' stated limits of detection. The data show that the variability of measurements at low concentrations was not greater than at the higher concentrations. Lead was not only detected in low concentration specimens, it was also quantified, even at concentrations near and below the limit of detection as it is usually defined, especially when data are interpreted collectively instead of as isolated individual values. The accuracy of measurements may be affected more by bias than by poor precision.

The method detection limit (MDL) is a misunderstood and often misused concept. The concept of an MDL does not deal with bias; it is only concerned with precision. It is a quantity that cannot be estimated with great precision. Throwing away measured values because they are “below the limit of detection” discards much useful information and it is recommended that this practice be discontinued.

Keywords: bias; lead; limit of detection; monitoring; precision

Document Type: Research Article


Publication date: July 1, 1993

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  • Water Environment Research® (WER®) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year.

    Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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