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14

Limit of Detection

KEY WORDS limit of detection, measurement error, method limit of detection, percentile, variance,
standard deviation.

The method limit of detection or method detection limit (MDL) defines the ability of a measurement method
to determine an analyte in a sample matrix, regardless of its source of origin. Processing the specimen by
dilution, extraction, drying, etc. introduces variability and it is essential that the MDL include this variability.
The MDL is often thought of as a chemical concept because it varies from substance to substance
and it decreases as analytical methods improve. Nevertheless, the MDL is a statistic that is estimated
from data. As such it has no scientific meaning until it is operationally defined in terms of a measurement
process and a statistical method for analyzing the measurements that are produced. Without a precise
statistical definition, one cannot determine a numerical value for the limit of detection, or expect different
laboratories to be consistent in how they determine the limit of detection.
The many published definitions differ in detail, but all are defined in terms of a multiple of the standard
deviation of measurements on blank specimens or, alternately, on specimens that have a very low concen-
tration of the analyte of interest. All definitions exhibit the same difficulty with regard to how the standard
deviation of blank specimens is estimated.
The U.S. EPA’s definition and suggested method of estimation is reviewed and then we look at an
alternative approach to understanding the precision of measurements at low concentrations.

Case Study: Lead Measurements

Lead is a toxic metal that is regulated in drinking water and in wastewater effluents. Five laboratories
will share samples from time to time as a check on the quality of their work. They want to know whether
the method detection limit (MDL) for lead is the same at each lab. As a first step in this evaluation, each
laboratory analyzed 50 test specimens containing lead. They did not know how many levels of lead had
been prepared (by spiking known amounts of lead into a large quantity of common solution), nor did
they know what concentrations they were given, except that they were low, near the expected MDL.
The background matrix of all the test specimens was filtered effluent from a well-operated activated
sludge plant. A typical data set is shown in Table 14.1 (Berthouex, 1993).

Method Detection Limit: General Concepts
The method detection limit (MDL) is much more a statistical than a chemical concept. Without a precise
statistical definition, one cannot determine a scientifically defensible value for the limit of detection, expect
different laboratories to be consistent in how they determine the limit of detection, or be scientifically honest
about declaring that a substance has (or has not) been detected. Beyond the statistical definition there must
be a clear set of operational rules for how this measurement error is to be determined in the laboratory. Each
step in processing a specimen introduces variability and the MDL should include this variability.
The EPA says, “The method detection limit (MDL) is defined as the minimum concentration of a
substance that can be measured and reported with 99% confidence that the analyte concentration is

© 2002 By CRC Press LLC