APPENDIX
MEANING OF QUANTITATION LIMITS
This guidance is Part IV of four parts. They are:
Part I: Implementation Memo No. 3—PQLs as
Cleanup Standards
Part II: Guidance For The Use of Tables
Part III: MDL, PQL, and Comparisons Tables
Part IV: Appendix—Meaning of Quantitation Limits (this
document)
In Part II, Guidance For The Use of Tables, an overview was given of the need for a site manager to have information on the lowest levels which can be routinely quantified and reported by a laboratory. These lowest levels are known as the “practical quantitation limits” (PQLs). The “method detection limit” (MDL) is used mostly by the laboratory analyst and not usually reported but can provide useful information to the site manager.
This document discusses the meaning of these two terms, PQL and MDL.
The MDL is defined by the EPA in Appendix B of 40 CFR 136 as “…the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero.” Appendix B includes detailed procedures for determining the MDL either in lab reagent water or in the sample matrix.
Detection should be based on the variability of the response of the measurement system (such as a gas chromatograph) to a sample with zero concentration of the analyte (blank response). Detection limits should account for the probabilities of false positives and false negatives. The MDL is based on the variability of the response of the measurement system to a low level standard or spiked sample and accounts only for false positives.
Concentrations of chemicals that exceed the MDL but do not exceed the PQL are often reported as estimates.
There is no single method for defining or determining the PQL. Some documents, including some by EPA, refer to “detection limits” without explanation of how they were derived. Many PQLs listed in the federal regulations are based on consensus rather than rigorous technical assessments. The following is an excerpt from guidance for statistical regulations (U.S. EPA 1988):
“The
PQLs listed were EPA’s best estimate of the practical sensitivity
of
the applicable method for the RCRA ground water monitoring purposes.
However,
some of the PQLs may be unattainable because they are based on
general
estimates for the specific substance.
Furthermore, due to site-specific
factors,
these limits may not be reached. For
these reasons, the agency feels
that
the PQLs listed in Appendix IX are not appropriate for establishing a
national
baseline value for each constituent for determining whether a release
to
ground water has occurred. Instead, the
PQLs are viewed as target levels
that
chemical laboratories should try to achieve in their analysis of ground water.”
Soils usually present even more difficulty for analysis than groundwater because they have a more complex matrix to separate the contaminants from, often there are more contaminants present, and usually a smaller analytical sample is used. There is also often a wider range of contaminant concentrations to deal with. For these reasons, PQLs for soils are even more subject to variation than for groundwater.
The Model Toxics Control Act (MTCA) defines Practical Quantitation Limits:
“…the
lowest concentration that can be reliably measured within specified limits
of
precision, accuracy, representativeness, completeness, and comparability
during
routine laboratory operating conditions, using department approved methods.”
(WAC 173-340-200 Definitions).
Or more simply, the minimum level of a substance for which the question of how much of that substance is present, can be answered with a high degree of certainty. PQLs often are determined by evaluating performance results of inter-laboratories studies where artificial samples are analyzed to test each laboratory’s ability to accurately measure a substance using a specific method.
Practical quantitation limits are expected to provide a lower bound on the technical feasibility of cleanup levels. Important factors that influence the quantitation limits include sample size, analytical method, instrument limits, and the analytical uncertainties in the sample matrix. Unfortunately, inner-laboratory studies cannot duplicate every matrix, especially those most difficult to analyze.
Ecology has put a threshold on the PQL in WAC 173-340-707 (2) Analytical considerations. The PQL must be the more stringent of the following conditions:
(a) The PQL may be no greater than ten times the method detection limit; or
(b) The PQL for a particular hazardous substance, medium, and analytical procedure may be no greater than the PQL established by the United States Environmental Protection Agency and used to establish requirements in 40 CFR 136, 40 CFR 141 through 143, or 40 CFR 260 through 270.
PQLs As Cleanup Levels
·
Method A may use PQLs as the compliance levels. See WAC 173-340-704 (2) (d) Use
of Method A.
· Method B or C may use PQLs as compliance levels for substances when the risk-based
cleanup standard is below the PQL. See WAC 173-340-700 (6) Natural background
and analytical considerations; WAC 173-340-707 (2) Analytical considerations;
and Part I, Technical Information Memo No. 3—PQLs As
Cleanup Standards for further discussion.
Survey Of Analytical
Laboratories
A survey of analytical laboratories was conducted by the Department of Ecology in March of 1992. The purpose of the survey was to assess the performance capabilities of analytical laboratories in support of investigations under MTCA.
The survey data was used, in part, to develop the tables in Part III: MDL, PQL, and Comparisons Tables.
The purpose of the survey was to identify MDLs and PQLs that
could be achieved by commercial laboratories on a regular basis. Laboratories have not been identified because
the individual responses were considered confidential. Ecology does not recommend any specific
laboratory. Someone requesting the
services of a laboratory should ascertain the qualifications and ability of the
laboratory to perform the desired work.
These tables should help provide a comparison for the MDLs or PQLs the
laboratory may provide.