Upper Midwest Environmental Sciences Center

Sediment-Contaminant Database

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Quality-Assurance Index

This qualitative index reflects the quality-assurance documentation during analyses of all contaminants within a specific data set in the database. The scale of the quality-assurance index ranges from 1 to 5, and reflects the relative completeness of quality-assurance documentation. An index of "1" indicates the most comprehensive documentation, and an index of "5" indicates the minimum level of documentation required for inclusion of a data set into the database. The five levels of the quality-assurance documentation are defined below.

1. The accuracy of analyses were characterized with estimates of precision (replicate samples) and bias (both spiked samples and reference materials). The detection limits were provided for all or most contaminants that were quantified.
   
   
2. The accuracy of analyses were characterized with an estimate of precision (replicate samples) and bias (either spiked samples or reference materials). The detection limits were provided for some contaminants that were quantified.
   
   
3. The accuracy of analyses were characterized with an estimate of precision (replicate samples) and bias (either spiked samples or reference materials). The detection limits were not available for any contaminants that were quantified.
   
   
4. The reliability of analyses were characterized with an estimate of bias (either spiked samples or reference materials). The detection limits were not available for any contaminants that were quantified.
   
   
5. The reliability of analyses were characterized with an estimate of precision (replicate samples).

Definitions pertaining to the Quality-Assurance Index

The following definitions were derived largely from Sections 1010, 1020, and 1030 in Standard Methods (American Public Health Association, American Water Works Association, and Water Environment Federation. 1992. Standard methods for the examination of water and wastewater, 18th edition. American Public Health Association, Washington, D.C.), which can be consulted for more detailed information.

Accuracy--A combination of bias and precision of an analytical procedure, which reflects the closeness of a measured value to a true value.

Bias--A consistent deviation of measured values from the true value, caused by systematic errors in a procedure. Bias is assessed by measuring the recovery of known additions (spiked samples) and the recovery of internal standards and laboratory control standards.

Detection limits--The common term that encompasses various analytical detection limits. Some of the common detection limits (in increasing order of concentration detected) include the instrument detection limit (IDL), the lower limit of detection (LLD), the method detection limit (MDL), and the limit of quantitation (LOQ). The LLD, for example, is the contaminant concentration that produces a signal sufficiently large that 99% of the trials with that amount will produce a detectable signal. The MDL differs from the LLD in that samples containing the contaminant of interest are processed through the complete analytical method. The relation among these limits is about IDL:LLD:MDL:LOQ = 1:2:4:10. Most of the studies in this database reported one or more of the detection limits. For purposes of assigning a quality assurance index value for a specific data set, any of these detection limits was considered acceptable.

Internal standard--A pure compound added to a sample extract just before instrumental analysis to permit correction for inefficiencies.

Laboratory control standard--A standard, usually certified by an outside agency, used to measure the bias in a procedure. Examples include the National Institute of Standards and Technology (NIST) Standard Reference Materials and the National Research Council of Canada (NRCC) reference materials.

Precision--A measure of the degree of agreement among replicate analyses of a sample (e.g., standard deviation, percent difference, or percent relative standard deviation).

Replicate--A repeated operation occurring within an analytical procedure. Two or more analyses for the same constituent in an extract of a single sample constitute replicate extract analyses.

Spiked samples--Also termed the recovery of known additions, a pure compound added to a sample in the laboratory so that the overall efficiency of a method can be determined. Spiked samples are used to assess the bias and verify the absence of matrix effects.

Content manager: Michelle Bartsch