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by Douglas M. Chatham
Senior Chemist


The matrix spiking program should be eliminated from EPA protocols except for projects which can justify the expense of spiking every sample. Surrogate recoveries are the best measure of accuracy on a sample-by-sample basis and laboratory control sample/laboratory control sample duplicate results (LCS/LCSD) for each batch and the laboratory control charts are the best measure of laboratory accuracy and precision. Elimination of MS/MSD samples could reduce analytical costs by 10%. For a project with analytical costs of $50,000, this represents a savings of $5,000.


Use surrogate recoveries to measure matrix effects for organic analyses.
Use Laboratory Control Spikes/Duplicates (LCS/LCSD) rather than MS/MSDs for determining precision and accuracy.
Use control charts for warning and control limits on precision and accuracy.
Avoid MS/MSD for metal analyses; metal analyses do not generally require a measure of matrix effects since the digestion and analytical methods destroy the matrix.


Matrix spike (MS) samples are analyzed to determine the effect of the sample matrix on the accuracy of the analytical results. The spike is the addition of a known amount of analyte to a normal sample in the lab. Matrix spike duplicates (MSD) are the second of a pair of lab matrix spike samples, and are analyzed to check the precision of analytical procedures. In order to evaluate the effect of the sample matrix on analytical data, triplicate volume is collected for one sample out of every group of 20. Two portions of the sample (the MS and MSD) are spiked with a standard solution. These spiked samples are analyzed, and the percent recovery and relative percent difference are calculated.
It has been estimated that up to 90 percent of all environmental measurement variability can be attributed to the sampling process.(1) The matrix spiking protocol assumes that one sample out of a batch of twenty is adequate to assess the effect of the matrix on accuracy and precision. Much of the variability of the sampling process is due to the variability of environmental media and the contaminants within that media; likewise, the matrix effect is as variable as each medium and its contaminants. To be effective in defining method accuracy and precision, matrix spiking would have to be done for all samples, which would be prohibitively expensive.
Soils can vary in their sand, clay, humus, and water content; the mineral, electrolyte, and acidity/alkalinity; the amount and types of naturally occurring organic matter present; and the amount and types of contaminants present. All of these factors can vary dramatically laterally and vertically (soil at the surface is different from soil at six inches deep) and can have an effect on the extractability of contaminants. Even with a batch of samples with similar sand, clay, humus, water, and electrolyte composition, variations in the total amount of contaminants can affect recoveries of specific contaminants, e.g., variations in the total amount of petroleum compounds can have a dramatic effect on recoveries of benzene, toluene, ethylbenzene, and xylenes (BTEX). The presence of contaminants produces a new and different matrix.

Data validation guidelines (2) include the following steps if MS/MSD results do not meet criteria:

1. No action is taken on MS/MSD data alone. (Decisions based on MS/MSD data must be supported by other types of QC, such as surrogate recoveries, which can stand alone.)

2. The data reviewer should first try to determine to what extent the results of the MS/MSD effect the associated data.

3. In those instances where it can be determined that the results of the MS/MSD effect only the sample spiked, then qualification should be limited to this sample alone. However, it may be determined through the MS/MSD results that a laboratory is having a systematic problem in the analysis of one or more analytes, which affects all associated samples.

Since data validation based on MS/MSD results is applied only to the sample spiked, the QA/QC value of MS/MSD samples is much lower than the value of surrogate recoveries and of laboratory control sample/laboratory control sample duplicate results (LCS/LCSD) or blank spike/blank spike duplicate (BS/BSD) results. Surrogates are added to every sample analyzed for organics and are the best measure of accuracy and matrix effects for an individual sample. LCS/LCSD results for each batch and the laboratory control charts are the best measure of laboratory accuracy and precision for organic analyses. The LCS/LCSD program is also the best measure of accuracy and precision for metals analyses. The digestion procedure for metals virtually destroys the matrix so that the only interferences normally encountered in ICP and atomic absorption methods are from high concentrations of other metals.
The matrix spike sample is analyzed to determine accuracy; the matrix spike duplicate sample is included as a measure of precision.

";When designing experiments or procedures, it is important to keep in mind that the overall objective is accuracy. It naturally follows that those in charge of a project should ask whether additional measurements really contribute to the accuracy of a method, or simply to its precision.
In today's business world cost is very important, and each extra measurement adds to the cost of a project. We all know that precision is important, but we need to take a closer look at the costs and benefits to the customer when expenses are increased for the sake of improving precision without necessarily increasing accuracy.";(3)


Homsher, M.T., Haeberer, Fred, Marsden, Paul J., Mitchum, R.K., Neptune, Dean, and Warren, John, Performance Based Criteria, A Panel Discussion, Environmental Lab, October/November 1991.

USEPA Contract Laboratory Program, National Functional Guidelines for Organic Data Review, June, 1991.
Phifer, Lyle H., Accuracy Versus Precision, Environmental Testing & Analysis, March/April 1995.

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