SUGGESTIONS FOR REDUCTION
OF ANALYTICAL COSTS BY
ELIMINATION OF UNNECESSARY
by Douglas M. Chatham
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
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
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)
LIST OF REFERENCES
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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