Traps, Mistakes, and Errors - Field Sampling: Principles and Practices in Environmental Analysis - page 287

11.8. SAMPLE ANALYSIS MISMATCH
Two of the most common mistakes in the commercial laboratory are not considering
either all the aspects of the sample that is being analyzed or the basic question that is
being asked; that is, can the sample be tested for the analyte in question or will the
sample preparation or analysis introduce too many problems? An example would be
looking for VOCs in a solid sample that needs to be ground before sample extraction. The
extra sample preparation and handling required will result in VOC loss unless steps are
taken to prevent it. The sample may have to be handled under an inert gas, at a reduced
temperature, or by manipulation within a sealed environment. The resulting headspace as
well as the original sample then needs to be analyzed.
Another example of a sample and test mismatch would be the extraction of a plastic or
resin polymer sample with an organic solvent that could partially or fully dissolve the
underlying polymer base. A sample that contains plastic fragments may have some
plastic dissolve while it is being extracted using a large volume of methylene chloride,
and this may interfere with subsequent analysis. Some of the dissolved plastic will
precipitate when using solvent evaporation methods to concentrate the solution to the
final analytical volume. This may result in a sample being impossible to analyze because
either the whole sample solidified or part of the component of interest is dissolved in or
sorbed by the precipitated plastic.
11.9. EXTRACTION OR METHOD ERRORS
Analyzing a sample for a target analyte may require solvent extraction and subsequent
concentration. At this point in the analysis the concentrated solvent may be incompatible
with the instrument or detector stipulated in the original method. As an example, a water
sample analyzed for pesticides is extracted using diethyl ether and is then concentrated
and dissolved in methyl tertiary butyl ether (MTBE). This concentrated extract is then
analyzed using a gas chromatograph and an electron capture detector. For ether extracts
and solvents detector response is acceptable and there is no interference with target
compounds.
If on the other hand an equally common extracting solvent, methylene chloride, is used
in the extraction and subsequent concentration, an acceptable detector response will not
be obtained with this prescribed detector. The extraction and concentration steps will
proceed well and the final concentrated analytical sample will look good. Indeed, it may
even look like the MTBE extract described above. However, when the sample is injected
into the gas chromatograph the electron capture detector, which is not compatible with
this solvent, will become saturated with methylene chloride, requiring several hours of
cleaning before it can again be used for the desired analysis.