Environmental Engineering Reference
In-Depth Information
concentration in a common container. Theoretically 50-75% of the sample contaminant
will be extracted by the first aliquot. Lighter or less complex sample matrixes might have
an extraction efficiency of upwards of 90% in the first fraction, while heavy molecular
weight compounds or complex matrixes or soils may have a far lower efficiency. This
will be indicated by recovery of the extraction surrogate. The standardized use of three
solvent aliquots with a fixed solvent contact time of 1 or 2 min acts as a point of
standardization for the extraction process, hopefully ensuring that if you have x amount
of a compound in your sample you will have extracted it all. If 100% is not extracted
because of matrix interference you will be aware of the decreased extraction efficiency
by the monitoring surrogate extraction.
10.8.2.
Methods for Extracting Solid Samples
Solid samples are extracted by adding an extractant, then mixing and either decanting the
extractant or filtering to separate solids from the extractant.
10.8.2.1. Methods for Solid Samples
Individual samples are taken from the field using a predetermined pattern and extracted.
The extracts are then analyzed. The handling of these solid samples will be illustrated
using soil as an example.
A soil sample is characterized and mixed to assure a uniform sample from the sample
container. A subsample is weighed and the original weight being extracted is recorded in
the laboratory logbook. A portion of the sample must be analyzed for its percentage of
solids because this factor must be applied to the resultant data for the soil sample to yield
the field's true component concentration. Extraction surrogates are added and the sample
is mixed to ensure even distribution. Mixing with a drying agent, such as sodium sulfate,
dries the sample.
An aliquot of the extraction solution is added and the sample is mixed for a fixed time,
either by stirring or through the use of an ultrasonic extraction device. Solvent is poured
off through a piece of filter paper into a flask and the sample is re-extracted twice more
for a total of three solvent aliquots. The extract can be passed through a cleanup phase;
that is, a fluorasile cleanup column (see Figure 10.5) that has been prerinsed with the
solvent used for the extraction. This or the original solvent extract is reduced in volume
to 1 or 2 ml and analyzed.
10.9.
DATA ASSESSMENT
The data produced using one of the analytical procedures described above is what is
termed “raw” data and must be related to some standard or reference to determine the
relationship of the analytical response to an actual value. This is usually done by
comparing the raw response to the response obtained from the instrument or method
when a series of known standards is analyzed. The instrument responses from these
standards are used to prepare a response or calibration curve for the instrument that
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