Environmental Engineering Reference
In-Depth Information
10.8.1. Methods for Liquid Samples
Water or liquid samples are measured by volume, the pH is adjusted, and an extraction
reference is added. (This extraction reference is traditionally called a surrogate, and is a
known compound or group of compounds with similar characteristics as the target
compounds.) This way there is a measure of extraction efficiency for the sample; that is,
if X amount of a surrogate with the same or similar characteristics to your target analyte
is added to the sample and after extraction the analysis of the sample extract shows a 90%
extraction efficiency it could be assumed that there was a 90% recovery of the target
compound. Unfortunately this does not hold to true for many compounds. Usually
extraction efficiency should not be used to correct the analytical results for extraction
efficiency; rather, the surrogate recoveries are used in comparison to the method-
determined limits to validate it. In the case of an invalid testing event the sample may
require reanalysis to confirm matrix or method interference. If enough of the original
sample remains, the entire sample preparation and analysis process may be repeated.
10.8.1.1. Extracting Liquid Samples
Liquid samples are extracted in one of the following ways, depending on whether they
are aqueous or nonaqueous. (Extraction methods are summarized in Table 10.3.)
Aqueous Samples:
Classic —Liquid/liquid or solid/liquid (i.e., liquid sample matrix/ organic
solvent or solid matrix/organic solvent).
Solid phase —A liquid sample is passed through an absorbent that retains the
analyte, which is flushed from the column for analysis.
Supercritical fluid —A sample is extracted by the use of a gas that has been
compressed to its liquid phase. This is usually carried out using liquid carbon
dioxide (CO 2 ) [10].
Nonaqueous Liquid Samples:
Small quantities of oil or some other nonwater miscible matrix or a solid—
extracted with either water or an organic solvent.
Large quantities of oils or nonwater immiscible matrixes are usually
separated from any water phase either by decanting or by using a drying agent
and then extracting the remaining material, usually with either water or an
organic solvent.
10.8.1.2. Multiple Extractions
Multiple sample extractions can be carried out in two ways. The first is the extraction of
the sample with multiple aliquots of solvent, usually three times. For example, three 50-
ml aliquots of methylene chloride are added to 1 liter of water sample, each aliquot is
shaken for 2 min, is allowed to separate, and is collected combined with other extracts for
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