An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 266

removed and introduced into the GC's carrier gas stream and analyzed [10].
The last general form of extraction is known as solid phase extraction (SPE). The use
of this form of extraction is rapidly increasing because of the smaller volumes of solvent
used in the extraction and the concurrent ease of sample concentration due to the smaller
original volumes involved. It allows for high selectivity in the components extracted and
retained and on the elution solvents, which are used to remove the component of interest.
A liquid sample is passed through a short disposable column containing an absorbent
material using either pressure to push or vacuum to pull the liquid through the column
(Figure 10.5). By choosing suitable column material and solvent, the selective retention
and subsequent elution of analytes of interest can be obtained, providing both sample
cleanup and concentration in one or two steps. This extraction method is only usable for
liquid samples (usually water) that have little or no particulate matter because suspended
solids will clog the extraction column before the entire sample has passed through.
FIGURE 10.5 An extraction column used for purification of analyte before
analysis.
Any material accidentally added to a sample during sampling, transportation, or storage,
or any error in sampling would also be “concentrated” during the above procedures. Even
small, seemingly inconsequential contamination can be magnified 10 or 100 times during
concentration steps. When this happens it can have a pronounced detrimental effect on
the analytical results and may produce entirely erroneous results, resulting in dramatically
increased costs of analysis and remediation [11-13].
10.7.5. Chromatographic Separation
There are a myriad of chromatographic methods in common use, all of which are
extremely powerful separation techniques and all of which operate on the same basic
principles. They have a stationary phase over which a mobile phase moves. When a
complex mixture of components is introduced into a chromatographic procedure, the
components are differentially partitioned between the two phases. Differential
partitioning results in separation of the components as they move under the influence of
the mobile phase. Once separated the individual pure components are identified and
quantified.
Two specific types, liquid chromatography (LC) and GC, are most commonly used in
environmental analysis. Liquid chromatography, also known as column chromatography,