Biology Reference
In-Depth Information
headspace analysis of volatile organic compounds. The way the method works is that
a syringe is equipped with a retractable length of fiber that is coated with a bonded
phase such as polydimethylsiloxane (PDMS). When the sample is exposed to the
PDMS, chemicals within the sample will adsorb to the surface of the fiber. The fiber
is then retracted and reinserted into the injection port of a gas chromatograph, where
the chemicals are thermally desorbed onto the column (
Ballesteros-Gómez, 2011
).
Recently a version of this technique, space-resolved SPME, was used to determine
tissue-specific concentrations of atrazine as well as three pharmaceutical compounds
in live rainbow trout (
Zhang et al., 2010
). The use of this, or other, passive sampling
technique provides the advantage of avoiding loss of analytes during the cleanup and
enrichment procedures prior to instrumental analysis.
Sample Extraction, Cleanup, and Enrichment
Once the samples have been collected, the analytes of interest (e.g., the pesticide and/or
its metabolites) must be separated out from the matrix that was collected (e.g., the blood,
urine, tissue, etc.). In most case this is done by bringing a suitable solvent into intimate
contact with the sample, generally in a ratio of 5 to 25 volumes of solvent to 1 volume
of sample or, more commonly, through the use of a solid-phase extraction (SPE).
The use of an electric blender is a common method of extraction of biological
materials. The weighed sample is placed in a container, solvent is added, and the tissue
is homogenized by motor-driven blades. Blending for 5 to 15 min followed by a repeat
blending will extract most pesticide residues. A homogenate in an organic solvent can
be filtered through anhydrous sodium sulfate to remove water that might interfere
with the quantification phase of the analysis. The use of sonication is another method
for extracting tissue samples, particularly when the binding of toxicants to subcellular
fractions is of interest. Sonicator probes rupture cells rapidly, thus allowing the solvent
to come into intimate contact with all cell components. In solid-phase extraction, liq-
uid samples are filtered through a cartridge or filter disc made of material such as C-18
or Oasis HLB™ (hydrophilic-lipophilic balance). Analytes of interest are retained on
the SPE filter and can be collected by eluting with various solvents or solvent mix-
tures. For example, the Oasis HLB™ can be used for the extraction of 29 currently
used pesticides in human serum or plasma (
Martínez Vidal and Garrido Frenich, 2006
).
Using one of the extraction techniques described above will remove the analytes of
interest from the bulk medium, but will also extract other matrix constituents (waxes,
lipids, inorganic components, etc.). These interfering compounds must be removed
prior to analysis, and there are various methods available to separate the desired com-
ponents from the matrix interferences. A little over 100 years ago, a Russian botanist
by the name of M.S. Tsweet published a paper describing a new method for separat-
ing out plant pigments by percolating a plant extract through a column of CaCO
3
Search WWH ::
Custom Search