Biology Reference
In-Depth Information
retained, dried, and weighed. Thus, analytical results from the sample can be reported
on a lipid weight basis where appropriate.
Enrichment
Finally the samples are concentrated to a very small volume of solvent, e.g., 250 µl, and
appropriate standards are added for quality assurance and quality control purposes.
Analysis
Once the samples have been extracted, cleaned up, and concentrated, the next step is to
analyze the extract using a variety of techniques, depending on the analytes of interest.
Gas Chromatography
GC is used most commonly for the separation and quantification of thermally stable
pesticides such as organochlorines or pyrethroids. This system consists of an injector
port, oven, detector, amplifier (electrometer), and supporting electronics. Gas chromato-
graphs use a capillary column to effect separation of complex mixtures of organic
molecules. The stationary phase is coated onto the inside of the capillary column. The
mobile phase in this system is an inert gas (called the carrier gas), usually helium or
nitrogen, that passes through the column. This technique is actually “gas-liquid chro-
matography,” deriving from the fact that the polymer coating that acts as the stationary
phase is technically a liquid.
Briefly, a sample is injected into a port that is at a temperature sufficient to vapor-
ize the sample components (generally ≈300°C). Based on the solubility and volatility
of these components with respect to the stationary phase, the components separate
and are swept through the column by the carrier gas to a detector, which responds to
the concentration of each component. The column is contained within an oven and the
temperature within the oven can be programmed by the analyst. Similar to the way the
solvent systems can be changed in column chromatography, the temperature program
can be altered to maximize the analyte separation while minimizing the run time per
sample. The electronic signal produced as the component passes through the detector
is amplified by the electrometer, and the resulting signal is sent to a computer, or other
electronic data-collecting device, for quantification. The time at which a specific com-
pound exits the column for a given set of conditions within the instrument is called
the retention time. Standard mixtures are run under the given conditions to determine
the retention time for each analyte of interest. This is then compared with the reten-
tion times of peaks in the unknown samples.
Increased sensitivity and component resolution have resulted from advances in
solid-state electronics and column and detector technologies. In the field of column
technology, the capillary column has revolutionized pesticide detection in complex
samples. This column generally is made of fused silica, 5 to 60 m in length, with a very
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