Introduction to Soil Chemistry

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gas chromatography 177
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Exit to
detector
Figure 9.2. Separation of a complex mixture by chromatography. The top segment shows the
column just after application of mixture. As the sample progresses through the column, separation
of mixture occurs as a result of the chromatography. The first component exits the column
and is detected, followed by the other two components.
introduced into a gas chromatograph, must either already be a gas or be
immediately turned into a gas on injection. A complete chromatogram is
usually obtained in less than 20min, although some analysis can take much
longer.
9.2.1. Sample Introduction
For gas chromatography a 1 to 0.1mL syringe, shown in Figure 9.3, which has
the entire sample contained in the needle, is frequently used for the introduction
of the sample into the injection port.The needle must be of the correct
type and length for the gas chromatograph being used, or poor results will be
obtained. Injection is effected through a septum made of special rubber;
Teflon-backed septa are also available and must be in good condition for accurate
chromatograms to be obtained. Injection ports without septa are also
available. When a new septum has been placed in the injection port, piercing
it with a syringe needle will be harder, and care must be taken not to bend the
needle or syringe plunger. When the needle is fully inserted, the plunger is
depressed and a recording of the detector output is started. This is the 0 time
used in calculating the retention times (R t, described below) of emerging
peaks. Needle insertion and depression of the plunger must be done in one
continuous motion to obtain a good chromatogram. GCs can be fitted with
automatic samplers that take and inject the sample automatically.