Gas Chromatography
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A Description
Gas chromatography is a method for separating substances in a mixture
and measuring the relative quantities of substances. It is a useful
technique for substances that do not decompose at high temperatures and
when a very small quantity of sample (micrograms) is available.
In this type of chromatography, a sample is rapidly heated and vaporized,
and then a stream of gas carries it along a column that contains a stationary phase.
The sample becomes distributed between the mobile gas phase and the stationary
phase. The higher a substance's affinity for the stationary phase,
the more slowly it comes off of the column. The substances are
detected as peaks on a chart recorder.
Preparing the Sample for Injection
Typically a microliter syringe is used to inject the sample into
the gas chromatograph. The syringe should be cleaned and rinsed
before use.
Video: Cleaning the syringe ( 3.52 M ) Text description
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True or False?
It is important to rinse the syringe with water immediately before
injecting sample into the gas chromatograph. Answer |
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Next, draw the sample and some air into the syringe.
Video: Drawing the sample ( 4.56 M ) Text description
Using the Gas Chromatograph and Chart Recorder
The column temperature of the gas chromatograph will be determined by your instructor. After the gas chromatograph has heated up, adjust the rate
of gas flow if necessary.
Video: Adjusting the rate of gas flow ( 4.45 M ) Text description
Turn on the chart recorder, and record its settings. Remove
the cap from the pen.
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| Turn on the chart recorder. | Remove the pen cap. |
Next, flip the "standby/record" switch to "record".
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| The "standby/record" switch. |
Finally, if the pen is not near the edge of the chart paper,
adjust the baseline position with the chart recorder's "zero" knob.
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| The "zero" knob. |
Next, inject the sample into the instrument and record the chromatogram. (Caution: The injection port may be HOT! The hot port will vaporize the sample, causing back pressure on the piston of the syringe. The increase in pressure may cause the piston to quickly and dangerously pop out of the syringe; therefore, keep your thumb or finger over the piston as you inject the sample.)
Video: Injecting the sample and recording the chromatogram ( 5.76 M ) Text description
If the chromatogram looks unusual
or no chromatogram is recorded, the settings on the chart recorder
may need to be adjusted. If the chart
recorder's settings are changed, the experiment should be repeated.
When the experiment is finished, turn off the chart recorder
and replace the pen cap.
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Explain what could be done to produce a better chromatogram
than the one shown below in the next experiment.
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Identify two different things that could be done in another experiment to produce
a better chromatogram than the one shown below.
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Interpreting Results
The retention time (tR) is a reflection of the substance's
affinity for the stationary phase. The longer the retention time,
the higher the substance's affinity for the stationary phase.
Substances with long retention times often give broad peaks in
the chromatogram.
The retention time is calculated according to the following equation:
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Imagine that the chromatogram shown below was printed on a
chart recorder that was operating at 1 cm/sec. Calculate the retention
times of the peaks.
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The sizes of the peaks also give useful information. The ratio
of the sizes of the peaks gives the ratio of the relative amounts
of substances in the sample. The sizes of the peaks may be found
by calculating the areas under the peaks
or by carefully cutting out the peaks and weighing them on an
accurate balance.
Copyright © 1995-1996 NT Curriculum Project, UW-Madison
