Gas
Chromatography
Experiment
Gas Chromatography
- Gas Chromatography (GC) is a common technique used to separate and
identify volatile organic compounds (analytes). As the gas moves the
analyte across the stationary phase, the analyte will be in equilibrium with
the gas and the liquid phase (usually suspended on a solid surface).
- This method depends upon the solubility and the boiling point
of the volatile organic liquid in order to separate them from a mixture.
- It’s both a qualitative (identity) and quantitative (how much) tool.
- The mobile phase is an inert gas. Commonly used gases include N2, He,
Ar, and CO2, depended on the type of detector.
- The stationary phase is a high-boiling liquid film supported on an inert
solid and packed in either a fused silica (12 to 30 meter) capillary column
or in a copper or stainless steel (1.5 to 3.0 meter) metal column.
GC Instrument
Schematic diagram for gas chromatograph
Injection Port
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Sample sizes for standard GC procedures typically involve 0.1 to 10.0
microliters of analyte solution injected into a heated sample port.
The needle must be inserted carefully to avoid bending or breaking.
Stationary Phase
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The stationary phase can either be polar (polyethylene glycol - PEG) or
non-polar (polydimethylsiloxane - PDMS).
For capillary columns, the diameter is typically less than 1 mm, so samples
sizes are small. For product separations (not just composition analysis),
larger columns can be used in order to isolate milligrams of product.
GC Chromatogram
The chromatogram shows:
1. the order of elution (order of components coming off
the column) related to boiling points and polarities of
the substances in the mixture,
2. the retention time (time of elution), and
3. the relative amount of the components in the mixture.
Experimental Procedure
1.
Instead of the acetates listed in the lab manual, we will
work with a series of alkanes. The gas chromatogram
for these alkanes has already been done.
2. The TA provides only 1-2 drops of the GC unknown.
Record the code in your lab notebook.
3.
Perform just one run of the unknown.
4. Compare your unknown chromatogram to the posted
chromatogram of the mixture containing all “knowns”.