GCSE SCIENCE CHEMISTRY HIGH SCHOOL - Test for Compounds - What is Gas Chromatography - Column - Retention Time - Chromatograph

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What is Gas Chromatography?
Instrumental Methods - Identifying Compounds.

In gas chromatography a small sample of a mixture
of compounds is vapourised (turned into gas).
A carrier gas takes the sample through the column.
The carrier gas (often nitrogen)
must be inert and not react with the sample.

The column is packed with a solid material which
slows the sample down. Different compounds travel
through the column at different speeds and leave
the other end of the column at different times. The
amount of time a particular compound takes to pass
through the instrument is called its retention time.
The retention time can help identify the compound.

A recorder draws a graph called a gas chromatogram
which shows a peak for each compound.
The number of peaks show the number of compounds
present in the sample and the position of the peaks
show the retention time. See the example shown below.

Gas Chromatogram

In the gas chromatogram above, there are five peaks
showing that there are five compounds present. The
height of the peak shows how much of the compound
there is in the mixture. A more accurate way
of identifying the different compounds which leave the
column can be made by using a mass spectrometer.