Gas chromatography

In gas chromatography (GC), the mobile phase is an inert gas (eg helium).

The stationary phase is a very thin layer of an inert liquid on an inert solid support - such as beads of silica packed into a long thin tube (this flexible tube is coiled many times inside a thermostatically-controlled oven to keep it at a constant temperature).

GC is used to separate complex mixtures. It is much better at this than thin-layer or paper chromatography.

This is because it is more sensitive - allowing the determination not only of what chemicals are in the mixture, but also how much of each chemical there is.

Gas chromatography set up. A thermostatic oven contains a coiled colum

The mixture to be analysed is injected into the stream of carrier gas. As it passes along the column (long thin tube) it separates into the different substances.

Substances with a greater affinity (attraction) for the mobile phase reach the detector at the end of the column more quickly. Substances with a greater affinity for the stationary phase move more slowly through the column.

Gas chromatography can be used to detect banned substances in urine samples from athletes, or by forensic investigators to detect the presence of fuels that may have been used to deliberately start fires.

A gas chromatogram might show the time along the x-axis and the strength of response along the y-axis.

The amount of time that a substance takes to pass through the column is called it retention time. The retention time of an unknown substance can be compared with standard reference data to help to identify it.

Three main pieces of information can be gathered from a gas chromatogram:

  • the number of compounds in the mixture - represented by the number of peaks
  • how much of each compound is present - represented by the height of the peak (higher = more)
  • the retention time - indicated by the position of the peak

A graph of time in minutes against strength of response for a gas chromatogram

This gas chromatogram shows that:

  • substance A was present in the smallest quantity (it has the smallest peak)
  • substance A had the shortest retention time
  • substances B and C were present in equal amounts
  • substance F had the longest retention time
  • substance F was present in the greatest quantity (it has the largest peak)
  • substance F had the greatest affinity for the stationary phase