6.3.1 (b) GLC interpretation

(b) interpretation of gas chromatograms in terms of:

(i) retention times

(ii) the amounts and proportions of the components in a mixture.

{To include creation and use of external calibration curves to confirm concentrations of components.}

{Peak integration values will be supplied.}

Traditional Column Chromatography relied on gravity to pull substances through a column packed with an inert powder - the stationary phase (didn't move)

A solvent (or eluent) was generally used to carry the substances through the powder.

This was the mobile phase because it moved.

Some substances adsorbed onto the powder - generally substances that dissolved less well in the eluent.

They moved slowly.

More soluble substances would travel through the column at almost the same speed as the eluent because they rarely interacted with the powder to be slowed down.

The equivalent of an Rf value for column chromatography was Retention Time - the time it took the substances to emerge from the column although this was controlled by a number of factors as well as the type of substance eg the type of powder, length of tube, the eluent, even the temperature.

Gas-Liquid Chromatography (GLC) works in a similar way but is easier to understand if you have a working knowledge of column chromatography even though Column chromatography isn't on the syllabus.

In GLC the column is actually a thin, curved tube and is much longer.

Gravity would not take substances through the tube so an inert "carrier gas" (often Nitrogen) is used and so this is the mobile phase.

The make-up of the tube itself isn't that important because the stationary phase is a liquid with a very high boiling point such as a long-chain hydrocarbon which is simply adsorbed onto the tube's surface.

As in all forms of chromatography, some components of the mixture will interact with the stationary phase more than others.

In this case the more soluble the component s in the hydrocarbon the slower it will travel through the tube.

Slow moving components will have long retention times (time from injection to detection).

So components can again be identified by retention time providing that there are not many components with very similar RTs.

A typical read out from the detector may look like below:

Retention time would allow you to identify the compounds in the sample.

The peak area (shown usually as (Peak Integration or just with a percentage above) allows you to determine the concentration of the component in that peak.

To do so you would first have to calibrate the machine - using known concentrations of the substances in the sample to compare against.

Here known Concentrations have been plotted by Peak Integration.

The red line represents the Peak Integration of the same substance in a mixture, allowing the user to read off its concentration from the x-axis