Gas Chromatography & Interpreting Chromatograms (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about gas chromatography and interpreting chromatograms.

A chromatogram identifies the components of a mixture by comparing a measured value against known standards, and what that value depends on is the same in every technique: the attraction between the component and the stationary phase, and its solubility in the mobile phase.

Gas-liquid chromatography separates a mixture and records the result as a chromatogram. Each component has a retention time, and those retention times are compared with data book values to identify unknown molecules. On a TLC plate the equivalent measurement is the Rf value, compared with standards in the same way. Both measurements depend on how strongly a component is held by the stationary phase and how readily it moves with the mobile phase.

We'll take how GLC separates a sample first, then retention times and what a gas chromatogram shows, and finally how Rf values are interpreted.

Gas-liquid chromatography is used for analysing gases, volatile liquids, and solids in their vapour form. The stationary phase is a long coiled column, and it is normally a non-volatile liquid. The mobile phase is an inert carrier gas, such as helium or nitrogen, which moves the sample molecules through the stationary phase. The sample is injected into the column through a self-sealing disc, and the vapour that forms is carried through by that carrier gas. How long each component takes to get through is set by how it interacts with those two phases.

Once sample molecules reach the detector their retention times are recorded. A retention time is the time taken for a component to travel through the column, and it depends on the attraction between the component and the stationary and mobile phases, as well as the volatility and nature of the component.

The retention times are recorded on a chromatogram, where each peak represents a volatile compound in the analysed sample. The relative sizes of the peaks, meaning their areas, are related to how much of each compound is present in the mixture. Retention times are then compared with data book values to identify unknown molecules, which is the comparison against standards that makes the chromatogram useful.

On a TLC plate, the less polar components travel further up the plate, so their Rf values are higher than those of components closer to the baseline. They are more soluble in the mobile phase and get carried forwards with the solvent. More polar components do not travel far up the plate, because they are more attracted to the polar stationary phase.

So the extent to which the molecules in a sample separate depends on their solubility in the mobile and stationary phases, exactly as a retention time does in gas chromatography. Knowing the Rf values of the compounds being analysed helps to compare the polarity of the various molecules.

Gas-liquid chromatography carries a vaporised sample through a coiled column using an inert carrier gas. Each component's retention time is the time it takes to cross that column, and comparing it with data book values identifies the compound, while the area of its peak shows how much is present. On a TLC plate the Rf value does the equivalent job, and it also tells you how polar a component is, because less polar components travel further. Both measurements are read the same way: compare what you measured against a known standard.

On Save My Exams, you'll find exam specific revision, made by trusted examiners. Study with revision notes, practice with exam questions, and improve your grade by targeting your weak spots. Visit the links in the description.