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  • Gas chromatography (GC), is a method of separating mixtures and is particularly suited to mixtures

  • that are fairly volatile liquid. As in all chromatographic methods, there is

  • a mobile phase, in this case a gas, that carries the compaonents of the mixture over a stationary

  • phase. In this case, the stationary phase is a tube,

  • called a column, packed with solid or coated with a high boiling point liquid.

  • The components of the liquid leave the column in order of volatility; the most volatile

  • first. This is the complete instrument, with a computer

  • control system on the right. The sample is injected here. From this injection

  • port the sample passes into the column which is kept in a temperature-controlled oven.

  • Columns are normally wound into a spiral to save space.

  • This capilliary column is 30 metres long. This is much shorter: about 1 metre.

  • The properties of the column and it's filling are chosen for the particular separation that

  • is to be carried out. The components of the mixture are carried

  • through the column by a stream of inert helium gas, the mobile phase.

  • The more volatile the component, and the less it interacts with the stationary phase, the

  • faster it travels through the column. At the other end of the column is a detector

  • that detects each component of the mixture as it comes out of the column and also measures

  • it's amount. This instrument has a flame ionisation detector

  • which consist of a hydrogen flame burning in air.

  • As a substance leaves the column it burns in this flame producing ions which can be

  • detected by measuring the electrical conductivity of the flame.

  • The hydrogen for the flame comes from this cylinder here.

  • Before beginning a separation, the operator must set the flow rate of the gases and the

  • temperature of the oven. The temperature of the inlet port is also

  • set at a level that ensures the sample is fully vaporised. The flame ionisation detector

  • must be lit. Here, we will separate a mixture of methanol

  • and methile benzine. About 0.1 microlitres is taken up in a hypodermic

  • syringe. It is then injected into the inlet port through

  • a self-sealing rubber disc, called a septum. The first peak is methanol, the more volatile

  • component. It's retention time, that is the time taken for it to pass through the column,

  • is about 1 minute. The second peak, with a retention time opf

  • about 1.5 minutes, is the less volatile methile benzine.

  • It's peak has a larger area, showing there is more of it in the mixture.

  • The computer calculates accurate retention times and peak areas.

  • The area under each peak is proportional to the amount of each component. The computer

  • calculates this. This instrument is used for undergraduate

  • practical sessions and samples are run singly as and when each student is ready.

  • In this, alternative instrument, the gas chromatograph is on the left and the detector is a mass

  • spectrometer which runs the mass spectrum of each component as it comes out of the column.

  • This can be particularly useful when analysing an unknown mixture as the mass spectrum can

  • help to identify each component. The combined technique is called gas chromatography

  • mass spectrometry (GCMS). This GCMS instrument has an autochanger for

  • the samples. Many samples can be loaded up and run automatically,

  • perhaps overnight.

Gas chromatography (GC), is a method of separating mixtures and is particularly suited to mixtures

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