Gas chromatography

Gas chromatography is a type of chromatography. The sample that is going to be tested is first turned into a gas, and then carried through a column by a nonreactive 'carrier' gas such as helium or other inert gas such as nitrogen. As the sample is carried through the column it is separated into its individual components. To achieve this the column is housed inside an oven, where the temperature is controlled to allow the individual components to exit the column at different times.

Instrument

Carrier gas

In selecting a carrier gas it is important to select a gas that will not react with the components of the sample. The carrier gas should also be able to withstand high temperatures due to the oven. The flow rate of the carrier gas is important in that if it is too high there is not enough time for the sample to interact with the column and no separation will be seen, if it is too slow the experiment may take a very long time.

Column

The main type of columns used in gas chromatography is a capillary column. Packed columns are created of fused silica or stainless steel. To increase separation of a gas sample columns must be created of a large length and are coiled to fit inside of the oven. Capillary columns are separated into two categories; wall coated open tubular and support coated open tubular. WCOT are capillary tubes with a thin layer of stationary phase, and in SCOT, the tube is lined with a thin film of support material. As the sample moves through the column the individual parts of the sample become contained with the material inside of the column and are then released. This action allows for a sample to become separated.

Oven

It is important that the oven is capable of maintaining a constant temperature. Since movement of the sample through the column relies on the boiling point of the sample being analyzed, the oven should be set to a temperature that is slightly higher than its boiling point. For samples with a large boiling range, a temperature program can be used, in which the column temperature is raised. By using a high temperature, the sample moves through the column faster, while at lower temperatures the sample moves slower, but better resolution is achieved.

Detector

Many types of detectors are used in gas chromatographic separations, with the most common are flame ionization, thermal conductivity, and mass spectrometry detectors. In a flame ionization detectors, the separated sample from the column is directed into a flame. By creating a voltage near the burner tip and the detector, the ions that are produced from the flame travel towards the detector. Flame detectors are not capable of detecting H₂O, CO₂, SO₂, and CO.

With a thermal conductivity detector, the sample from the column is passed into an area that is electrically heated. The thermal conductivity of the column is reduced when the sample passes over by. When this occurs the detector heats up and measures the change in resistance. The thermal conductivity detector is capable of detecting all types of compounds.

A mass spectrometer measures the mass to charge ratio of fragmented ions of a sample. The output of a column can be feed directly into the ionization chamber of the mass spectrometer. A mass spectrometry detector is capable of being able to obtain information from incompletely separated components. Two types of mass spectrometer detectors used are quadrupole and time of flight mass analyzers. In a quadrupole detector, a voltage is produced which makes ions of a specific mass to travel to the detector. In a time of flight mass analyzer, the speed of an ion is measured allowing the mass to charge ratio to be known.

Applications

Gas chromatography is regularly used to describe what is inside a complex sample. The information obtained from gas chromatography is placed into a graph of detector response versus the time the sample leaves the column. If the separate parts of a complex sample come out at different times far apart from one another it is possible to determine what came out of the column.

If a sample is compared to a standard calibration it is possible to know how much of a separated part of the sample makes up the sample. This is useful in monitoring quality of a product such as medicine, beverage, or perfume.