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Gas chromatographs are really rather simple. The apparatus consists of a pressurized tank (cylinder) of carrier gas, a pressure regulator to control the flow rate of the gas through the chromatograph, a sample inlet, the column, a detector with associated electronics, some kind of interface to the outside world such as a recorder, and a flow meter to measure the flow rate of carrier gas. Chromatographs also provide heating for the column, the sample inlet, and the detector. The temperatures of these three components can usually be controlled independently.

A gas chromatograph uses a flow-through narrow tube known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas, mobile phase) at different rates depending on their various chemical and physical properties and their interaction with a specific column filling, called the stationary phase. As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, and the temperature.

The procedure involves vaporizing a sample and sweeping it through a column with a moving stream of gas termed the mobile phase or the carrier gas. The gases are commonly supplied by compressed gas cylinders. The sample is introduced into the injection port. The most common type of analysis involves the injection of 1 to 3 micro liters of a liquid sample into a heated inlet, either manually or by an automated injection device. The injection port is interfaced to the column where the actual separation takes place. In most cases capillary columns are used to obtain the best possible separation. The downside to capillary columns is that they have a limited "capacity". In other words not much sample can be separated at a time. Therefore a split injection port is often used to allow only a fraction of the injected volume of sample onto the capillary column. The capillary column's inner walls are coated with either a porous solid or a viscous liquid material. This inner coating will interact with different solute molecules to different extents. Those molecules which interact more strongly with the stationary phase spend on average a higher percentage of their time associated with the stationary phase than those solutes which do not interact strongly. Those compounds which do not often associate with the stationary phase pass more quickly through the column than those compounds which have strong interactions with the stationary phase, and a separation of the components in the mixture is achieved. Since the compounds have different mobilities, they exit the column at different times; i.e., they have different retention times, tR. The retention time is the time between injection and detection. There are numerous detectors which can be used in gas chromatography. It is a device that senses the presence of components different from the carrier gas (mobile phase) and converts that information to an electrical signal. For qualitative identification one must rely on matching retention times of known compounds with the retention times of components in the unknown mixture. It is important to remember that any changes in operating conditions will affect the retention time which will affect the accuracy of identification. Thus GC is most often used when one is performing a target compound analysis, where one has a good idea of the compounds present in a mixture so reference standards can be used for determining retention times. For a sample of largely unknown composition qualitative identification can be determined by gas chromatography-mass spectrometry. A mass spectrum of any or all peaks in the chromatogram is compared with spectra contained in spectral libraries on the system's computer.

GC truly tells you is at which relative time a component eluted from the column and that the detector was sensitive to it. To make results meaningful, analysts need to know which components at which concentrations are to be expected; and even then a small amount of a substance can hide itself behind a substance having both a higher concentration and the same relative elution time. Last but not least it is often needed to check the results of the sample against a GC analysis of a reference sample containing only the suspected substance.

The modern gas chromatograph is a fairly complex instrument mostly computer controlled. The samples are mechanically injected, the analytical results are automatically calculated and the results printed out, together with the pertinent operating conditions in a standard format.

The major advantage of gas chromatography is the very high resolving (i.e., separating) power provided by the capillary columns that are typically used. A broad selection of stationary phases are available that provide different retention characteristics giving us many choices for our particular analytical application.