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The basic apparatus for gas chromatography consists of a carrier-gas supply and flow controller; a sample inlet system through which the sample can be introduced to the chromatography columns; the chromatographic column with its associated column oven; a detector system; and a recorder. The carrier gas is most commonly nitrogen or helium, but carbon dioxide argon, xenon and hydrogen are sometimes used. Use of a carrier gas of higher molecular weight will improve column efficiency. For this reason, nitrogen, or a gas with an even higher molecular weight is better than helium if a detector other than a thermal conductivity detector is to be used.
A rotometer is sometimes used in the carrier-gas system to give an indication of the gas flow rate. It consists of a graduated tube with slowly increasing inside diameter, and a glass or metal ball that is suspended in the gas flow within the tube at a height dependent on the flow rate. The position of the ball is a function of both the flow rate and the column back pressure, and when positioned at the column inlet, the rotometer can be used to give a rough approximation of the flow rate. A soap
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