Introduction
Gas chromatography is a powerful technique for separating the components of a gaseous nature. A valuable feature of the technique is that after the separation it is usually possible to measure quantitatively the amounts of the components present. An instrument which does this is called an analytical chromatograph. In general, only very small amounts of the sample are required for an analysis which is a further important property of the technique.
Gas chromatography - specifically gas-liquid chromatography - involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.
Have a look at this schematic diagram of a gas chromatograph:
Instrumental components
Carrier gas
The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.
Sample injection port
For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapor - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro syringe is used to inject sample through a rubber septum into a flash vaporizer port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a micro liter up to 20 micro liters. Capillary columns, on the other hand, need much less sample, typically around 10-3 μL. For capillary GC, split/split less injection is used. Have a look at this diagram of a split/split less injector sample. For packed columns, sample size ranges from tenths of a micro liter up to 20 micro liters. Capillary columns, on the other hand, need much less sample, typically around 10-3 μL. For capillary GC, split/split less injection is used. Have a look at this diagram of a split/split less injector
