Gas chromatography

Gas chromatography - particularly gas-liquid chromatography - involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed on the surface of an inert solid. The carrier gas should be chemically inert. Commonly used gases include nitrogen, argon, helium, and carbon dioxide. The choice of carrier gas is dependent upon the type of detector which is used. The carrier gas system has a molecular sieve to remove water and other impurities.

For optimum column efficiency, the sample should not be very 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 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 around 50°C higher than the B.P. of the least volatile component of the sample. For packed columns, sample size variss from tenths of a micro liter up to 20 micro liters. Capillary columns, on the other hand, require much less sample, typically around 10^-3 mL. For capillary GC, split less injection is used. The injector can be used in 1 of the 2 modes; split or split less. The injector contains a heated chamber containing a glass liner into which the sample is injected by the septum. The carrier gas enters in chamber and can leave by 3 routes .The sample vaporizes to create a mixture of carrier gas, vaporized solvent and vaporized solutes. The septum purge outlet prevents septum components from entering the column. There are 2 general types of column, packed and capillary. Packed columns contain a divided, inert, solid support material coated having the liquid stationary phase. Most of the packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.