The Column in Gas Chromatography

Separations are possible in gas chromatography if the solutes differ in their vapor pressure and/or intensity of solute-stationary phase interactions. As a minimum requirement the sample, or some convenient derivative of it, must be thermally stable at the temperature required for vaporization. The fundamental limit for sample suitability is established by the thermal stability of the sample and system suitability by the thermal stability of column materials. In contemporary practice an upper temperature limit of about 400°C and a sample molecular weight less than 1000 is indicated, although higher temperatures have been used and higher molecular weight samples have been separated in a few instances. 

Five column types have been used in gas chromatography. Classical packed columns have internal diameters greater than 2 mm and are packed with particles in the range 100 to 250 p,m. Micropacked columns have diameters less than 1 mm and a similar packing density to classical packed columns (dp / dc less than 0.3, where dp is the average particle diameter and dc the column diameter). Packed capillary columns have a column diameter less than 0.6 mm and are packed with particles of 5-20 p,m diameter. SCOT 

Representative properties of different column types for gas chromatography Hmin = minimum plate height at the optimum mobile phase velocity Uopt 

Column type Phase ratio f min (mm) Uopt (cm/s) Permeability (lO. cm ) 

Column type Length (m) Internal diameter (mm) Film thickness (p.m) Phase ratio Retention factor (k) min (mm) Column plate number Plates per meter 

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In addition to the availability of different methods of sample introduction into the column in gas chromatography, there are two different types of injectors that can be used to input the sample. These are the splitless injector and the split injector. 

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