Summary of parameters governing capillary GC performance

GC traces of an extract from a decongestant syrup. (A) Underivatised and (B) after treatment with trifluoroacetic anhydride. 

Column Pressure Temperature (C) T (s) Flow rate Temp Flow rate (C) T (s) 

As column temperature increases the degree of resolution between two components decreases because the degree of interaction with the stationary phase is reduced as the vapour pressure of the analytes increases. Lower temperatures produce better resolution. 

The separating power of a column varies as the square root of its length. Thus if a two-fold increase in resolution is required and a four-fold increase in column length would be required, this would result in a four-fold increase in analysis time. The increased resolution afforded by length can often be replaced with a decrease in temperature ensuring that more interaction with the stationary phase occurs, especially if the stationary phase has characteristics that enable it to select one analyte more than another. 

The greater the volume of stationary phase the more a solute will partition into it. If the film thickness or loading of stationary phase doubles then in theory the retention of an analyte should double. Thus thicker films are used for very volatile materials to increase their retention time and to increase resolution between analytes without increasing the column length. 

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