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GC COLUMNS

Selecting a GC Column

The relationship between polarity, selectivity, and sample loading capacity can be illustrated using an ex-

ample. Consider the analysis of benzene and butanol, which have nearly the same boiling point, on an Rtx®-20

column (diphenyl dimethyl polysiloxane stationary phase). Since the benzene molecule is structurally more

similar to the diphenyl phase than butanol is, benzene will solvate into the stationary phase more readily

than butanol based on the concept that “like dissolves like.” Since benzene solvates more readily with the

stationary phase, it has more interaction with the stationary phase as it elutes through the column and will

be retained longer. Since butanol solvates less with the stationary phase, it has fewer interactions with the

stationary phase and less will be retained. Therefore, the elution order of these two compounds on an Rtx®-20

column will be butanol first and benzene second. In addition, since benzene is more soluble in the diphenyl

phase, the column has more capacity for benzene. This results in a more symmetrical peak shape for benzene

than for butanol. A more polar column, such as a polyethylene glycol (PEG) column, will provide retention

and better peak shape for butanol compared to benzene.

Due to their influence on separation factor, polarity and selectivity are primary considerations when se-

lecting a column. However, temperature limits must also be considered. In general, highly polar stationary

phases have lower maximum operating temperatures, so choosing a column with the appropriate maximum

operating temperature, as well as optimal polarity and selectivity for the type of compounds being analyzed,

is crucial.

Retention Factor (k)

The retention factor (k) of a column is based on the time an analyte spends in the stationary phase relative

to the time it spends in the carrier gas. It is influenced primarily by column inner diameter (ID), phase film

thickness, and temperature. Retention factor is sometimes referred to as capacity factor, which should not

be confused with sample loading capacity. As a general rule, the thicker the film and the smaller the inner

diameter, the more an analyte will be retained. Note that as temperature increases, k decreases; therefore, at

higher temperatures analytes stay in the carrier gas longer and are less retained.

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