Topics - Selectivity
In chromatography, the selectivity of a phase system refers to its capacity to retain certain types of solutes to a significantly greater extent than others. For example, a gas chromatographic system, employing a long chain hydrocarbon as the stationary phase, would retain hydrocarbons very strongly, due to their mutual, strong dispersive interactions. In contrast, short chain alcohols would elute very rapidly due to the relatively small dispersive interactions between the alcohols and the hydrocarbon stationary phase. The phase system would be said to ‘selectively’ retain hydrocarbons relative to short chained alcohols. However, if the stationary phase was polyethylene glycol, the short chained alcohols would be retained very strongly due to the strong polar interactions between the hydroxy groups on the solute and those of the stationary phase. In contrast, due to the small dispersive capability of the polyethylene glycol, hydrocarbons (including long chained hydrocarbons) would interact very weakly with the stationary phase and would only be slightly retained. Thus, the stationary phase would be said to selectively retain alcohols relative to hydrocarbons. The cases sited are extreme and chosen to illustrate selectivity, but, in fact, selectivity can be far more subtle when separating similar types of compounds.