Principles and Practice of Chromatography - Chromatography Applications > Gas Chromatography Applications > High Temperature GC Stationary Phases > Page 72

High Temperature GC Stationary Phases

The major limitation of gas chromatography is the stability of the stationary phase at high temperatures. The higher the polarity and the higher the molecular weight of the solutes, the higher the temperature necessary to provide adequate solute partial vapor pressure to allow a gas chromatographic separation to be realized. Similarly, the stability of the solutes at high temperature can also become a problem. The solute must be thermally stable so that the partial pressure is sufficiently high to allow elution in a reasonable time. Nothing can be done with respect to the solute stability as this is determined by the nature of the sample.

There are certain materials hat can be used as stationary phases at remarkably high temperatures. These materials are based on the polymerization of carborane substituted siloxanes. An example of the empirical formula of a carborane silicone polymer is as follows,

 

where represents the meta–carborane nucleus.

There are three commonly used carborane stationary phases, The first a dispersive phase, Dexsil 300 where the carboranes are linked with a methylsilicone polymer and can be used up to a temperature of 450˚C (an exceedingly high temperature for chromatographic separations). Some induced polarizability has been introduced into the carborane polymer by employing a methyl phenyl silicone and this resulted in the second carborane having intermediate polarity that can be operated up to a temperature of 400˚C. The introduction of a phenyl group makes it slightly less thermally stable. The third and most polar carborane is Dexsil 410 that contains methyl, b–cyanoethyl silicone (the polarity being contributed by the cyano group) which can be used up to 375˚C.