Capillary Chromatography - High Temperature Stationary Phases
High Temperature Stationary Phases
A capillary column contains very little stationary phase and it follows that the partial pressure of the coated material must be extremely low, otherwise it will be vaporized and rapidly lost. The chromatographic effect of loss of stationary phase will be a continues reduction in retention times until finally all the solutes will be eluted at the dead volume. As a consequence, it is extremely difficult to find stationary phases that have sufficiently low vapor pressures to allow them to be used much above 250 ûC. The stationary phases that have the highest operating temperatures are, without doubt, the carborane/silicones. Typically, Dexsil 300 carborane methyl silicone, a largely dispersive stationary phase, can operate satisfactorily at 540oC whereas, the weakly polar Dexsil 400 carborane/methyl phenyl silicone is much less thermally stable and can only be used at 450oC. A diagram of the carborane structure is shown in figure 34.
Figure 34 The Structure of Carborane
There are no strongly polar stationary phases that operate at very high temperatures, although Dexsil 410 carborane methyl cyanoethyl silicone (which is still just moderately polar) will only function at 400oC. The more polar the stationary phase the less thermally stable it becomes. The carboranes have been used to separate a wide variety of high boiling substances some examples of which (including low temperature operation) are as follows.