Polar columns, coating efficiency

These columns (i.d. < 200 pm) are characterized by a high specific efficiency (number of theoretical plates per meter and per second). Cartoni and co-workers [44] described the technique for preparing glass and fused-silica capillary columns (100 pm i d ), which were precoated with a very thin layer of graphitized carbon black and then coated with polar liquid phases. The layer of carbon black increased the wettability of the capillary columns walls and a very uniform coating was obtained. Columns coated with Carbowax 20 M, 40 M and 600 M were prepared. Polar liquid phases were strongly retained on carbon black, and these column showed higher temperature stability. 

Nonpolar phases are more inert than polar phases, bleed less, have a wide range of operating column temperatures, and have higher coating efficiencies. 

A wide variety of stationary phases and bonded-phase particles for SFC are available. Capillary columns are coated with substituted and cross-linked polysiloxanes, which exhibit good inertness, efficiency, and stability. There are three main classes of capillary column stationary phases for SFC apolar, polarizable, and polar. 

Relative retentions..the a values..usually vary Inversely with column temperature, but are most strongly affected by the choice of liquid phase. In packed column chromatography, the choice of liquid phase Is usually the most effective route by which separation efficiency Is Influenced. In capillary GC, however, there Is normally such an abundance of theoretical plates that the choice of liquid phase Is a relatively unimportant parameter for many analyses. In some cases however. It does become desirable (or even necessary) to select a liquid phase in which the relative retentions of certain solutes Is larger. Until quite recently, this posed a real problem with the fused silica capillary column, because the more polar liquid phases, l.e. those In which relative retentions are usually greater, coated fused silica only reluctantly, and produced columns whose useful lives were quite limited. The development of stable bonded phase columns ( ) eventually overcame this difficulty (vide Infra). 

Poor wettability of glass columns was a major obstacle to the reliable column technology in the past. With the advent of surface corrosion techniques [11,97,98], it has become feasible to coat uniform stationary films of even polar liquids on the inner column surface. Similarly, coated dispersions of solids [99] can aid in a uniform distribution of the stationary liquids for efficient separations. 

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