Columns

Although a full range of columns can be used in SFC, the technique is most usually divided into two categories based on column type, with advantages and disadvantages outlined in Table 2. Packed columns for SFC have reflected developments in HPLC: (a) 'conventional' packed columns (up to 4.6 mm i.d.); (b) microbore columns; and (c) packed capillaries with smaller i.d. Most of the work has been carried out on 4.6-mm columns, with 3-10 |im particle size packing materials - usually bonded-silica particles (Table 3). It was realized early that the unreacted silanol groups on silica particles which have little activity in reversed-phase HPLC led to peak tailing because of the low polarity of the mobile phase. Such effects led to the almost exclusive addition of organic modifiers (alcohols, ethers, etc.) to the carbon dioxide mobile phase in packed column SFC. These modifiers can influence selectivity and increase the solubility of analytes. A number of specially end-capped silica packings were developed, along with polymer-based

Table 3 Bonded stationary phases in SFC

Packed SFC

Capillary SFC

Octadecyl

Methyl

Octyl

Octyl

Phenyl

Phenyl

Hydroxyl

Biphenyl

Diol

Cyanopropyl

Cyano

Liquid crystal

Amino

Polyether

Chiral

Chiral

packings, but most of these have had limited use. The low-pressure drop across packed columns has led to the coupling together of columns, with consequent improvement in selectivity and efficiency.

Capillary SFC was only made possible by the development from 1979 onwards of fused silica capillary columns for GC. Because of the slower diffusion of analyte in supercritical fluids, open tubular columns for SFC must have internal diameters less than 100 |im to achieve resolution comparable to that on 200 |im i.d. columns for GC. Lee's group undertook an extensive series of developments of columns for both GC and SFC between 1980 and 1990, basing stationary phase structures on a polysiloxane polymer backbone with pendant groups 'designed' for particular applications. As well as the more usual methyl- and phenyl-containing phases (Table 3), polymers with biphenyl, cyanopropyl and ethylene glycol, etc., groups were synthesized; more exotic phases had bonded liquid crystal or chiral groupings. Capillary columns for SFC are highly inert and are usually operated with CO2 alone as mobile phase.

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