Column, capillary optimized packed columns

This simple illustration shows how the basic choice of capillary versus packed columns determines the nature of the resulting separation and how optimization of flow and temperamre can produce further improvements. Now we will leave packed columns behind and consider in more detail the various avenues available for optimization of capillary-column separations. 

In addition, solute focusing is possible by maintaining a low initial temperature (e.g. 40 °C) for a long period of time (8-12 min ) to allow the mixture of decompressed carbon dioxide, helium gas and the solutes to focus on the GC column. The optimization of the GC inlet temperature can also lead to increased solute focusing. After supercritical fluid analysis, the SF fluid effluent is decompressed through a heated capillary restrictor from a packed column (4.6 mm i.d.) directly into a hot GC split vaporization injector. 

Supercritical fluid chromatography is a very important chromatographic technique still underestimated and underutilized. It presents characteristics similar to both GC and HPLC, although having its own characteristics. Whereas the column temperature control is the way to achieve a good separation in GC and the solvating power of the mobile phase is controlling factor in HPLC, in SFC the density of the fluid is the major factor to be optimized. Both packed (LC-like) and capillary (GC-like) columns have been used in this technique, which has found applications in practically all areas in which GC or HPLC has shown to be the selected separation technique. 

In the case of capillary columns, finding appropriate solutions for sample introduction is much more challenging than with packed columns. Since packed columns have dimensions similar to normal HPLC, optimal sample introduction can be achieved with ordinary sample loop injectors (see Section 12.2.4.2). 

FIGURE 3.15 Optimized separations of peppermint oil on (a) 6-ft x 0.25-in.-i.d. packed column, (b) 500-ft x 0.03-in.-i.d. stainless-steel capillary column and (c) 50-m x 0.25-mm-i.d. glass capillary column. Stationary phase on each column was Carbowax 20M. [W. Jennings, J. Chromatogr. Sci. 17, 637 (1977). Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, a Division of Preston Industries, Inc. ... 

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