Loading and Injection of Samples

One area of preparative SFC that would benefit from further investigation is the sample injection technique. With the exception of on-line extraction/chromatogra-phy, the sample is usually introduced as a solution in an organic solvent aind injected onto the column by means of a loop rotary valve. The sample loop is then flushed with a high density liquid that eventually becomes the supercritical fluid mobile phase on entering the heated column. 

Cretier et al. [18] describe an injection technique whereby the sample solution is passed from the sample injection loop to a packed pre-column. The solvent is then 

8 Scaling-up of Supercritical Fluid Chromatography to Large-Scale Applications 

In practice the volume and consequently the amount of sample that can be injected are limited by the volume of the pre-column. However, the loading and venting cycle can be repeated a number of times before carrying out the elution with liquid carbon dioxide and the final preparative separation. 

When the volume of injection solvent is small, the dilution with the supercritical fluid mobile phase leads to conditions of elution for the solutes which are almost identical to the conditions of elution with the supercritical fluid itself. As the volume of injection is increased there is less dilution of the bolus of solvent by the mobile phase and the solvent acts as a second eluting mobile phase for part of the solute, giving rise to an adjacent earlier eluted peak. The amplitude of the band broadening depends on the nature of the sample solvent. 

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