Density gradients in packed columns: II. Effects of density gradients on efficiency in supercritical fluid separations

To investigate how fluid compressibility affects efficiency in supercritical fluid separations, band dispersion along a packed capillary column was measured from on-column elution rate profiles obtained under solvating gas chromatography (SGC) conditions; this allowed efficiency to be determined with respect to position along the column. Theoretical efficiency was also modeled. The model indicates that the primary cause of band broadening in SGC is high mobile phase velocity near the column outlet. However, the experimental results show that significant band broadening also occurs near the column inlet in a region that corresponds to high elution rates of the analyte. On-column detection also revealed spatial focusing of the analyte as it moves down the column density gradient.