EFFECT OF THE SAMPLE SOLVENT AND INSTRUMENT DESIGN ON THE REPRODUCIBILITY OF RETENTION TIMES AND PEAK SHAPES IN PACKED-COLUMN SUPERCRITICAL-FLUID CHROMATOGRAPHY

Factors affecting the reproducibility of retention times and peak shapes in supercritical fluid chromatography on a cyano-bonded silica packed column have been studied. These included the inclusion of low proportions of modifier in the eluent and the solvent used to inject the analyte. With carbon dioxide as the eluent, polar sample solvents were found to cause residual effects, which changed subsequent separations. These effects were lost when an eluent modifier was present suggesting that they resulted from temporary masking of silanol groups on the silica surface. If the mobile phase was cooled to near the critical point between the oven and a spectroscopic detector, small changes in conditions caused baseline fluctuations, which was considered to be due to changes in the refractive index of the solution.