Effect of density on kinetic performance in supercritical fluid chromatography with methanol modified carbon dioxide

In a relatively recent reevaluation of the van Deemter Equation, Guiochon concluded that the mass transfer resistance in the mobile phase is independent of the retention factor. In the process he showed reduced plate heights ≈ 2 for a nearly unretained peak (k = 0.4) in high performance liquid chromatography (HPLC). In the present work, using supercritical fluid chromatography (SFC), efficiency was measured at various pressures, densities, and modifier concentrations. The highest efficiency, with a reduce plate height of h_r = 1.63, was recorded with the lowest retention factor (k < 0.8). This is an extremely low h_r for totally porous particles, at very low k, and appears to support Guiochon's analysis. The density of methanol/carbon dioxide mixtures were calculated using the REFPROP program from the National Institute of Standards and Technology (NIST) over a wide range of pressures and % methanol. The density of higher methanol concentrations (>20%), commonly used in SFC, was found to be lower than the density of lower concentrations (<20%). At low methanol concentrations, density varies widely with pressure. However, at high methanol concentrations there is very little change in density, and very little change in retention with pressure. With increasing modifier concentration, density decreases, while viscosity increases (ΔP increases). The pump and back pressure regulator (BPR) pressures are not necessarily good indicators of pressures or densities in the column. At high flow rates the extra-column pressure drop (ΔP) can be much larger than the column ΔP and can be unevenly distributed in front of and behind the column. In one extreme the ΔP after the column was 3 times higher (105 bar) than the actual column ΔP (32 bar).