Long Packed Column Supercritical Fluid Chromatography: Influence of Pressure Drop on Apparent Efficiency

Abstract

Supercritical fluid chromatography, when performed on a packed column, is a powerful and fast separation technique. To enhance the number of theoretical plates (TP) available, long packed columns (>1 m) have been used successfully, despite controversy over the effect on column efficiency of the density gradient induced by the pressure drop. Peak broadening and deformation were reported, and packings with larger particle diameter than those used in liquid chromatography (10 μm instead of 3−5 μm) were advised in order to reduce the column pressure drop. Velocity gradient induced by the density gradient was reported to reduce efficiency. This paper presents the results of investigations on the influence of density gradient on the apparent efficiency obtained on a series of four 25-cm × 4.6-mm-i.d. Nucleosil C18 columns connected in series (particle size, 5 μm). Apparent column efficiency is found to vary from less than 10 000 TP to more than 100 000 TP versus the density and the density gradient. The higher the density gradient, the higher the efficiency loss. A model is presented which accounts for the effect of linear velocity and density gradients on peak broadening. It confirms that it is the linear velocity variation rather than the variation of the density which causes band broadening and allows prediction of conditions for which apparent efficiency loss occurs. To reduce the density gradient induced by column pressure drop, one can compensate for pressure gradient by a superimposed temperature gradient (multitemperature control of the mobile phase via three column ovens). It allows one to obtain the highest efficiency and to use CO2 at lower density without any loss of efficiency. When methanol is added to the CO2, no pressure drop compensation is required in order to obtain the highest apparent efficiency. As density gradient compensation via multitemperature control of the mobile phase provides higher apparent efficiency and, consequently, higher resolution than in isothermal operation, it is especially useful for separation of complex oil samples.