Maximizing performance in supercritical fluid chromatography using low-density mobile phases

The performance of a 3.0 mm x 150 mm column packed with 1.8 mu m fully porous HSS-SB-C-18 particles was investigated in supercritical fluid chromatography (SFC) with low-density, highly expansible carbon dioxide. These conditions are selected for the analysis of semi-volatile compounds. Elevated temperatures (>100 degrees C) were then combined with low column back pressures (<100 bar). In this work, the inlet temperature of pure carbon dioxide was set at 107 degrees C, the active back pressure regulator (ABPR) pressure was fixed at 100 bar, and the flow rate was set at 2.1 mL/min at 12 degrees C (liquefied carbon dioxide) and at an inlet column pressure close to 300 bar. Nine n-alkylbenzenes (from benzene to octadecylbenzene) were injected under linear (no sample overload) conditions. The severe steepness of the temperature gradients across the column diameter were predicted from a simplified heat transfer model. Such conditions dramatically lower the column performance by affecting the symmetry of the peak shape. In order to cope with this problem, three different approaches were experimentally tested. They include (1) the decoupling and the proper selection of the inlet eluent temperature with respect to the oven temperature, (2) the partial thermal insulation of the column using polyethylene aerogel, and (3) the application of a high vacuum (10(-5) Torr provided by a turbo-molecular pump) in a housing chamber surrounding the whole column body. The results reveal that (1) the column efficiency can be maximized by properly selecting the difference between the eluent and the oven temperatures, (2) the mere wrapping of the column with an excellent insulating material is insufficient to fully eliminate heat exchanges by conduction and the undesirable radial density gradients across the column i.d., and (3) the complete thermal insulation of the SFC column under high vacuum allows to maximize the column efficiency by maintaining the integrity of the peak shape. (C) 2016 Elsevier B.V. All rights reserved.