Supercritical fluid chromatograph for studies of retention and efficiency with a brief study of n-alkanes showing evidence of significant temperature drop with pressure drop

Abstract

An instrument designed specifically to examine retention and efficiency in packed-column supercritical fluid chromatography is described. Built on a gas chromatograph, it incorporates a syringe pump, helium-actuated injector, flame ionization detector, two outlet restrictors, and direct control of column inlet and outlet pressures. A supercritical fluid solvent injection system provides chromatograms with no solvent peak or solvent-induced effects on retention and efficiency. Mass flow-rates obtained by reading the volumetric displacement at the syringe pump are accurate to within 1.4%, and the repeatability of retention times and plate numbers with the pump operated in constant pressure and constant flow modes is examined. Isopycnic plate height curves for elution of n-dodecane, n-tetradecane, n-hexadecane, and n-octadecane with carbon dioxide at 50°C and a reduced density of 1.0 on a 150×2.0-mm I.D. column packed with 10-μm Spherisorb C 8 show a broad minimum with reduced plate heights of about 2 at reduced velocities from 3 to 7, with sharp increases at higher velocities for the heavier solutes. Retention volumes decrease with pressure drop, which is attributed to a significant temperature drop from the inlet to the outlet of the column.