Efficiency for unretained solutes in packed column supercritical fluid chromatography

At near-critical temperatures and pressures, experimental results for elution of methane with neat carbon dioxide on a 150 mm × 2.0 mm I.D. column packed with 5 μm porous silica with a bonded octylsilica stationary phase show much greater efficiency losses than predicted by theory if isothermal conditions are assumed. Experiments with insulated, air- and water-thermostatted columns demonstrate that significant axial and radial temperature gradients are produced by Joule-Thomson cooling of the mobile phase, and that radial temperature gradients can be a major cause of band spreading at low temperatures and pressures. The use of thermal insulation on the column can greatly improve efficiency under these conditions.