Achieving quasi-adiabatic thermal environment to maximize resolution power in very high-pressure liquid chromatography: Theory, models, and experiments

• The chromatographic column was isolated thermally from the external air environment.
• This was achieved by applying high vacuum (<10−4 Torr) inside cylindrical column housing.
• Heat transfer mechanism due to natural convection and air conduction was fully eliminated.
• Heat transfer mechanism due to radiation was reduced using low-emissivity aluminum tape.
• Column efficiency is increased by up to 35% at 13,000 psi pressure and 0.6 mL/min flow rate.

A cylindrical vacuum chamber (inner diameter 5 cm) housing a narrow-bore 2.1 mm × 100 mm column packed with 1.8 μm HSS-T3 fully porous particles was built in order to isolate thermally the chromatographic column from the external air environment. Consistent with statistical physics and the mean free path of air molecules, the experimental results show that natural air convection and conduction are fully eliminated for housing air pressures smaller than 10−4 Torr. Heat radiation is minimized by wrapping up the column with low-emissivity aluminum-tape (emissivity coefficient ϵ = 0.03 vs. 0.28 for polished stainless steel 316). Overall, the heat flux at the column wall is reduced by 96% with respect to standard still-air ovens. From a practical viewpoint, the efficiency of the column run at a flow rate of 0.6 mL/min at a constant 13,000 psi pressure drop (the viscous heat power is around 9 W/m) is improved by up to 35% irrespective of the analyte retention. Models of heat and mass transfer reveal that (1) the amplitude of the radial temperature gradient is significantly reduced from 0.30 to 0.01 K and (2) the observed improvement in resolution power stems from a more uniform distribution of the flow velocity across the column diameter. The eddy dispersion term in the van Deemter equation is reduced by 0.8 ± 0.1 reduced plate height unit, a significant gain in column performance.