Elevated temperature liquid chromatography using reversed-phase packed capillary columns

Reversed-phase packed capillary column liquid chromatography (LC) was investigated at column temperatures of 24°C, 40°C, 60°C, and 80°C. For a mixed mobile phase containing acetonitrile and water (72:28, v/v), the viscosity of the mobile phase decreased 2.2 times and the diffusion rate of the solute increased 2.6 times in the range of temperatures investigated. The faster diffusion rate of naphthalene in the mobile phase at 80°C resulted in a 2.6-fold faster optimum linear velocity and a 2.5-fold decrease in the C term of the van Deemter equation compared to values obtained at 24°C. Better efficiencies were obtained at faster than optimum linear velocities at elevated temperatures, while at the same time retention times were much shorter than at room temperature. It was also found that temperature could have a better or worse effect on selectivity, depending on the retention factors, enthalpies, and entropies of the compounds analyzed. The pressure drop decreased by one half at an elevated temperature of 80°C compared with that at room temperature for the mobile phase investigated. It is estimated that smaller particles or longer columns could be used at elevated temperatures in an LC system to obtain even higher total plate numbers.