Heat Exchanges in Fast, High-Performance Liquid Chromatography. A Complete Thermodynamic Study

Abstract

The successive physical transformations of the mobile phase that take place in very high pressure liquid chromatography were studied based on the formalism of classical thermodynamics. The eluent is initially under atmospheric pressure (P⁰) and at ambient temperature (T_ext). In a first step, it is compressed to a high pressure (P_max of the order of 1 kbar) in the pump heads of the chromatograph. In a second step, the pressurized eluent is transferred to the inlet of the chromatographic column, along which, in a third step, it is decompressed to atmospheric pressure. Both the compression and the decompression of the fluid were considered to take place under conditions that can be either adiabatic or nonadiabatic and either reversible or irreversible. Applications of the first and second principles of thermodynamics allow the determination of the heat and energy exchanged between the eluent and the external surroundings during each transformation. Experimental data were acquired using acetonitrile as the mobile phase. The true state equation, ρ(P, T), of liquid acetonitrile was used in the theoretical calculations. A series of four different flow rates (0.55, 0.85, 1.15, and 1.45 mL/min, corresponding to inlet pressures of 357.2, 559.5, 765.1, and 972.9 bar, respectively), were applied to a 2.1 × 100 mm column packed with 1.7-μm bridged ethane−silicon hybrid particles. Thermocouples were used to measure the eluent temperature before and after its passage through the column. These data provide estimates of the variation of the internal energy of the eluent. The heat lost through the external wall of the column during the eluent decompression was estimated by measuring the surface temperature of the column tube under steady state. Both the compression and the decompression of acetonitrile were found to be nonadiabatic and irreversible transformations. The results showed that, during the eluent decompression, the heat released by the friction forces serves four different purposes: (1) it increases the eluent entropy at constant temperature (for ∼35%); (2) it increases the temperature of the eluent (for ∼5%); (3) it provides heat to the laboratory atmosphere (for ∼5%); and (4) it provides some work inside the column (for ∼5%). This quantitative heat balance description accounts well for the actual performance of the new, very high pressure liquid chromatographic technique.