Reversed-phase retention thermodynamics of pure-water mobile phases at ambient and elevated temperature

The use of pure water at superheated temperatures, between 100 and 200degreesC, as a mobile phase for reversed-phase separations is explored. The thermodynamics of the retention process at low temperature (15-55degreesC) are compared to the thermodynamics at elevated temperature (125-175degreesC). Significant differences in the enthalpy of the retention process are observed between the two temperature ranges. This is possibly due to changes in the hydrogen-bond network of the pure-water mobile phase, which would change the solvation, and therefore retention, of non-polar solutes. The change in thermodynamic values between the two temperature regions invalidates extrapolation of retention as a function of temperature between the two temperature regions for the prediction of room-temperature pure-water retention factors. The thermodynamic changes observed as the temperature is increased are similar to those seen when mobile phase composition is changed (by adding organic modifier) at constant temperature. (C) 2004 Elsevier B.V. All rights reserved.