Direct examination of separation processes in liquid chromatography: Effect of temperature and pressure on solute retention

In this study, retention processes in reversed‐phase liquid chromatography were examined in situ along an optically transparent column by means of laser‐induced fluorescence. Using a homologous series of derivatized fatty acids as model solutes, the capacity factors were systematically measured as a function of temperature in the range from 10 to 60°C and pressure in the range from 980 to 4,430 psi. A phase transition was observed for the octadecylsilica stationary phase as temperature was decreased and as pressure was increased. From evaluation of its thermodynamic properties during this phase transition, the stationary phase appears to be more similar to a randomly oriented alkane solvent than a highly ordered system such as a phospholipid membrane. Because this phase transition occurs within the range of routine operating conditions in liquid chromatography, it has important implications for both fundamental and practical applications.