For much of the early development of liquid chromatography, separations were carried out at ambient temperature and many laboratories did not attempt to regulate or control the temperature of the column. Frequently, the column would be mounted on the side of the pump or detector and thus would be subjected to changes in the room temperature or changes due to external factors, such as sunlight. However, the influence of temperature on the retention times of analytes was well known and had been studied by a number of groups—in particular, Melander et al. [1]. They demonstrated that for most analytes. | 18 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS Roger M. Smith THE INFLUENCE OF TEMPERATURE ON CHROMATOGRAPHY For much of the early development of liquid chromatography separations were carried out at ambient temperature and many laboratories did not attempt to regulate or control the temperature of the column. Frequently the column would be mounted on the side of the pump or detector and thus would be subjected to changes in the room temperature or changes due to external factors such as sunlight. However the influence of temperature on the retention times of analytes was well known and had been studied by a number of groups in particular Melander et al. 1 . They demonstrated that for most analytes there was a linear relationship between the retention factor of an analyte and the inverse of the absolute column temperature see Chapter 1 . However for a few samples there has been an increase in retention with increasing temperature usually attributed to entropy effects. In the case of polyethylene glycol oligomers the optimum separation was achieved with a negative temperature gradient 2 . The retention of leucine-phenylalanine at low pH and high acetonitrile also increased with increasing temperature 3 . As a result temperature can play an important role in pharmaceutical analysis. The precise and accurate control of temperature can improve reproducibility and method transferability Section . In recent years the use of HPLC for Pharmaceutical Scientists Edited by Yuri Kazakevich and Rosario LoBrutto Copyright 2007 by John Wiley Sons Inc. 811 812 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS elevated or unconventional temperatures have been examined as methods to alter selectivity and column efficiency either with conventional mobile phases Section or with solvent free systems such as superheated water Section . Although normally the interest has been in elevated temperatures subambient chromatography has provided a number
