A study of the critical criteria for analyte stability in high-temperature liquid chromatography.
There are three major impediments to the use of high-temperature ultrafast liquid chromatography. First, the stationary phase must be thermally stable. Over the past decade, a series of thermally stable, highly efficient stationary phases have been developed that can withstand temperatures exceeding 200 degrees C. Second, the temperature mismatch between the incoming eluent and the column must be minimized (<5 degrees C), because such a mismatch is a very serious cause of peak broadening, especially in ultrafast separations. The thermal mismatch problem can be significantly ameliorated at high column linear velocities by using narrow-bore columns (2.1-mm i.d.). Third, analytes that are exposed to high temperatures must be thermally stable on the time scale of the chromatographic run. We report here a study of the ability of a number of pharmaceuticals to withstand superambient temperatures on the time scale of fast separations. We propose criteria by which a particular analyte may be rejected as a candidate for high-temperature analysis, and we demonstrate that complex molecules are amenable to quantitation, even at temperatures in excess of 100 degrees C in the aqueous media. We also show that as the time an analyte spends on hot column decreases, the extent of on-column reaction decreases for those analytes that do react. Although the seminal work of Antia and Horvath addresses these issues from a theoretical perspective, we hope to further alleviate fear of the use of high temperatures in liquid chromatography through the empirical approach used here.