Thermodynamic study of retention in liquid exclusion–adsorption chromatography

HPLC separation of acetaminophen and its impurities using a mixed-mode reversed-phase/cation exchange stationary phase

Determination of acetaminophen and its main impurities: 4-nitrophenol, 4'-chloroacetanilide, as well as 4-aminophenol and its degradation products, p-benzoquinone and hydroquinone has been developed and validated by a new high-performance liquid chromatography method. Chromatographic separation has been obtained on a Hypersil Duet C18/SCX column, using gradient elution, with a mixture of phosphate buffer (pH = 4.88) and methanol as a mobile phase. Analysis time did not exceed 14.5 min and good resolutions, peak shapes and asymmetries have resulted. The linearity of the method has been tested in the range of 5.0-60 µg/mL for acetaminophen and 0.5-6 µg/mL for the other compounds. The limits of detection and quantification have been also established to be lower than 0.1 µg/mL and 0.5 µg/mL, respectively. The method has been successfully applied for the analysis of commercial acetaminophen preparations.

A thermodynamic study of retention of poly(ethylene glycol)s in liquid adsorption chromatography on reversed phases

The retention behaviour of poly(ethylene glycol)s (PEGs) on various reversed phase columns has been studied in liquid interaction chromatography at different temperatures. Similar separations could be achieved in different mobile phases. In methanol-water practically no temperature dependence was observed, while in acetone-water retention decreased with increasing temperature. From the van't Hoff plots the thermodynamic parameters were calculated, which showed, that retention is driven by enthalpy in methanol-water, while in acetone-water the driving force is entropy.

Liquid chromatography of polyoxyethylenes under critical conditions: A thermodynamic study

Polyethylene glycols (PEG), fatty alcohol ethoxylates (FAE) and fatty acid polyglycol esters were analyzed by liquid chromatography under critical conditions (LCCC) on typical reversed phase columns in mobile phases consisting of methanol-water and acetone-water in the region of the critical adsorption point (CAP) for the oxyethylene unit. Critical conditions can be adjusted by varying the mobile phase composition or temperature. In methanol-water the temperature dependence is almost negligible, while it is much more pronounced in acetone-water. Critical conditions for the oxyethylene unit were realized on octadecyl phases in methanol-water containing 85.8 wt.% methanol and in acetone-water at 89.6 wt.% acetone. In the van't Hoff plots the logarithm of the distribution coefficient was plotted versus the reciprocal temperature. In all plots straight lines were found, from slope and intercept of which the entropy and enthalpy changes were calculated. The relation of the thermodynamic parameters -deltaG/RT, -deltaH/RT, and deltaS/R to the numbers of A and B chain units in block copolymers AB and BAB at the CAP for A is studied both theoretically and experimentally. Various approaches for obtaining the critical temperature are discussed.