Thermodynamics of retention for small molecules in the polystyrene/toluene and polystyrene/tetrahydrofuran systems

Abstract

The enthalpy of lnteractlon between test solutes and swollen polystyrene has been measured for toluene and tetrahydrofuran (THF) moblle phases. All solutes thermodynarnlcally prefer the statlonary phase when toluene Is the mobile phase, and pure slze-exciusion chromatography (SEC) never occurs. This surprislng result contradicts the "iike~issolves-like" approach to choosing moblie phase systems for size-excluslon chromatography. Tetrahydrofuran functions well for SEC, probably due to hydrogen-bonding effects In the mobile phase. However, nitromethane prefers the statlonary phase in this system. As wlth earlier work uslng a chloroform mobile phase, a deuterated moblie phase ylelds van't Hoff plots wlth nonzero slopes. The enthalplc contrlbutlon to retention Is different for each of the three mobile phases.The polystyrene-divinylbenzene (PS-DVB) gels are undergoing a period of renewed interest as alternative stationary phases to silica. These materials offer several advantages, including chemical stability and utility for a much wider spectrum of mobile phases than silica. The PS-DVB gels may be used as stationary phases for either normal-phase, reversed-phase, or size-exclusion chromatography (SEC).The latter application represents the traditional use for PS-DVB gels and has received considerable attention. This is supported by the amount of material devoted to gel stationary phases in recent monographs (1,2). However, a clear understanding of the total separation mechanism has yet to be elucidated. In addition to the conventional size-exclusion effects, other interactions such as adsorption or partitioning have been proposed, but only empirical information is available.Much of the attention has centered around the solventswollen gels. The use of "good" solvents should produce liquid chromatography systems where SEC is the dominating mechanism. We recently reported a study of the good solvent chloroform (3), where it was shown that the system separated most solutes by SEC. However, phenols and carboxylic acids interacted with the PS-DVB stationary phase, as evidenced by nonzero slopes from van't Hoff plots. Furthermore, the magnitude of the interaction between chloroform and the gel represented the threshold interaction energy necessary for a solute to interact with the gel.We report here an extension of this thermodynamic investigation to two other good solvents, toluene and tetrahydrofuran. Nonexclusion (enthalpic) effects are identified from the slope of van't Hoff plots, according to the equation (4)Ink=---.-+ --l n P z [is ]

Integrated On-Line System for DNA Sequencing by Capillary Electrophoresis: From Template to Called Bases

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Multimode separation of polycyclic aromatic compounds by size exclusion chromatography with poly(divinylbenzene)

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Petroleum Size Exclusion Chromatography: Mechanisms Explaining the NMP Front Peak

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