Effect of the Type of Organic Modifiers on Mass Transfer in Reversed-Phase Liquid Chromatography

Abstract

Methanol/water and acetonitrile/water mixtures have generally been used as a mobile phase in reversedphase liquid chromatography (RP-LC). Chromatographic separations are significantly affected by the mobile-phase conditions, such as the type and composition of the organic modifiers. A number of studies have been made on the influence of the mobile-phase conditions on the retention behavior in RP-LC in connection with the specification of the separation mechanism and with the establishment of optimization strategies for chromatographic conditions.It is also essential to analyze the chromatographic processes from the viewpoints of mass-transfer kinetics and the thermodynamic properties as well as the retention behavior in order to understand the separation mechanism in RP-LC. However, there have not been very many kinetic studies on mass-transfer phenomena in RP packing materials [1][2][3][4] , in contrast to the extremely extensive studies on chromatographic retention. The authors have studied mass-transfer phenomena in RP-LC using mainly methanol/water and octadecylsilyl (ODS)-silica gel as mobile and stationary phases. 5-8Pulse-response experiments were made under various conditions of the solutes as well as the stationary and mobile phases. Their type, composition, and concentration were changed. Some items of information about the retention behavior, mass-transfer rates, and thermodynamic properties in RP-LC were obtained.This paper is concerned with the influence of the type of organic modifiers in the mobile phases on the chromatographic properties in RP-LC. The information about the retention behavior, mass-transfer rates, and thermodynamic properties were determined by using a mixture of acetonitrile and water (70/30, v/v) and an ODS-silica gel as a mobile and stationary phase. In a previous paper 9 , the authors reported on results of a similar study using alkylbenzenes and p-alkylphenols as solutes. In this study, the characteristics of the RP-LC system were analyzed in more detail using more solutes than before. In particular, it was attempted to correlate the surface diffusion coefficient (D s ) with various physical parameters of the solutes. The results in 70 vol% acetonitrile were compared with those in 70 vol% methanol. On the basis of the results, a few estimation procedures of D s have been proposed. In this paper, the mass-transfer and concentration phenomena of solutes from a bulk mobile phase to the surface of ODS-silica gel is represented by the word "adsorption" in a wide sense. The mechanism of the mass transfer, i.e., "partition or adsorption", is not defined. Experimental Column and reagentsSeveral properties of an ODS column (YMC) and the experimental conditions are given in Table 1. An ODS column packed with relatively large ODS-silica gel particles was used in order to make it easy to quantitatively analyze the influence of the mass-transfer rates on the peak spreading. The volumetric composition of acetonitrile was 70%, which was selected as a typical value of mobile-phase compositio...

A comparative study of hydrophobicity of octadecyl and alkylamide bonded phases based on methylene selectivity

“…The data listed in Table 6 demonstrate also a differentiated ordering of near-surface structures, as well as a different nature of mass exchange between the stationary and mobile phases in the case of both Nu-AP phases. According to Pesek [42], Miyabe [43], and Dorsey [44,45], no such dramatic changes in selectivity are observed for phases characterized by specific structural properties in a lower temperature range. In this temperature range and in the presence of the binary mobile phase components, a more rigid and more ordered arrangement of ligands is observed, enabling better exchange of linearly condensed naphthacene molecules.…”