Liquid-solid chromatography retention models

Several different physicochemical models have been proposed to predict and explain the retention behavior in liquid-solid chromatography. The models can be divided into two groups depending on the assumptions made concerning the fundamental mechanism of the chromatographic process. The two assumptions are as follows ... 

Different models have been developed to describe the retention of substances in adsorption chromatography. The Snyder model (Fig. 4.15) assumes that in liquid-solid chromatography the whole adsorbent surface is covered with a monolayer of solvent molecules and the adsorbent together with the adsorbed monolayer has to be considered as the stationary phase (Snyder, 1968 Snyder and Kirkland, 1979 Snyder et al. 1997). Adsorption of the sample occurs by displacement of a certain volume of solvent molecules from the monolayer by an approximately equal volume of sample,... 

The retention model developed by Eon and Guiochon [7,8] to describe the adsorption effects at both gas-liquid and liquid-solid interfaces, which was later modified by Mdckel et al. [6] to account for the retention at chemically bonded reversed-phase materials in HPLC, is not applicable to ion chromatography. But if the dependence of the capacity factors of various inorganic anions on the column temperature is studied, certain parallels with HPLC are observed. The linear dependences shown in Fig. 3-2 are obtained for the ions bromide and nitrate when the In k values are plotted versus the reciprocal temperature (van t Hoff plot). However, in the case of fluoride, chloride, nitrite, orthophosphate, and sulfate, the k values were found to be constant within experimental error limits in the temperature range investigated. Upon linear regression of the values in Table 3-1, the following relations are derived for bromide and nitrate ... 

The fundamental references in gas-solid adsorption are the works by Fowler and Guggenheim [12], Everett [13], and Hill [14,15], and the books by Young and Crowell [16], de Boer [17], Kiselev [4], and more recently by Ruthven [18] and T6th [19], who gives a clear, logical, and simple presentation of this topic. We present first a few theoretical results obtained in the study of gas-sohd adsorption, results that have been extended semiempirically to liquid-solid adsorption [18]. Then, we describe the various isotherm models that have been used in the study of retention mechanisms in liquid chromatography. 

Countercurrent chromatography (CCC) is a liquid-liquid partition technique that eliminates various complications arising from the use of solid supports. The highspeed CCC, the most advanced form in terms of partition efficiency and separation time, has been used for the separation and purification of a wide variety of natural products. The recent model of high-speed CCC, which facilitates the stationary phase retention for polar solvent systems, is particularly useful for the separation of hydrophilic, highly polymerized procyanidins. 

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