Chromatographic distribution coefficient

A convenient way to classify solvents of chromatographic interest in terms of their polarity and the specific chemical interactions is the empirical scheme proposed by Snyder [214,215]. This scheme is based on experimental (gas chromatographic) distribution coefficients for three test solutes ( probes ) on a large number of stationary phases, which were published by Rohrschneider [216]. The probe compounds are ethanol (e), 1,4-dioxane (d) and nitromethane (n). The experimental values for the distribution coefficients undergo several empirical modifications ... 

The chromatographic distribution coefficients K is defined as bound protein-free protein and can be modified to... 

The chromatographic distribution coefficient, K, , which is used here as a tentative measure of the affinity of the sugars for polystyrene, was calculated by... 

The chromatographic distribution coefficient (K ) of monosaccharides and maltodextrins obtained with polystyrene gel (Bio-Beads SM-4) in water and salt solutions at 25°C . 

It is clear that the separation ratio is simply the ratio of the distribution coefficients of the two solutes, which only depend on the operating temperature and the nature of the two phases. More importantly, they are independent of the mobile phase flow rate and the phase ratio of the column. This means, for example, that the same separation ratios will be obtained for two solutes chromatographed on either a packed column or a capillary column, providing the temperature is the same and the same phase system is employed. This does, however, assume that there are no exclusion effects from the support or stationary phase. If the support or stationary phase is porous, as, for example, silica gel or silica gel based materials, and a pair of solutes differ in size, then the stationary phase available to one solute may not be available to the other. In which case, unless both stationary phases have exactly the same pore distribution, if separated on another column, the separation ratios may not be the same, even if the same phase system and temperature are employed. This will become more evident when the measurement of dead volume is discussed and the importance of pore distribution is considered. 

Temperature programming was introduced in the early days of GC and is now a commonly practiced elution technique. It follows that the temperature programmer is an essential accessory to all contemporary gas chromatographs and also to many liquid chromatographs. The technique is used for the same reasons as flow programming, that is, to accelerate the elution rate of the late peaks that would otherwise take an inordinately long time to elute. The distribution coefficient of a solute is exponentially related to the reciprocal of the absolute temperature, and as the retention volume is directly related to the distribution coefficient, temperature will govern the elution rate of the solute. 

The distribution coefficient can be determined by batch experiments in which a small known quantity of resin is shaken with a solution containing a known concentration of the solute, followed by analysis of the two phases after equilibrium has been attained. The separation factor, a, is used as a measure of the chromatographic separation possible and is given by the equation,... 

The volume distribution coefficient is also a useful parameter for chromatographic calculations and is defined as... 

Consequently, the solutes will pass through the chromatographic system at speeds that are inversely proportional to their distribution coefficients with respect to the stationary phase. The control of solute retention by the magnitude of the solute distribution coefficient will be discussed in the next chapter. 

Solute retention, and consequently chromatographic resolution, is determined by the magnitude of the distribution coefficients of the solutes with respect to the stationary phase and relative to each other. As already suggested, the magnitude of the distribution coefficient is, in turn, controlled by molecular forces between the solutes and the two phases. The procedure by which the analyst can manipulate the solute/phase interactions to effect the desired resolution will also be discussed in chapter 2. 

The chromatographic column has a dichotomy of purpose. During a separation, two processes ensue in the column, continuously, progressively and virtually independent of one another. Firstly, the individual solutes are moved apart as a result of the differing distribution coefficients of each component with respect to the stationary phase in the manner previously described. Secondly, having moved the individual components apart, the column is designed to constrain the natural dispersion of each solute band (i.e. the band... 

It has been shown that, in LC, the size of the distribution coefficient of a solute between the two phases determines the extent of its retention. As a consequence, the difference between the distribution coefficients of two solutes establishes the extent of their separation. The distribution coefficients are controlled by the nature and strength of the molecular interactions that takes place between the solutes and the two phases. Thus it is the choice of the phase system that primarily determines the separation that is achieved by the chromatographic system. 

The lower isotherm represents the overload condition that can occur in liquid/liquid or gas/liquid systems under somewhat unique circumstances. If the interactions between solute molecules with themselves is stronger than the interactions between the solute molecules and the stationary phase molecules, then, as the concentration of solute molecules increases, the distribution coefficient of the solute with respect to the stationary phase also increases. This is because the solute molecules interact more strongly with a solution of themselves in the stationary phase than the stationary phase alone. Thus, the higher concentrations of solute in the chromatographic... 

Hafkenscheid, T.L., Tomlinson, E. (1983) Correlations between alkane/water and octan-l-ol/water distribution coefficients and isocratic reversed-phase liquid chromatographic capacity factors of acids, bases and neutrals. Int l. J. Pharmaceu. 16, 225-239. 

The choice of operating temperature can have a profound effect on a chromatographic separation due to the temperature dependence of the distribution ratio D of each solute or to be strict, of the distribution coefficient A , (cf. solvent extraction, p. 56). The relation is an exponential one,... 

Kd = the solute s organic solvent water distribution coefficient. k = chromatographic capacity ratio (k — tr — t0/t0, tr and t0 being solute retention time and mobile phase holdup time, respectively), a and b = coefficients whose magnitudes depend on the LL distribution and RPLC systems. 

As already discussed, the enhanced conversion is due to the separation of the products from the reaction zone. This can be realized via different distribution coefficients of the compounds (and consequently, a separation of the components) or via (selective) adsorption on a support. Since in the first case the compound travels through the reactor with different speeds, a continuous feed would cause repeated mixing of the separated compounds. Therefore, no improvement can be expected. In the second case, a regeneration of the adsorbent is needed after a certain operative period. This is an inherent drawback of the discontinuous operation of the fixed-bed chromatographic reactor. 

The chromatographic separation of technetium from molybdenum is based on the different extent to which molybdate and pertechnetate are adsorbed from alkaline and acid solutions. The distribution coefficient of molybdate between the anion exchanger Dowex 1-X8 and 3 M NaOH is 12, while it is 10 for pertechnetate under the same conditions. Molybdate is also adsorbed to a much lesser extent from hydrochloric acid solutions than pertechnetate. Thus, molybdemun can be eluted by hydroxide or HCl solutions while nitric acid, perchlorate or thiocyanate are used for the elution of technetium . 

Przyjazny, A., Janicki, W., Chrzanowski, and Staszewki, R. Headspace gas chromatographic determination of distribution coefficients of selected organosulphur compounds and their dependence of some parameters, J. Chromatogr. A, 280 249-260, 1983. 

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