Chromatography distribution coefficient

Why discuss distribution coefficients Most everyone is familiar with the demonstration of iodine distributed between an organic and an aqueous layer. However, distribution equilibria are at the heart of many separation processes from liquid-liquid extractions to virtually every type of chromatography in which the distribution of the solute between the mobile phase and the stationary phase determines the effectiveness of the separation. In the practice of analytical chromatography, distribution coefficients are often called partition coefficients but the concept is identical, only the names have changed. The temperature dependence of a distribution coefficient is at the heart of temperature programming in gas-liquid chromatography (GC), and analyses of the temperature behavior depend on the heats of solution of the distributed solutes. Indeed, GC measurements have been used to measure heats of solution. 

Ion-exchange chromatography involves more variables than other forms of chromatography. Distribution coefficients and selectivities are functions of pH, solute charge and radius, resin porosity, ionic strength and type of buffer, type of solvent and temperature. The number of experimental variables makes lEC a very versatile technique but a difficult one because of the effort needed to optimise a separation. 

Distribution Coefficients. Gel-permeation stationary-phase chromatography normally exhibits symmetrical (Gaussian) peaks because the partitioning of the solute between mobile and stationary phases is linear. Criteria more sophisticated than those represented in Figure 8 are seldom used (34). 

Salt Effects. The definition of a capacity factor k in hydrophobic interaction chromatography is analogous to the distribution coefficient, in gel permeation chromatography ... 

By measuring the retention volume of a solute, the distribution coefficient can be obtained. The distribution coefficient, determined over a range of temperatures, is often used to determine the thermodynamic properties of the system this will be discussed later. From a chromatography point of view, thermodynamic studies are also employed as a diagnostic tool to examine the actual nature of the distribution. The use of thermodynamics for this purpose will be a subject of discussion in the next chapter. It follows that the accurate measurement of (VV) can be extremely... 

Summarizing, the greater the forces between the molecules, the greater the energy (enthalpy) contribution, the larger the distribution coefficient, and the greater the retention. Conversely, any reduction in the random nature of the molecules or any increase in the amount of order in the system reduces the distribution coefficient and attenuates the retention. In chromatography, the standard enthalpy and standard entropy oppose one another in their effects on solute retention. Experimentally it has... 

For chromatography purposes the product of the distribution coefficient and the volume of stationary phase, or stationary phase surface area, gives the corrected retention volume,/, e.,... 

It should be recalled that the distribution coefficients are referenced to the solvent mixture and not the stationary phase and are thus the inverse of the distribution coefficient employed in the chromatography elution equation. 

The basic principle of chromatography separations can be described by thermodynamics using the distribution coefficient K (12) ... 

One of the most rational means for displacing a broad zone is electrolyte desorption under the conditions of decreasing degree of ionization, i.e., when counterions are converted into dipolar ions, uncharged molecules and coions. This conversion corresponds to a sharp decrease in distribution coefficients of the desorbed substance. Hence, the displacement of equilibrium parame ters at a high rate of mass-exchange is one of the methods of selective stepwise chromatography. 

Chemical forces are normally irreversible in nature (at least in chromatography) and thus, the distribution coefficient of the solute with respect to the stationary phase is infinite or close to infinite. Affinity chromatography is an example of the use of chemical forces in a separation process. The stationary phase is formed in such a manner that it will chemically interact with one unique solute present in the sample and thus, exclusively extract it from the other materials... 

The capacity ratio of a solute (k ) was introduced early in the development of chromatography theory and was defined as the ratio of the distribution coefficient of the solute to the phase ratio (a) of the column. In turn the phase ratio of the column was defined as the ratio of the volume of mobile phase in the column to the volume of stationary phase in the column. 

The upper curve shows the adsorption isotherm that normally occurs in liquid chromatography separations where the concentration of solute in the system is very low. The isotherm is linear and thus the distribution coefficient is constant at all concentrations of solute in either phase. It follows that as the peak velocity is inversely related to the distribution coefficient, all solute concentrations travel at the same velocity through the column and the peak is symmetrical. 

Figure 5.1 Pesticides included in the systematic investigations on APCI-MS signal response dependence on eluent flow rate the parameter IsTow represents the distribution coefficient of the pesticide between n-octanol and water. Reprinted from J. Chromatogr, A, 937, Asperger, A., Efer, 1., Koal, T. and Engewald, W., On the signal response of various pesticides in electrospray and atmospheric pressure chemical ionization depending on the flow rate of eluent applied in liquid chromatography-mass spectrometry , 65-72, Copyright (2001), with permission from Elsevier Science.

In chromatography the components of a mixture are separated as they pass through a column. The column contains a stationary phase which may be a packed bed of solid particles or a liquid with which the packing is impregnated. The mixture is carried through the column dissolved in a gas or liquid stream known as the mobile phase, eluent or carrier. Separation occurs because the differing distribution coefficients of the components of the mixture between the stationary and mobile phases result in differing velocities of travel. 

The ratio of the distribution coefficients of pertechnetate and perrhenate is about 1.6 to 2, comparable to adjacent rare earth metals. Technetium and rhenium may be separated by ion-exchange chromatography. However, efficient separations require some care and tend to be slow. On the other hand, cation exchange resins adsorb technetiiun only to a negligible extent so that pertechnetate can be rapidly separated from cationic elements . 

Gel Permeation Chromatography. Samples were filtered on columns of Bio-Gel P-6DG (Bio-Rad Laboratories), and columns of Sephadex G-10, G-25, and G-50 (Pharmacia Corp.) using deionized water as eluant. Gel filtration properties are expressed in terms of the distribution coefficient calculated from the relationship = (V - V )/ (V - V ) where the the volume at which a component elutes, is the void volume, and is the total volume of the system. Blue dextran 2,000 and xylose were used to determine and Vj respectively. 

Size exclusion chromatography is a unique separation technique based on molecular size (hydrodynamic volume) differences among solutes. The distribution coefficient of an eluting solute is defined as... 

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