Distribution coefficients zone chromatography

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. 

Fig. 7. Interrelation between molecular weight and retention volume for macromolecules of different functionality at chromatography in the exclusion (1-3), the critical (4), and the adsorption (5) separation modes 59). In the general case, the distribution coefficient Kd is a function of the pore size D, the chain length N, the interaction energy with the pore wall of the backbone segments 0 and the terminal segment 0f containing the functional group (zones 1,2 and 3 correspond to the cases shown in Fig. 2)...

The concept of plate theory was originally proposed for the performance of distillation columns (12). However, Martin and Synge (13) first applied the plate theory to partition chromatography. The theory assumes that the column is divided into a number of zones called theoretical plates. One determines the zone thickness or height equivalent to a theoretical plate (HETP) by assuming that there is perfect equilibrium between the gas and liquid phases within each plate. The resulting behavior of the plate column is calculated on the assumption that the distribution coefficient remains unaffected by the presence of other... 

Chromatographic and related electrophoretic methods for the separation of transition metal complexes or their ligands were reviewed . Micellar electrokinetic chromatography (MEKC) presents a new development in the field of capillary zone electrophoresis (CZE). The use of micellar solutions expands the application of CZE to electronically neutral solutes, as well as charged ones. Thus, electrically neutral / -diketonates Cr(dik)3, Co(dik)3, Rd(dik)3, Pt(dik)2 and Pd(dik)2 were separated by CZE in micellar solutions of sds. A linear log-log relationship was found between the distribution coefficient and the partition coefficient of the complex between dodecane and water, which was used for prediction of both the distribution coefficients and the migration times of different metal complexes . 

Zone chromatography is a variant of the zone melting method, in which the mixture being separated is introduced into a column with a solid solvent and a molten zone is passed repeatedly along the length of the column to separate mixtures into separate bands of their components. Zone chromatography has been used for the separation of mixtures of lanthanides for preparative and analytical purposes The chelates used were mixtures of hydrated / -diketonates and their adducts with 2,2 -bipyridyl (bipy) and acety-lacetonimines. The distribution coefficients of different chelates and binary mixtures have been determined . 

As mentioned earher, the plate theory has played a role in the development of chromatography. The concept of "plate" was originally proposed as a measmement of the performance of distillation processes. It is based upon the assumption that the column is divided into a number of zones called theoretical plates, that are treated as if there exists a perfect equilibrium between the gas and the Hquid phases within each plate. This assumption imphes that the distribution coefficient remains the same fi-om one plate to another plate, and is not affected by other sample components, and that the distribution isotherm is hnear. However, experimental evidences show that this is not true. Plate theory disregards that chromatography is a dynamic process of mass transfer, and it reveals httle about the factors affecting the values of the theoretical plate number. In principle, once a sample has been introduced, it enters the GC column as a narrow-width "band" or "zone" of its composite molecules. On the column, the band is further broadened by interaction of components with the stationary phase which retains some components more than others. Increasing... 

This is the most direct and simple theory of chromatography. The transport of the solute down the column will depend upon the distribution constant (partition coefficient), K, and the ratio of the amounts of the two phases in the column. Band (zone) shape does not change during this movement through the column. The system could be visualized as illustrated in Figures 1.10, 1.11. 

The first and most often encountered separation mechanism in CE is based on mobility differences of the analytes in an electric field these differences are dependent on the size and charge-to-mass ratio of the analyte ion. Analyte ions are separated into distinct zones when the mobility of one analyte differs sufficiently from the mobility of the next. This mechanism is exemplified by capillary zone electrophoresis (CZE) which is the simplest CE mode. A number of other recognized CE modes are variations of CZE. These are micellar electrokinetic capillary chromatography (MECC), capillary gel electrophoresis (CGE), capillary electrochromatography (CEC), and chiral CE. In MECC the separation is similar to CZE, but an additional mechanism is in effect that is based on differences in the partition coefficients of the solutes between the buffer and micelles present in the buffer. In CGE the additional mechanism is based on solute size, as the capillary is filled with a gel or a polymer network that inhibits the passage of larger molecules. In chiral CE the additional separation mechanism is based on chiral selectivity. Finally, in CEC the capillary is packed with a stationary phase that can retain solutes on basis of the same distribution equilibria found in chromatography. 

Partition chromatography has several valuable features. First of all. the partition coefficient is concentration-independent over a broad range of concentrations. The distribution (partition) isotherm is thus usually linear. In such a case, the zones of substances chromatographed are symmetrical and their position is independent of concentration. 

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