Partition coefficient, chromatographic function

The correlation between partition coefficient and chromatographic retention has been examined by many workers in an attempt to predict log F, the logarithm of water-octanol partition coefficient used in establishing structure-function relationships, from retention data or vice versa. The data (92, 203, 215) seem to imply changes in log k are linear in changes in log P but that log k changes more slowly. Some literature data suggest that the retention is directly proportional to the partition coefficient. This apparent equality disappears, however, if the data are recalculated to identical experimental conditions. 

Since the evaluation of this parameter is very important, it was also measured using the micellar HPLC technique (9), which allows a better estimation of the partition coefficients in the presence of quite high concentrations of surfactant. The chromatographic parameter P was measured for each ligand as a function of surfactant concentration, according to Equation 2 ... 

Another important parameter for characterizing the retentitm of a compound is the capacity factor, k, which is defined as the mass of solute in the stationary phase divided by the mass of solute in the mobile phase as shown in Eq. 4. In contrast with partition coefficient, k is not a state function and is dependent on the cohunn s characteristic phase ratio, which is equal to the volume of the stationary phase (yj divided by the volume of the mobile phase (ym). A greater stationary phase volume increases interactiOTis with the stationary phase, thereby increasing the retention time and A . It is important to note that the capacity factor is also equivalent to the adjusted retention time divided by the void time, which are obtained directly from the chromatogram. This important parameter can be intuitively described as a measure of the strength of an analyte s interaction with the stationary phase in a particular chromatographic system. 

The relationship between the chemical structures of compounds and their chromatographic behavior has been considered by many scientists and was first reported by Martin in partition chromatography, where the partition coefficient is regarded as an additive value. The hypothesis of the additivity has raised worldwide discussion hence, additivity rales were formulated. Deviation from the / M additivity results from the complex character of the chromatographic process (change of a composition and volume proportions of phases), constitutional effects in molecules, because of reciprocal interactions of functional groups (internal hydrogen-bond effects, steric and electromeric effects) as well as ionization of substances. 

Plate Theory elution equation equation that gives the concentration of a given analyte in the last plate of the column (adjacent to the detector) as a function of the initial concentration in the first plate before elution has begun, the total mobile phase volume required for analyte elution, the number of theoretical plates N, and basic physico-chemical properties of the system (the partition coefficient) see Equation [3.11] as the Plate Theory description of a chromatographic peak like that in Figure 3.2. 

Solute retention in reversed-phase HPLC is dependent on the different distribution coefficients established between a polar mobile and a nonpolar stationary phase by the peptidic components of a mixture. Although there are many similarities between reversed-phase HPLC separations of peptides and the classical liquid-liquid partition chromatographic methods, it is debatable whether the sorption process in reversed-phase HPLC arises due to partition or adsorption events, i.e., whether the nonpolar stationary phase functions as a bulk liquid or as an adsorptive monolayer. These aspects and the theoretical models for reversed-phase HPLC are discussed in a subsequent section. 

The partition and activity coefficients are used for determining the excess thermodynamic functions of the mixing of chromatographed substances on stationary phase free... 

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