Chromatography capacity ratio

Hammers WE, Meurs GJ, De Ligny CL (1982) Correlations between liquid chromatography capacity ratio data on lichrosorb RP-18 and partition coefficients in the octanol-water system. J Chromatogr 247 1-13. 

The alternative expression for resolution given in equation (7) demonstrates that the plate resolution, as in other forms of chromatography, depends on the number of theoretical plates, the selectivity and the capacity ratio of the solute for the particular plate concerned. In practice, however, the expression given in equation (7) appears to be the more practically useful for TLC. separations. 

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. 

Jandera, P. and Churacek, J., Gradient elution in liquid chromatography. I. The influence of the composition of the mobile phase on the capacity ratio (retention volume, band width, and resolution) in isocratic elution — theoretical considerations, /. Chromatogr., 91, 207, 1974. 

Analogous to chromatography, the ratio of bound to free substrate molecules is defined as capacity factor k ... 

The log Ofj term reflects the differences in capacity ratios of the two peptide solutes Sj and Sj which differ by a functional group and is analogous to the term used to predict selectivity differences for the classical liquid-liquid partition chromatography of peptides. The influence of functional group behavior on the retention of polar solutes in reversed-phase HPLC has been the subject of several recent articles and similar trends are apparent with peptide derivatives (29-31). 

The LFER-based retention parameter in high-performance liquid chromatography (HPLC) is the logarithm of the phase capacity ratio or retention factor k. The capacity... 

Capacity factor (k J. The fundamental dimensionless measure of retention in liquid chromatography is the capacity ratio (or capacity factor) (k ), which is defined as the ratio of the number of molecules of solute in the stationary phase, N, to the number of molecules in the mobile phase,... 

Retention in reversed-phase liquid chromatography decreases with decreasing concentration of water in the mobile phase. The logarithm of the capacity ratio for a given solute is linearly related to the volume fraction of the organic modifier in the mobile phase, O, according to equation (3.12) ... 

Several other models have been proposed to account for retention in reversed phase chromatography of these two have found some degree of popularity and include the concepts of molecular connectivity and interaction indexes . The value of the molecular connectivity index has been shown to be proportional to the capacity ratio and the solubility of the solute in water (Karger et al., 1976). 

Figures Supercritical fluid chromatography (SFC) with COj as a mobile phase capacity ratio k as a function of density p and pressure p see text Q0H22 = decane, Ci,Ha = hexadecane, CaoH4a = eicosane, CjoH 2 = 2,6.10,15,19,23-Aexame/A /-tetracosane)...

In order to optimize a separation and produce it in the minimum time, the capacity ratios and separation ratios must be measured for a given pair of enantiomers under known conditions of mobile phase composition and temperature (this will be discussed in detail later in this chapter). Unfortunately, when two peaks are eluted close together, which frequently occurs in chiral chromatography, the positions of the peak maxima are distorted due to the immediate presence of the other peak. An example of this problem is shown in figure 10.1, where the peaks are simulated and added, and the composite envelope plotted over the envelope of each individual peak. It is seen that the actual retention difference, if taken from the maxima of the envelope, will give a value of less than 60% of the true retention difference. Unfortunately, this type of error will probably not be taken into account by most data processing software. It follows, that if such data is used in an attempt to calculate the... 

Retention time in gas chromatography is related to a combination of retention and volatility, similar to solubility in liquid chromatography. Predicting volatility is as difficult as predicting solubility. Volatility has been explained as the enthalpy of vaporization (Avap f), and a method for predicting volatility has been proposed." If the A apH values are available, it may be possible to predict retention time. Unknown A apH values have been calculated from the relationship between the van der Waals volume and reference AyapH values. The values are summarized with the corresponding reference values in Table 1 of the Appendix (p. 278). Values of A apH have also been related to capacity ratios. The correlation coefficients were 0.896 and 0.852 ( = 48) for DBl and CPSilS columns, respectively, which appear to be acceptable correlation coefficients, except that the relationship for allq l alcohols deviated from those of other compounds as seen Figure 4.4. 

The coefficient of correlation between the calculated individual molecular interaction (MI) energies and the logarithmic capacity ratio indicated the contribution of individual factors to the retention. MIVW was the main contributor to the interaction in reversed-phase liquid chromatography, and MIES was the main contributor to the retention in ion-exchange liquid chromatography. Steric hindrance affected the molecular interaction in enantiomeric separation. ... 

The retention time of phenolic compounds in reversed-phase liquid chromatography was predicted via molecular interaction energy values calculated using the MM2 program. The precision of the capacity ratios predicted by this new method was equivalent to a former method in which the retention time was predicted by log P calculated using the MOPAC program. Furthermore, the prediction of capacity ratios of phenolic compounds in reversed-phase... 

The capacity ratios of partially ionized compounds can be predicted using eqn (9). For evaluation by the above approach, k and iti were replaced with energy values calculated using eqns (4) and (5), respectively. The predicted p Ta values from the atom partial charge related to the measured p/fa values in liquid chromatography were used for the calculation. The results are summarized in Figure 6.22. 

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