Retention volume, adjusted temperature dependence

The dependence of retention volume on temperature was illustrated in Figure 9.1. Let us determine what temperature increase is necessary to reduce the adjusted retention volume by 50% that is ... 

The most commonly used retention parameter in gas chromatography is the Kovats index. When the adjusted retention times are used to calculate Kovats indices, retention parameters are obtained which depend only on the column temperature and the stationaiy phase used. Kovats indices are highly reproducible, and with a well designed experimental technique and an accurate timing mechanism, an inter-laboratory reproducibility of one unit for larger values of Kovats indices and two units for indices below 400 is possible [14]. Instead of Kovats indices, sometimes in QSRR studies the logarithms of retention volumes of solutes are used. 

Vpi2 are adjusted retention volumes of compounds 1 and 2, respectively, a depends on the stationary phase used and is a function of temperature, a is also the sepeiration factor. 

For example, in RP chromatography very often organic solvents and water are used. The solubility in the organic solvent is higher and water is used to adjust retention and selectivity. Figure 3.33 shows a typical plot for the dependence of the solubility of a pharmaceutical intermediate on both the volume fraction (p of methanol in a mixture with water and the temperature. Generally, solubility increases with temperature and, in most cases, it increases for high fractions of methanol. 

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