Stationary phase constant

To classify all stationary phases in order to simplify the choice of the column best adapted to a particular kind of separation problem. The chemical nature of the phases and polarities do not allow prediction of which column will be optimal for a given separation. For this, the behaviour of stationary phases with respect to several reference compounds should be examined, preparing the way toward stationary phase constants. 

The system of stationary phase constants introduced by Rohrschneider [282,283] and later modified by McReynolds [284] was the first widely adopted approach for the systematic organization of stationary phases based on their selectivity for specific solute interactions. Virtually all-popular stationary phases have been characterized by this method and compilations of phase constants are readily available [28,30]. Subsequent studies have demonstrated that the method is unsuitable for ranking stationary phases by their selectivity for specific interactions [29,102,285-287]. The solvation parameter model is suggested for this purpose (section 2.3.5). A brief summary of the model is presented here because of its historical significance and the fact that it provides a useful approach for the prediction of isothermal retention indices. 

From the obtained data, 10 compounds representing different classes of organic compounds were selected in order to test correlation between the various sets of stationary phase constants and the retention data at 120°C. 

Kersten, Poole, and Furton ° found that many ambiguities in the determination of the polarity with the McReynolds stationary phase constants are due to the use of n-aUtanes as the reference series, and estimated on 15 columns spanning a wide polarity range the 2-alkanones as the universal retention index markers to replace the w-alkanes which do not partition with polar phases. Ketones were also suggested by many authors as a good alternative series to -alkanes however, Mathiasson et al. found that, because of variation in retention volume with the amount injected, alkanols and 2-alkanones are unsuitable on both polar and non-polar columns. He suggested the use of alkylbenzenes as reference compounds, as these compounds seem to behave almost ideally on liquid phases of different polarities. 

Table 11.13 McReynolds Constants for Stationary Phases in Gas Chromatography...

Similar stationary phases Temp., °C McReynolds constants USP code... 

The constant is not a tme partition coefficient because of difference, — V, includes the soflds and the fluid associated with the gel or stationary phase. By definition, IV represents only the fluid inside the stationary-phase particles and does not include the volume occupied by the soflds which make up the gel. Thus is a property of the gel, and like it defines solute behavior independently of the bed dimensions. The ratio of to should be a constant for a given gel packed in a specific column (34). 

Thus, the larger the value of (K), the more the solute will be distributed in the stationary phase. (K) is a dimensionless constant and, in gas/liquid and liquidAiquid systems, (Xs) and (Xm) can be measured as mass of solute per unit volume of phase. In gas/solid and liquid/solid systems, (Xs) and (Xm) can be measured as mass of solute per unit mass of phase. 

In an attempt to explain the nature of polar interactions, Martire et al. [15] developed a theory assuming that such interactions could be explained by the formation of a complex between the solute and the stationary phase with its own equilibrium constant. Martire and Riedl adopted a procedure used by Danger et al. [16], and divided the solute activity coefficient into two components. 

It is seen that there is a good correlation between experimental and calculated values. The scatter that does exist may be due to the dead volume of the column not being precisely independent of the solvent composition. The dead volume will depend, to a small extent, on the relative proportion of the different solvents adsorbed on the stationary phase surface, which will differ as the solvent composition changes. A constant value for the dead volume was assumed in the computer program that derived the equation. 

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