Retention activity coefficient ratio effect

Activity coefficient ratio [Eq. (18a)] accounting for effect of solvent-solute interactions on retention (apart from hydrogen bonding)... 

Another problem, that of adsorption at any of the several interfaces (such as gas-Uquid and gas-solid), may arise with minute samples. Martin found gas-chromatographic retention volumes to be markedly affected by such adsorptions and in some cases to be the cause of a larger contribution to retention volume than solution in the bulk stationary phase. He both measured and calculated the extent of such adsorptions for a variety of solutes. Adsorption at a gas-liquid interface is serious for polar stationary phases when surface areas are relatively high, when the ratio of stationary phase to solid support is low, and when the temperature is low. The effects of adsorption are minimized by the use of relatively highly loaded columns and nonpolar stationary phases and by avoiding solute-stationary phase pairs in which the infinite-dilution activity coefficients deviate markedly from unity. Ottenstein indicated that adsorption on a liquid surface can be considered negligible in packed columns when liquid loadings exceed 5% and activity coefficients are less than 10. 

It is therefore very important, in g.l.c. work of this nature, to determine retention volumes on a number of columns of different solvent/solid support ratio. This will either point to the non-existence of surface effects or the results can be used to determine a retention volume at zero mean column pressure, K r, and hence an activity coefficient, which is independent of surface effects. 

The HPLC technique has certain advantages that allow partition coefficients to be determined relatively rapidly in comparison to the conventional shaking-flask method. The elution process also makes it feasible to run mixture of solutes and yield results relatively free of solute-solute or solute-impurity interactions with a column giving good separation. Basically, the precision of the method depends on whether the value of yw/7o solute in the octanol-water system [Eq. (5)] can be effectively simulated by the column composition. It is therefore important that efforts be expanded to ensure that the properties of the column environment accurately reflect the bulk solvent-water (e.g., octanol-water) characteristics. If the solute can truly be equilibrated between mobile and stationary phases during elution, its retention time is determined by the ratio of the activity coefficients (i.e., partition constant) in the mobile and stationary phases. When octanol and... 

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