Effect of Alcohol Modifiers

The effect of methanol and ethanol on the separations could possibly be explained by stronger solvation of the sample solutes in the mobile phase. However, this explanation becomes unlikely for an eluent containing only 5% 1-butanol, the remainder being water. 

Scott and Simpson studied the adsorption of ahphatic alcohols, aldehydes and carboxylic acids in binary mixtures with water by ODS-2 sUica [9]. They found that the distribution coefficient increases exponentially with the carbon number of the moderator. When using an aliphatic moderator having a chain length of four or five carbon atoms, the surface of a bonded phase could be completely covered with a monolayer. They stated further that the chromatographic characteristics of the surface could be changed by choosing appropriately active groups. 

Adsorption of a layer of alcohol on the polymeric resin surface is believed to explain the dramatic effects observed in our separations of carboxyhc acids. Butanol has the highest distribution coefficient of the alcohol moderators studied, and only a low concentration in the aqueous eluent is needed to coat the resin surface. Partitioning of the various solute acids between the predominantly aqueous eluent and the coated resin surface is much different from that in the case of an uncoated polystyrene surface. 

It was demonstrated that carbon dioxide or carbonic acid in a 1-butanol/water eluent equilibrates with the resin to form an adsorbed layer that is responsible for conversion of carboxylate salts to the molecular form. 

Adsorbed H2CO3 -1- RCO2 — Adsorbed RCO2 -1- RCO2H 

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The effects of alcoholic modifiers and pH on the chromatographic properties of the OVM CSP have been investigated using acidic, basic, and neutral solutes (103). A series of primary, secondary, and tertiary alcohols and pH s ranging from 3.5-6.0 were used in this study. The results indicate that both the shape and hydrophobicity of the alcoholic modifier affect retention (k ) and enantioselectivity (a). In general, an increase in the hydrophobicity of the modifier results in a decrease in k s and a s. However, this is not the case when (-butanol is the modifier, suggesting that the size of the alkyl moiety attached to the carbinol carbon also contributes to the chromatographic results. The pH studies indicated that... 

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