Salting-out gradients in centrifugal partition chromatography for the isolation of chlorogenic acids from green coffee beans. J. Chrom. A Special Issue CCC2008

This article highlights two innovative CPC/CCC issues: 1) the use of inorganic modifiers in biphasic solvent systems, and 2) the use of gradients in CPC/CCC. Adding an acid to a system before solvent system equilibration typically has the effect of decreasing the solubility of organic acids in the aqueous phase. This paper is somewhat unusual in the respect that the pH modulation is not the issue, but instead the salts modify the non-pH dependant solubility characteristics of the aqueous (mobile) phase. The amount of method development relegated to these separations is truly impressive. First, a series of six salts at various concentrations was used to determine the partition coefficient of 5-caffeoylquinic acid in a ethyl acetate-water solvent system. In general, as the salt concentration increases the solubility of the organic acid in the aqueous phase decreases. A superficial way of thinking about this is that the salt makes the water more polar and drives out more of the organic acid. What is more, the nature of the salt made a significant difference in the corresponding partition coefficient. This effect was very handily explained by noticing that the ability of the salt to force the organic acid from the aqueous phase generally follows the Hofmeister series that describes the ability of an ion to attract water molecules. Second, lithium chloride was chosen as a reasonable salt and then tested in 6 different concentrations in 3 different solvent systems. The partition coefficients for 10 different chlorogenic acids were recorded for each solvent system / salt concentration combination. The addition of salts adds another layer of complexity to the already challenging task of selecting a solvent system based on combining 3 or 4 different solvents. I also wonder about the ease of desalting the products obtained. All in all, it furthers yet another interesting avenue of exploration for CCC/CPC.