Approaches to gradient elution in ion-exclusion chromatography of carboxylic acids

Abstract

A range of eluents has been examined with a view to determining which can be used to manipulate the selectivity of retention in ion-exclusion chromatography when applied to the separation of common water-soluble carboxylic acids. The column used was a Bio-Rad HPX-87H Organic Acids column and the eluents examined included water and dilute solutions of sulfuric acid, phosphoric acid, p-toluenesulfonic acid, methanesulfonic acid and benzoic acid. Both conductivity and ultraviolet detection were utilized. The use of water alone as an eluent gave poorly shaped peaks, whilst the remaining eluents gave satisfactory peak shape. The best performance was obtained using methanesulfonic acid as eluent. Studies on gradient elution in ion-exclusion chromatography were also undertaken. Three approaches to gradient separation were investigated. The first approach was to utilize a concentration gradient in which the eluent concentration was decreased over the run, thereby increasing the degree of solute ionization and thus solute retention times. This method proved to be of limited utility. The second approach involved increasing the amount of an organic modifier (acetonitrile) in the eluent, and satisfactory gradients were produced by this method. In the third approach, a varying concentration of -cyclodextrin was introduced into the eluent as a means of reducing the retention times of aromatic solutes through the formation of inclusion compounds with -cyclodextrin. Once again, satisfactory gradient separations were produced with this approach.