ИСТИНА

Nowadays, a lot of important tasks of food, beverage, and pharmaceutical industry require the simultaneous determination of mono- and divalent carboxylic acids in the presence of inorganic anions. Suppressed ion chromatography (IC) with electrolytic eluent generation is the most convenient method for solving the above-mentioned task. However, due to the incomplete resolution of some carboxylic acids (e.g. glycolic/acetic/lactic/formic, malic/succinic, etc.) with modern commercially available anion exchangers, determination of full organic acid profiles in the real samples still remains one of the serious challenges, which defines the need for developing novel stationary phases for IC with improved selectivity toward organic acids. In this work we demonstrate clear algorithm, which can be used for manipulating and adjusting chromatographic properties of covalently-bonded hyperbranched anion exchangers in order to achieve the desired selectivity toward carboxylic acids. For revealing the general trends in selectivity shifts for both mono- and divalent organic acids, the set of 16 anion exchangers was prepared by repeating the modification cycles including alkylation with 1,4-butanediol diglycidyl ether and amination with various amines: primary and secondary alkyl- or alkanolamines, amino and aminosulfonic acids, di- and polyamines. The study of separate and joint effects of structure, hydrophilicity, and basicity of amino acids and other amines located in different positions in the functional layer allowed us to discover the dominating factors responsible for the separation of particular pairs of organic acids and to exclude the insignificant parameters from the consideration. The chromatographic study and comparison of the prepared anion exchangers was performed at different temperatures within the range of 30-50 °C, which also revealed considerable temperature effect on the retention and elution order of organic acids. The resulting algorithm for constructing functional layers is based on the cooperative effect of 3 general parameters: the type of amine in the internal and external parts of the functional layer, and the temperature of separation. In most cases using the established regularities allows one to shift selectivity for certain pairs of carboxylic acids significantly by changing just one of three parameters. Following the developed algorithm, we managed to achieve baseline resolution formic, lactic, galacturonic, and propionic) together with divalent (malic, succinic, oxalic, fumaric) and trivalent (citric, isocitric, trans-aconitic) ones in the presence of standard inorganic anions and oxyhalides. The obtained anion exchangers are superior to the commercially available stationary phases for suppressed IC in terms of resolution for carboxylic acids in real samples such as juice, wine, beer, etc.