Carboxyl-converting titration versus other methods for the uronic acid content analysis of alginate oligosaccharide

This study presents a carboxyl-conversing titration method for determining the uronic acid content of alginate oligosaccharide (AOS). All C6 carboxyl groups on the monosaccharide residues of AOS were initially converted into carboxylates, which were subsequently transformed into carbonates through carbonization and incineration. Quantification was finally done through acid titration. The AOS content, which was represented by the amount of monosaccharide residues (ManA and GulA residues) in AOS sample, was calculated based on the stoichiometric relationship between the C6 carboxyl group and monosaccharide residue. To evaluate its performance, the carboxyl-conversing titration method was compared with the sulfuric acid-carbazole method and 1-phenyl-3-methyl-5-pyrazolone (PMP) pre-column derivatization high performance liquid chromatography (PMP-HPLC) method. The content of the uronic acid in AOS sample were detected 96.24 % ± 0.42 % using the carboxyl-conversing titration method, 186.85 ± 0.49 % (ManA/GulA = 1/1 as the reference standard) using the sulfuric acid-carbazole method, and 3.00 ± 0.08 % using the PMP-HPLC method. The carboxyl-conversing titration method demonstrated the highest accuracy of the content value. The validation of this method was also confirmed in the terms of specificity, limits of detection and quantification, recovery, repeatability, intermediate precision and durability. Compared to the sulfuric acid-carbazole and PMP-HPLC methods, the carboxyl-conversing titration method was proved easy to operate, reliable, and independent of reference standards. This innovative method offers a promising solution for accurately determining AOS content in laboratory and industrial settings.