Acid hydrolysis and high-performance liquid chromatography of xanthan

Abstract

A method for the routine determination of the hexose and organic acid content of xanthan preparations based on acid hydrolysis of the exopolysaccharide followed by high-performance liquid chromatography (HPLC) of the hydrolysate on two cation-exchange columns was investigated. A kinetic model was fitted to the recoveries of these components over a range of hydrolysis conditions using non-linear regression, affording values for the rate constants and the initial concentration of each compotent in the unhydrolysed polysaccharide. The ratio of glucose to mannose was greater than the assumed 1:1 ratio in unhydrolysed xanthan, suggesting that the polysaccharide may contain fewer terminal mannosyl residues than previously proposed. Quantification of glucuronic acid was complicated by its lactonisation and consequent elution as two peaks and by interference from its degradation products. The possibility of determining this component as its aldobiouronic acid [β- d-GlcA p-(1→2)- d-Man p] precursor was investigated. Acetic acid recovery was not significantly affected by the hydrolysis conditions over the range studied. The difference between the maximum recoveries and the fitted initial concentrations of each component indicate that compositional analyses of xanthan samples based on single time and acid strength conditions should be treated with caution as they are unlikely to yield quantitatively accurate values.