Study of sugar acids separation by high-performance anion-exchange chromatography–pulsed amperometric detection using alkaline eluents spiked with Ba 2+, Sr 2+, or Ca 2+ as acetate or nitrate salts

Abstract

The influence of Ba 2+, Ca 2+ and Sr 2+ ions on the anion-exchange chromatographic separation of some carboxylated sugar acids such as d-gluconic acid, N-acetylneuraminic acid, muramic acid and d-galacturonic, d-mannuronic and d-glucuronic acids was investigated. The chromatographic technique was coupled with pulsed amperometric detection using gold as a working electrode. Since acidic carbohydrates are strongly retained on the anion-exchange column, acetate and nitrate as counterions were used to regulate retention within 30 min. The addition of alkaline-earth metal ions at a millimolar concentration to the alkaline eluents does impart a noticeable decrease in the retention, especially for N-acetylneuraminic and uronic acids. Complexation of these compounds with free divalent metal ions presumably occurs in the alkaline eluent. The extent of decreased retention is related to each divalent metal ion, and a good correlation was found between the retention modulus ( η= k′/ k o′) and the stability constant of each sugar–metal ion complex. As expected, calcium ion induced a slightly greater retention compared to strontium and barium ions, and this is consistent with the fact that alduronate–calcium complexes are relatively more stable ( η<1.00). We demonstrate also that upon the addition of Ba 2+ and Sr 2+ ions in the alkaline eluent, but the same cannot be claimed for calcium-containing eluents, there is a gain in sensitivity for all compounds investigated. The increment on the peak height when the column was eluted with NaOH spiked with Ba(NO 3) 2 was generally higher (5–75%) than that with Ba(OAc) 2.