Suppressed ion chromatography of inorganic anions and divalent metal cations with pyromellitic acid as eluent

Abstract

Suppressed conductimetric detection ion chromatography (IC) using pyromellitate eluent was investigated for the simultaneous determination of common inorganic anions and divalent cations. When pyromellitate and sample anions (Cl −, NO 3 − and SO 4 2−) in the eluent were introduced into a cation-exchange membrane suppressor in the H + form after anion-exchange separation, pyromellitate and the anions were converted to pyromellitic acid and corresponding acids (HCl, HNO 3 and H 2SO 4), respectively. Since these acids are stronger than pyromellitic acid (p K a1=1.92), these acids were detected conductimetrically as an increase of the concentration of H + in the eluent. In contrast, divalent cation–pyromellitate complexes (Mg 2+–, Ca 2+–, Sr 2+–, Ba 2+–, Fe 2+–, Mn 2+–, Co 2+–, Ni 2+–, Zn 2+– and Cd 2+–pyromellitate complexes) were converted to pyromellitic acid by the suppressor and these divalent cations were detected as an increase of the concentration of pyromellitic acid in the eluent. When using both 0.15 m M pyromellitate eluent at pH 5.4 and a 25-cm anion-exchange column (TSKguardgel QAE-SW), both highly sensitive conductimetric detection and excellent simultaneous separation of inorganic anions (Cl −, NO 3 −, I −, SCN −, SO 4 2− and S 2O 3 2−) and divalent cations (Mg 2+, Ca 2+, Mn 2+, Co 2+, Ni 2+, Zn 2+ and Cd 2+) were achieved in 30 min. The detection limits ( S/ N=3; injection volume, 20 μl) for the anions were 0.73–2 μ M and those for the divalent cations were 0.21–4.7 μ M.