Enhanced conductivity detection of common inorganic anions in electrostatic ion chromatography using water eluent.

Abstract

To enhance the conductivity detection sensitivity of common anions (Na-anions) in electrostatic ion chromatography (EIC) by elution with water, a conductivity enhancement column packed with strong acid cation exchange resin in the H-form was inserted between an octadecyl silane (ODS)-silica separation column modified with zwitterionic surfactant (CHAPS: 3-{(3-cholamidopropyl)-dimethylammonio} propanesulfonate) and a conductivity detector. Specifically, the Na-anion pairing is converted to H-anion pairing after the EIC separation and then detected sensitively by the conductivity detector. The effects of conductivity enhancement and suppression in the EIC by the enhanced conductivity detection were characterized for the common strong acid anions such as SO4(2-), Cl(-), NO3(-), I(-) and ClO4(-) and weak acid anions such as F(-), NO2(-), HCOO(-), CH3COO(-) and HCO3(-). For the conductivity enhancement effect in the EIC, it is found that the conductivity of measured for all strong acid anions (Na-anions) was enhanced according to the theoretical conductivity predicted for H-anions and that of the measured for weak acid anions was suppressed depending on their pKa of H-anions. For the calibration linearity in the EIC, the strong acid anions were linear (r2 = 0.99 - 1.00) because the degree of dissociation is almost 1.0 over all the concentration range and that of the weak acid anions was non-linear because the degree of dissociation decreased by increasing the concentration of the weak acid anions. In conclusion, the EIC by enhanced conductivity detection was recognized to be useful only for the strong acid anions in terms of conductivity detection and calibration linearity.