Determination of anions using monolithic capillary column ion chromatography with end-to-end differential contactless conductometric detectors under resonance approach

Abstract

An end-to-end differential measurement approach with capacitively coupled contactless conductivity detection (C⁴D) was applied to anion-exchange monolithic capillary column ion chromatography. The column was prepared by thermally initiated radical polymerization of poly(glycidyl methacrylate) in a fused-silica capillary of 320 μm i.d. and modified by quaternary ammonium latex surface coating. Two C⁴Ds were placed near both ends of the capillary column and the output difference between them was measured. With 15 mM potassium hydrogen phthalate used as the eluent, good separation of a mixture of inorganic anions (F⁻, Cl⁻, NO₂⁻, NO₃⁻) was achieved. The detection limits of conventional C⁴D are 1.6, 0.28, 0.53, and 0.47 mg L⁻¹ for F⁻, Cl⁻, NO₂⁻, and NO₃⁻, respectively. To further enhance the sensitivity, the capacitive impedance from C⁴D was neutralized by an inductive impedance from a piezoelectric resonator. An increase in sensitivity by a factor of 7–8 was achieved in the resonating C⁴D in comparison with the conventional C⁴D. The detection limits of the resonating C⁴D are 0.23, 0.041, 0.065, and 0.059 mg L⁻¹ for F⁻, Cl⁻, NO₂⁻, and NO₃⁻, respectively. The response of the resonating C⁴D was analyzed based on an equivalent circuit model.