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

An end-to-end differential measurement approach with capacitively coupled contactless conductivity detection ((CD)-D-4) 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 mu m i.d. and modified by quaternary ammonium latex surface coating. Two C(4)Ds 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 (CD)-D-4 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 (CD)-D-4 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 (CD)-D-4 in comparison with the conventional (CD)-D-4. The detection limits of the resonating (CD)-D-4 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 (CD)-D-4 was analyzed based on an equivalent circuit model.