Sensing method based on impedance variation of minicolumn packed with cation-exchanger under electric field

Voltage-induced impedance variation of the minicolumn (i.d. 0.53 mm, length 2 mm) packed with cation exchanger was investigated to develop a sensing method. An aqueous sample solution containing the metal cations was continuously supplied to the minicolumn during the impedance measurement with the simultaneous application of both alternating current voltage (amplitude, 1.0 V; frequency, 200 kHz to 6 Hz) and direct current (DC) offset voltage (0.1 to 1.0 V). On a complex plane plot, the profile of the column impedance consisted of a semicircle (200 kHz to 100 Hz) and a straight line (<100 Hz), of which slope varied with the magnitude of the applied DC offset voltage (V(DC)). The slope-V(DC) relation depended on the kind of the metal cation and its concentration; in particular, the slope-V(DC) relations of monovalent cations (Na(+) and K(+)) and divalent ones (Mg(2+) and Ca(2+)) were significantly different. With the change in the concentration of minor divalent salt of MgCl(2) or CaCl(2) (60 to 140 microM) in the sample solution containing 10 mM NaCl, the slopes showed almost linear relationships between those with application of V(DC) = 0.1 V and 1.0 V both for magnesium and calcium additions. In the case of plural addition of both MgCl(2) and CaCl(2) to the solution, the data points in the slope(0.1 V)-slope(1.0 V) plot were located between the two proportional lines for single additions of magnesium and calcium, reflecting both the mixing ratio and net concentrations of the divalent cations. Thus, simulations determination of Mg(2+) and Ca(2+) can be attained on the basis of the slope(0.1 V)-slope(1.0 V) relation obtained by the impedance measurements of the minicolumn. Actually, the contents of both magnesium and calcium cations in the bottled mineral waters determined simultaneously using the proposed method were almost equivalent to those obtained by the atomic absorption spectrometric measurement.