Application of a low impedance contactless conductometric detector for the determination of inorganic cations in capillary monolithic column chromatography

Poly(glycidyl methacrylate) cation exchange monolithic column was prepared in fused-silica capillaries of 320 μm i.d. by thermally initiated radical polymerization and utilized in capillary ion chromatography. With 15 mM methanesulfonic acid as the mobile phase, the separations of a mixture of inorganic cations (Li+, Na+, NH4+, K+) was tested by using a capacitively coupled contactless conductivity detector (C4D) and a low impedance C4D (LIC4D). The LIC4D is the series combination of a C4D and a quartz crystal resonator. At the resonant frequency of the series combination, the capacitor impedance from capillary wall was offset by the inductance impedance from the quartz crystal resonator. A minimum impedance was obtained in the impedance–frequency curve of the combination. The responses of the C4D and LIC4D were analyzed based on an equivalent circuit model. It was shown that the sensitivity of the C4D to the change in analyte concentration is rather poor due to the high ratio of the impedance from the capillary wall capacitor to the solution impedance. The LIC4D has the similar sensitivity as a contact conductivity detector but a much smaller cell volume. The on-column detection model was realized by LiC4D without preparation of optical detection window in monolithic column.