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

摘要

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~+, NH_4~+, K~+) was tested by using a capacitively coupled contactless conductivity detector (C~4D) and a low impedance C~4D (LIC~4D). The LIC~4D is the series combination of a C ~4D 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 C~4D and LIC~4D were analyzed based on an equivalent circuit model. It was shown that the sensitivity of the C~4D 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 LIC~4D has the similar sensitivity as a contact conductivity detector but a much smaller cell volume. The on-column detection model was realized by LiC~4D without preparation of optical detection window in monolithic column.