Contactless conductivity detection of small ions in a surface micro-machined CE chip

A microchip is presented which is capable of CE separations and is built using exclusively thin film deposition techniques, fully compatible with microelectronics batch processing. Standard photolithography provides control of the spacing between electrodes used in conductivity measurement and overall channel geometry. Fluid channels are arranged as a double-T injector with a 50 μm offset at the arm intersection. The chip's performance was tested using concentrations of sodium chloride and calcium chloride ranging from 1 μM to 1 mM in a 5-mM MES/histidine buffer. Separations were performed by applying different voltages to reservoirs positioned at the four fluid channel openings. Conductivity detection was performed by applying a small AC voltage (1 Vrms) to the insulated electrodes positioned inside the fluid channels. A computer running LabVIEW controlled the AC signal generation, data acquisition and storage. Measurements indicated that the chip's detection limit was below 1 μM for both sodium and calcium cations.