A contactless conductivity detection cell for flow injection analysis: Determination of total inorganic carbon

New, very simple cells for contactless conductivity detection have been designed and tested in flow injection analysis. The cells consist of two insulated wire electrodes placed inside a PTFE tubing at a close mutual distance and oriented either across the tube or along the tube axis; the detection takes place within the space limited by the two electrodes. The electrodes are insulated by a 5 μm thick film of a polyimide (the dielectric). The electrodes with thin film of dielectric represent capacitors with higher capacitance which allows a higher ac current to flow between the electrodes, as compared to contactless conductivity cells used, e.g., in capillary electrophoresis, and the measuring sensitivity is thus enhanced. The cells were tested on the determination of total inorganic carbon in model water samples by the flow injection analysis combined with a gas diffusion separation. A concentration range from 0.01 to 1 mmol l−1 NaHCO3 was tested with a linear dynamic range of up to 0.1 mmol l−1. Detection limits of 0.6 and 6 μmol l−1 with RSD values of 4.1 and 4% were obtained for detectors with the electrodes placed axially and across the tube, respectively. A throughput of 15 samples per hour was attained under an experimental procedure used. The analytical parameters were compared with those obtained using a contactless conductivity cell with tubular electrodes placed outside the PTFE tubing.