A flow cell for on-line monitoring of metals in natural waters by voltammetry with a mercury drop electrode

A cell was developed to enable flow analysis with voltammetric detection using a hanging mercury drop electrode (HMDE). The flow cell has a bore of 0.7 mm in which the mercury drop is centred, and has ports for a reference and a counter electrode, and for the capillary of the HMDE. The mercury drop has a diameter of 0.46 mm and a surface area of 0.67 mm2. Approximately 60 measurements are carried out per hour, a ten-fold increase compared with automated analysis using a conventional voltammetric cell. The flow cell is combined with on-line UV-digestion to destroy organic matter, a peristaltic pump to carry sample and reagents, on-line de-oxygenation, and a three-way valve for standard additions to produce a system for on-line metal analysis. The system was tested using on-line determinations of cobalt during a cruise in the North Sea. The sensitivity of the system was intermittently calibrated under on-line conditions by switching to a reagent stream with added metal. The distribution of cobalt along the coast was correlated to geographical position and salinity, and possible sources of the metal could be evaluated. The possibility of integration with other high resolution measurements illustrates the advantage of on-line monitoring of trace metals by flow analysis with detection by cathodic stripping voltammetry (CSV).