A voltammetric sensor developed for in situ trace metal analysis in natural waters is presented. It consists of an array of 100 mercury-plated, iridium-based microdisk electrodes, coated with a 300−600-μm-thick 1.5% agarose gel membrane. This membrane acts as a dialysis membrane by allowing the diffusion of metal ions and complexes and by hindering the diffusion of colloids and macromolecules. Chronoamperometry and square wave anodic stripping voltammetry (SWASV) have been used to characterize the diffusion of hexacyanoferrate(III), lead, and cadmium in the agarose gel. For these species, the diffusion coefficients have been found to be half of the diffusion coefficient in free solution, and the time necessary for complete equilibration with the test solution varied with the gel thickness in accordance with the theory and can be lowered to 5 min for a gel thickness of 300 μm. The same techniques have been used to demonstrate the efficiency of the membrane against fouling and convection. Pressures in the range 1−600 bar have been found to have no effect on the sensor response. In contrast, variations in temperature in the range 4−22 °C considerably affected diffusion and charge-transfer kinetics, the resulting currents obeying a simple Arrhenius equation. These results confirm the suitability of the voltammetric sensor for in situ analysis of heavy metals in natural waters.

The application of a novel voltammetric in situ profiling system (VIP System) for real-time continuous monitoring of Mn(II) in anoxic lake waters is described. The heart of the submersible voltammetric probe is a gel-integrated microsensor including either a single or an array microelectrode. The integration of a microelectrode in a gel is a novelty which protects its surface from fouling and allows its application for direct measurements in complex media. The main features of this kind of microsensors for in situ measurements in natural waters are discussed. Reliability and validity of concentration profiles obtained using the VIP System have been demonstrated by comparing in situ and on-field voltammetric data with those obtained using classical techniques. The advantages of combining the VIP System with classical techniques for environmental study are also illustrated. In particular, (i) specific measurements of the mobile forms (i.e., hydrated Mn 2+ and small complexes with size smaller than a few nanometers), the colloidal forms (a few nanometers to 1 µm) and the particulate forms (>1 µm), which are three key fractions for understanding Mn biogeochemical cycles as well as (ii) the discrimination between seasonal and temporal variations can be performed with minimum sample handling and analysis time.

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