Rotating sample system: An equivalent of a rotating electrode for microliter samples

Voltammetric experiments in sample droplets of 20 mu L volume were carried out with gold ring electrodes microfabricated on Pyrex substrates. The droplet studied was centered and kept in place by a hydrophobic ring deposited on the substrate around the ring electrode, which also ensured that each sample assume a semispherical shape. A small hole in the center of the substrate filled with an agar gel membrane served as a junction. A mild jet of humidified nitrogen gas directed tangentially at the droplet caused it to rotate at a high rate. Cyclic voltammetry and constant-potential electrolysis of potassium ferricyanide were used to characterize this rotating sample system and to calculate diffusion layer thickness. The results clearly demonstrate that rotating of a semispherical microsample placed above a stationary ring electrode with a mild gas jet can be as effective as a rotating electrode system. As a practical example, voltammetric stripping analysis of 20-120 pmol mercuric nitrate samples was performed with a calibration of good linearity (tau(2) = 0.988) and a time constant for exhaustive electrolysis of similar to 2 min.