Ultramicroelectrodes (UMEs) have had great impact on electrochemistry. A novel hydrodynamic UME has now been introduced where mass transfer to the electrode is significantly enhanced by the convective flow of a solution (1, 2). In the microjet electrode (MJE) (1), a jet of solution is fired at ∼ 100 m s^-1 from a nozzle (diameter ∼ 25-100 μm) onto a disc UME, typically Pt (diameter 25 μm). The MJE has well defined, variable and high mass transfer rates.

In the radial flow microring electrode (RFMRE) (2), solution flows from a nozzle, placed close to a planar substrate. A Pt ring UME (thickness ∼ 100-500 nm) is positioned around the capillary edge, followed by an epoxy resin layer, so only a ring of metal at the capillary end is exposed to solution. The Pt is applied as a paint. As fluid leaves the capillary, it is forced into the nozzle/substrate gap (∼ 5-40 μm) and flows radially past the ring electrode. At moderately low volume flow rates, the device has produced the highest steady-state mass transfer rate of any hydrodynamic technique (∼ 2 cm s^-1).

The high mass transport rates of the MJE and RFMRE have resulted in kinetic applications and possible uses in electrochemical flow systems. When coupled with hydrodynamic modulation voltammetry (HMV) higher detection limits and electrode stability in flow systems can be obtained (1b, 2b).

The MJE-HMV offers the lowest concentration detection limits in a flow system of any hydrodynamically-modulated technique. The mass transport rate is modulated by oscillating the jet to hit and miss the electrode surface. The technique can discriminate against background processes, and IrCl₆^3- at 5 x 10^-8 mol dm^-3 has been detected. The technique is being developed as an electrochemical detector for liquid chromatography.