Coulombic determination of enzyme substrates in solution

Electrode system for coulometric determination of an electrochemically convertible analyte (A) in a liquid has a tubular working electrode having internal diameter 0.1-1 mm, of porous conductive material with the pores filled with electrolyte solution. The working electrode (including the electrolyte in the pores) is separated from the (A) solution inside the tubular electrode by a layer or membrane which is permeable to (A). Also claimed are: (1) a coulometric assay for a substrate in a solution, in which the sample solution is supplied by an automatic feed device through a capillary form enzyme reactor, the automatic feed is stopped until the enzymatic reaction of the sample solution (consuming or forming a secondary analyte) is complete, the converted solution is supplied by the feed device to a measuring electrode system as described above, the feed device is stopped, the secondary analyte is electrochemically degraded and the charge caused to flow is determined; (2) an alternative coulometric assay, in which sample solution is fed into a measuring cell provided with immobilised enzyme, the substrate is completely converted to a secondary analyte, the secondary analyte is electrochemically degraded in the same cell and the charge caused to flow is determined, and (3) a coulometric assay system consisting of: (a) a potentiostatic-coulometric measuring electrode system (3) as described above; (b) a capillary-form reactor (4), in which \- 1 enzyme or microorganism is immobilised, connected directly to the measuring cell and supplied with (A) solution from the other side; (c) an automatic feed device, located before or after the measuring cell/enzyme reactor combination, to feed (A) solution through the apparatus, and (d) a device for determining the charge caused to flow through the measuring cell.