the redox mediator the accuracy of the method is further improved each of electrodes 5 6 has a predefined area in the embodiments of fig1 to 4 cell zone 4 is defined by edges 9 10 11 of the membrane which correspond with edges of electrodes 5 6 and by leading with respect to target area 7 edges 12 13 of the electrodes in the present example the electrodes are about 600 angstrom thick and are from 1 to 5 mm wide optionally both sides of the membrane are covered with the exception of the target area 7 by laminating layers 14 omitted from plan views which serves to prevent evaporation of water from the sample and to provide mechanical robustness to the apparatus evaporation of water is undesirable as it concentrates the sample allows the electrodes to dry out and allows the solution to cool affecting the diffusion coefficient and slowing the enzyme kinetics although diffusion coefficient can be estimated as above a second embodiment according to the invention shown in fig5 and 6 differs from the first embodiment by inclusion of a second working electrode 25 and counter reference electrode 26 defining a second cell zone 24 therebetween these electrodes are also spaced apart by less than 500 u03bcm in the present example second electrodes 25 26 are situated intermediate cell zone 4 and target area 7 in this embodiment the redox mediator is contained in the membrane below or adjacent to target area 7 or.