of for example 150 u03bcm an external diameter of for example 180 u03bcm and a height of for example 50 m the reaction chambers 10 10 u2032 are filled for example with reaction starting material e g an enzyme substrate dissolved in an electrolyte 7 the electrolyte 2 being supplied to the individual reaction chambers via a supply volume 4 in fig2 the reaction chambers 10 10 u2032 can be closed off by a housing top part 5 by a mechanical ram 6 in the open state a supply volume 4 holding a liquid electrolyte is located above the cavities in fig2 the reaction spaces 10 10 u2032 as chambers which are open when the housing top part 5 is removed are filled with an electrolyte starting material 7 flowing through them the reservoir for the electrolyte 7 not being shown in detail in this figure after the reaction cavities 10 10 u2032 have been filled with electrolyte starting material 7 the housing top part 5 which may comprise for example a silicone membrane is placed onto the walls 11 11 u2032 which as mentioned above may be formed of polyimide by the ram 6 in this way the reaction spaces 10 10 u2032 are closed off so that mass transfer is then prevented in fig3 the lower region is of similar construction to that shown in fig2 in a particular embodiment which is not visible in the drawing presented in fig3 the walls.