electrolyte may be water glycols glycerine and mixtures thereof as is well known in the art fig2 shows a membrane in cross sectional detail the nonhomogeneous membrane has a thin very dense layer 21 a less dense middle layer 25 and a thick least dense or porous layer 22 which layers together form the composite structure the enzyme shown symbolically by dots is dispersed uniformly in the thick portion or strata of the membrane however some of the enzyme may diffuse into the thin layer or middle layer during preparation of the membrane before the solvent for the cellulose acetate has evaporated membrane surface 24 is in electrical contact with the electrode the membrane comprises the nonhomogeneous combination of the three layers and the enzyme the outer free surface of which 23 represents the test surface which is to be brought into contact with the test sample to be analyzed in the preferred embodiment the inner surface 24 which is an electrical contact with the electrode is about 65 microns in thickness the middle layer is about 23 microns in thickness and has an average pore diameter of about 15 nanometers and the outer layer in contact with the sample to be analyzed is about 2 microns the overall.