the electrodes in the array are divided into segments and can be made of the same or different materials however in order to function with selectivity the array must contain at least two different electrode materials electrodes can be made from platinum gold carbon vanadium iridium and the like centrally located in the core area 172 of the microelectrode array 106 is a platinum counter reference electrode 178 which has transversely situated leads 180 located at various points the leads 180 are situated approximately equidistant between the electrodes 170 projecting from the periphery of the core area 172 into the center thereof thus is provided a multielement array of microelectrodes in close proximity to each other but insulated from each other by suitable means in the embodiment shown in fig4 the length of the core area 172 is approximately 2 54 centimeters and the width of the connecting pad 176 array is approximately 2 03 centimeters the number of electrodes 170 or the number of different types of materials that can be present is limited only by the availability of physical methods for deposition and the resolution of the microlithographic technique the platinum electrode 178 running down the center of the core area 172 has a potentially versatile role.