The device of Claim 12 wherein the second reactant is selected from the group consisting of iodoacetic acid, oxidized 2-mercaptoethanol, chloroacetone, dichloro???acetone, methyl iodide, dibenzylsulfide, 2-hydroxy-???methyl-6-methoxy-1,4-benzoquinone and diiodo-4-pyridone-???N-acetic acid and the salts thereof or the second reactant is a chelating compound for the competing substrate, such as zinc