The enzymatic determination of mercury and copper using acid urease. The effects of buffers

We report on a quick and simple test based on enzyme inhibition for the det ection of mercury and copper using free acid urease coupled to an optical s ensor system. Lipophilized Nile Blue was incorporated in plasticized poly(v inyl chloride) (PVC) to produce an ammonium-sensitive layer with a thicknes s of around 4 mu m. The layer was fixed on one side of a disposable cuvette . A solution of buffer, enzyme and heavy metals was placed into the thermos tated cell. Enzymatic hydrolysis was started upon addition of urea and the formation of ammonium was monitored. Mercury and copper were the strongest inhibitors; for this reason the inhibitory efficiency of these metals was e xamined in citrate, acetate and trismaleate buffers. The cuvette test was m ost sensitive and selective for mercury in a citrate buffer. The limit of d etection for mercury(II) ions was as low as 1 mu g/L. Copper ions do not in terfere because of complexation by citrate. The inhibitory effects of metal combinations on the activity of acid urease and the effects of optimum pH of the enzyme and the transducer on the dynamic range of the cuvette test a re presented.