“…The nitrate exerts a catalytic effect on the reduction and reoxidation of thallium in cyclic and differential-pulse voltammetry, leading to a peak enhancement, hence NO 3 2 can be determined indirectly in the redox process of Tl(0) to Tl(I). Helms and Scholz 12 built a reactive electrode with a graphite solid ion exchanger and paraffin to determine Cd 2+ , Cu 2+ , Pb 2+ and Hg 2+ in aqueous samples, achieving detection limits of 10 27 mol l 21 , after 20 h of accumulation. In more recent work, Nistor and co-workers 13 improved the stability and selectivity of a tyrosinase biosensor immobilized in Eastman AQ, a polyester-sulfonic acid cation exchanger, to determine phenolic compounds, maintaining 80% of its initial activity after 42 d.…”