Complex formation of 2-(5-nitro-2-pyridylazo)-5-(N-propyl-N-sulfo-propylamino) phenol with lead, cadmium and manganese for their sensitive spectrophotometric detection in flow injection and ion chromatography systems

2-(5-Nitro-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (Nitro-PAPS) reacts with Pb(II), Cd(II) and Mn(II) in weakly alkaline media, yielding water soluble metal complexes having molar absorptivities of 7.6 × 104 (at λmax = 590 nm), 14.7 × 104 (λmax = 570 nm), and 10.7 × 104 (λmax = 574 nm) 1 mol−1 cm−1, respectively. A flow injection analysis (FIA) system utilizing these complexes was proposed for determining traces of Pb, Cd and Mn, which was successfully extended to ion chromatography of Pb, Cd and Mn as means of a sensitive post column reaction detection. Chromatographic separation of those metals with good baseline resolution was achieved by using a column packed with strongly acidic cation-exchange resin having high ion-exchange capacity with 0.15 M malic acid solution (pH 5.0) as eluent, which was directly coupled in-line with spectrophotometric detection at 574 nm using 6.0 × 10−5 M Nitro-PAPS as chromogenic reagent. The detection limits are 4.8, 1.9 and 1.5 ng ml−1 for Pb, Cd and Mn, respectively, with use of a 588 μl injection of sample solution. Relative standard deviations of < 3.2% were obtained for a synthetic sample containing each 50 ng ml−1 of metals in a mixture. Results for the analysis of a rice flour sample (NIES-CRM No. 10) after microwave digestion were in good agreement with the certified values for Cd and Mn.