Determination of sub-nanomolar concentrations of lead by anodic-stripping voltammetry at the silver electrode

The determination of lead at the nanomolar level by square-wave anodic-stripping voltammetry (SWASV) without removal of oxygen has been extended to the sub-nanomolar concentration range. The detection limit has been lowered with the use of a method of differences (SWASV-D) to 0.05 nM (10 ng l^-1) at 60 s electrodeposition. In this method the analytical signal is the difference between the voltammogram of the sample and that obtained with no electrolysis, recorded sequentially. Measurements at the sub-nanomolar concentration level, including standard-addition plots, are performed in a rapid sequence enabled by the ''Project'' option of the GPES-μ Autolab software. The repeatability of consecutive SWASV-D runs is good (0.5% at 4 μg l^-1 for 30 s electrolysis; 2% at 0.4 μg l^-1 for 60 s electrolysis; 5% at 0.06 μg l^-1 for 60 s electrolysis). Hundreds of runs can be carried out without any pretreatment of the electrode. The high stability is attributed to a process that takes place during the electrodeposition step in a two-electrode cell: the silver counter/quasi-reference electrode generates silver ions that codeposit with lead at the Ag-RDE, thus ensuring a continuously renewed surface of the latter. The analysis of lead in rivers and drinking water has been performed. Surfactants distort the SWASV. In order to ensure surfactant-free solutions, the pretreatment of the samples included digestion with HNO3 and H2 SO4 , evaporation to dryness and heating at 65°C. The detection limit is about 0.5 nM (0.1 μg l^-1) and is limited by the purity of the reagents used in the digestion procedure. (C) 1999 Elsevier Science B.V. All rights reserved.