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 a 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-mu Autolab software. The repeatability of consecutive SWASV_D runs is good (0.5% at 4 mu g l(-1) for 30 s electrolysis; 28 at 0.4 mu g l(-1) for 60 s electrolysis; 5% at 0.06 mu g 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 H2SO4, evaporation to dryness and heating at 650 degrees C. The detection limit is about 0.5 nM (0.1 mu g 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.