New methodological approach for deep penetrating geochemistry and environmental studies, Part 2: field determination of Co(II)/Ni(II) and Pb(II)/Zn(II) of on-site soil extractions by electrochemical stripping techniques

In the framework of deep penetrating geochemistry for mineral exploration, we report a field electroanalytical method using disposable screen-printed working electrodes able to quantify metal pathfinders in partial extracts of soil samples at the nanomolar level on-site. Four metal pathfinders (Co, Ni, Pb and Zn) were chosen due to their geological interest and suitability of electrochemical detection. Square-wave anodic stripping voltammetry (SW-ASV) was developed for the simultaneous detection of Pb(II) and Zn(II) providing limits of detection (LOD) of 1.2 and 5.3 nM, respectively. A linear-scan cathodic stripping voltammetry (LS-CSV) methodology was developed for the joint detection of Co(II) and Ni(II) yielding LOD of 1.8 and 3.5 nM, respectively. Method validation was carried out on 14 upper B-horizon soil samples during a field survey. Two wet extraction procedures were applied for soil samples – a strong extraction using nitric acid (HNO₃) and a weak extraction using calcium chloride (CaCl₂). Excellent agreement between electroanalytical methods and inductively coupled plasma mass spectrometry (ICP-MS) was found for Pb(II), while a small underestimation was observed for Zn(II). For Co(II) and Ni(II) the LS-CSV measurements in the HNO₃ extractions were impossible due to the change from low pH to pH to 7.15 which induces precipitation of iron and aluminium. For the CaCl₂ extractions, the Co(II)/Ni(II) results obtained by LS-CSV were in excellent agreement with ICP-MS measurements.