Field soil extraction for on-site trace metal and rare earth elements quantification

Using soil extractions in the field to solute metal ions for direct measurements and quantification would offer a fast efficient method of great interest for environmental monitoring and mineral exploration applications, namely deep penetrating geochemistry. In this study, a field extraction approach was made on boreal soils in Finland using two kinds of extractants: 0.01 M calcium chloride (CaCl2) and 0.43 M nitric acid (HNO3). CaCl2 is commonly used as an extractant of mobile and directly avalaible elements while 0.43 M HNO3 extracts reactive pool metals including a part of metals that are more closely bound to soil particles. These two extractants were chosen because they have (i) no specific constraints for electrochemical or spectrometric analysis in metal quantification, and are (ii) more environmentally friendly extractants than the ones often used for anomaly detection of Au mineralization, e.g. Aqua Regia (AR) or Ionic leachTM (IL, ALS). Fifty-five elements were analyzed with Inductively Coupled Plasma – Mass Spectrometer including major elements, trace metals and rare earth elements. In order to validate the efficiency of on-site protocols, field extractions on fresh soils (FIELD) were compared to standard laboratory extractions on air-dried soils (LAB).