Environmental Advances (Jul 2024)
Relevance of using portable X-ray fluorescence to identify gold hyperaccumulator plants
Abstract
Field Portable X-Ray Fluorescence (FPXRF) was investigated as an alternative analysis tool for the detection and quantification of gold in hyperaccumulator plants in the field. First, synthetic gold standards were prepared with materials chosen to best simulate plant material, using microcrystalline cellulose (mcc) or ash by adding known amounts of gold. Powder samples were obtained containing gold in amounts ranging from 10 to 1000 ppm of gold. They were analysed, as well as blank samples of mcc and ash, using a NITON xl3t GOLDD+ FPXRF spectrometer in AllGeo and Soil modes with an analysis time of 60 s. Results were compared to Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) analysis of the synthetic gold standards after digestion in aqua regia. Gold was detected and accurately quantified in synthetic gold standards containing more than 25 ppm of gold in AllGeo mode. At lower concentration, results were much more inaccurate, with no gold being detected at 10 ppm. In Soil mode, similar results to AllGeo mode were obtained with mcc synthetic standards. However, with ash synthetic standard gold quantification was very poor over the whole concentration range and gold was detected in the blank ash sample, which was confirmed as a false positive by the subsequent ICP analysis. Samples of Phyllanthus stipulatus, suspected gold hyperaccumulators, collected on gold-bearing sites in French Guiana were then analysed following the same protocol. The same false positive phenomenon was observed in some of them, with gold being detected by FPXRF in Soil mode but not by ICP analysis. Furthermore, analysis of the FPXRF spectra revealed that none of the characteristic gold emission ray was present. This false positive was attributed to an interference caused by zinc, present in significant amount in the ash of the synthetic standard and the collected plants.
