Comparison of analytical results for gold in vegetation with and without high-temperature ashing

Abstract

Two approaches to the analysis of vegetation for Au have been taken and their results compared. One method is most suited to the direct determination of Au in the dry form of the sample while the other, comprising simultaneous analysis for Pt and Pd, requires wet chemical digestion of the ashed sample. The widely employed procedure based upon instrumental neutron activation ana- lysis (INAA) of a dried 8-g pellet is evaluated with respect to analysis by inductively coupled plasma mass spectrometry (ICP-MS) following ashing at 875 °C and dissolution in HF-aqua regia. Samples were collected from an area of known Pd-Pt-Au mineralization at Ferguson Lake, Northwest Territories, Canada and comprise twig and leaf tissue of Labrador tea ( Ledum palustre ssp. decumbens) and dwarf birch ( Betula glandulosa). The two sets of data for 149 samples were statistically tested for bias, specifically to determine whether Au was lost on ashing at the high temperat- ure of 875°C. This was found not to be the case; indeed, a slight rotational bias was evident above 1 ppb Au (dry Weight) indicating higher results by 1CP-MS. This was probably due to differences in calibration strategies in the two analytical techniques and implies that one method should be employed consistently throughout a survey area. The larger control data showed superior precision by the ICP-MS method, suggested to be a reflection of the larger sampling weight and five-fold lower detection limit of 0.02 ppb Au. Both methods delineated the zone of mineralization well.