Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Very small analytical masses (mg to ng) are dictated by modern analytical instruments, either because of their high sensitivity, analytical protocols or because of the small sampling volume of micro-beam techniques. But small sample masses can create problems when trace elements are major constituent in some minerals (e.g. PGE as PGM) and are irregularly distributed in samples or reference materials. Accidental inclusion or exclusion could change the spatial concentration realised simply due to spatial heterogeneity issues [1,2]. Such effects influence mass balance calculation and element budgets. Recommended minimum mass for reference materials is generally determined by analysing smaller sample mass until variance become unacceptable. Is there a way to determine minimum sample mass directly related to RM heterogeneity? And is it identical for all analytes, or unique for each analyte, in each sample? In order to systematically investigate these questions, pressed-pellets of PGE-bearing reference materials (CHR-Bkg, CHR-Pt+, MASS-1, MASS-3, WMS-1, WMS-1a) were analysed with an EDAX EAGLE III micro-XRF. Approximately 10 000 contiguous measurements were made with a beam of 50 µm covering a total area of about 25 mm2 . Up to 25 analytes were measured including precious metals. Reproducibility was determined by analyzing the same location 1 000 times. In order to express the empirical heterogeneity, Angle Measure Technique (AMT) was employed [2]. The AMT transform describe the elemental map complexity as a function of geometrical scale from local to global. AMT provides a quantitative measure of the empirical heterogeneity for each element, RM, or analytical sample. From such results a minimum analytical mass for each analyte is proposed to ensure that analytical results are representative all the way down to the scale of RM fields-of-view. The proposed technique offers the advantage of defining the effective minimum mass for each analyte with better elemental sensitivity by allowing for a more meaningful estimation of the global minimum analytical mass.

[1] Savard, Barnes & Meisel (2010) Geostandards and Geoanalytical Research 34, 281-291. [2] Huang & Esbensen (2000) Chemometrics and Intelligent Laboratory Systems 54, 1-19.