Evaluation of portable X-ray fluorescence (pXRF) in exploration and mining: Phase 1, control reference materials

This paper describes a project sponsored by the Canadian Mining Industry Research Organisation (CAMIRO) to evaluate the strengths and weaknesses of portable XRF for use in mineral exploration and mining and to develop best-practice protocols in the analysis of rocks, soils, sediments and drill-core. Phase I focussed on the analysis of pulp control reference materials (CRMs) to determine the figures of merit, principally accuracy and precision, of the technique before introducing the confounding parameters associated with in-situ analysis such as heterogeneity, particle size and moisture. Five instruments (three handheld and two portable benchtop) from three manufacturers were used to carry out replicate analyses (n = 10) of a diverse suite of 41 CRMs, from barren granites, through soils and sediments, to ores. Standard factory calibration, in mining and soil modes, was used. The performance of the instruments was evaluated using x-y plots of results versus established element concentrations for the CRMs and the values of goodness of fit (r(2)), slope and intercept documented for both full and restricted concentration ranges. For many elements, the performance across the instruments varied markedly, as did the ability to correct for spectral interferences. Numerous interferences were encountered, particularly from the rare-earth elements (REEs) on transition elements, but also for well-known interference pairs such as Pb on As, Zn on Au, U on Mo, and Th on Bi. In general, major elements with the exception of the light element Mg were well determined, as were Mn and Ti. Sensitivity was inadequate for Cl and P; however, S could be measured with acceptable precision to c. 0.05% S. Performance for the trace elements was categorized as follows: very good for As, Cu, Nb, Pb, Rb, Sr, and Y (r(2)>0.9 for more than 1 instrument); good for Ba, Mo, Sn, Zn and Zr (r(2)>0.9 for 1 instrument); moderate for Cr, Sb, Se, Th and U (r(2) = 0.6-0.8); poor for Ag, Cd, Co, Ni and V; and very poor for Au, Bi, Cs, Hf, Hg, Pd, Sc, Ta, Te and W. Of the REEs determined (La, Ce, Nd, Sm, using the standard calibration by the manufacturer, i.e. not REE-specific), only La showed adequate sensitivity and precision (3-5% RSD), however, only at concentrations approaching c. 1000 ppm (50-100% RSD at La <100 ppm). Slopes of the best-fit lines, where r(2) >= 0.6, ranged from 0.5 to 5.0, indicating that calibration is required by the user for both the soil and mining modes. The precision, shown by 10 replicate readings, was excellent and usually better than 10% RSD except where close to detection limit or where major interferences were present. The beam time study showed that, in most situations, 60 s was a good compromise between productivity and precision but also highlighted cases of significant drift and a lack of improvement in precision with longer beam time. A study of the thin-film sample cover used for cups demonstrated that 4.0-mu m Prolene (R) is superior to the same thickness of Mylar (R) in both transmittance (especially for Mg, Al, Si) and contamination properties.