STUDY OF ELEMENT FINGERPRINTING IN GOLD DORÉ BY GLOW DISCHARGE MASS SPECTROMETRY

Transcription

1 STUDY OF ELEMENT FINGERPRINTING IN GOLD DORÉ BY GLOW DISCHARGE MASS SPECTROMETRY MICHAEL W. HINDS, Ph.D. 1 ELEMENT FINGERPRINTING In 1994, John Whatling* used laser ablation inductively coupled mass spectrometry (LA-ICP-MS) distinguish gold ore between different mine sites in Australia. Discriminated on basis of: Elements present or pattern of elements Concentrations at different ranges Major, high minor, low minor, high trace Extended to: Glass Lead bullets *Whatling, R.J., Herbert, H.K., Delev, D., Abell, I.D., Spectrochimica Acta, Part B, 1994, 49B, Session 5 (Continued) - Hinds 1

2 LASER ABLATION ICP-MS How it works: - Laser makes fine particles of the material which go into the ICP torch - Particles reduced to atoms and ions (Au and Au+) in the ICP torch - A small fraction of ions (Au+) make to the mass spectrometer to be detected - Matrix matched standards required for quantitation 3 ELEMENT FINGERPRINTING FOR Au DORÉ Element pattern information lost in the extraction and smelting process. Element pattern information may vary due to nature of the gold being processed: Alluvial vs Ore deposit LA-ICP-MS requires matrix matched standards for best quantitative results If no standards: precision varies and semi-quantitative results Glow Discharge Mass Spectrometry may be useful 4 Session 5 (Continued) - Hinds 2

3 GLOW DISCHARGE MASS SPECTROMETRY Used to measure wide concentration of element simultaneously (ppb - %) Used for a wide range matrices High purity metals (Au, In, Cd, Ga, Te,...) Semi-conductor materials (GaAs, GaP,...) Based on element sensitivity factors established by analysing a variety of known reference materials Excellent reproducibility and good accuracy Expensive ($475 for an analysis of 60 elements) Ottawa has two GD-MS at National Research Council 5 GLOW DISCHARGE MASS SPECTROMETRY Sample cleaned by high purity acids Sample placed into instrument via high vacuum port Sample negatively charged High energy Ar+ ions collide sputtering off atoms of the sample into high vacuum Material removed to clean the sample before data collection Slow sputtering rate used for data collection Sputtered atoms ionized in the glow discharge plasma (M+) Positive ions accelerated towards the detector via volt potential difference Magnets separate ions based on mass/charge ratio and continue on to the detectors 6 Session 5 (Continued) - Hinds 3

4 GLOW DISCHARGE SAMPLING 7 GLOW DISCHARGE - MASS SPECTROMETER DIAGRAM 8 Session 5 (Continued) - Hinds 4

5 GD-MS FACILITY NRC OTTAWA 9 SELECTED RESULTS 5 DIFFERENT AU DORE MINES Concentration, ppb (unless indicated) Mine S Fe Ni Cu Zn Ga Se Pd Cd Au Os Pb A % % % <1 0.2% B % % < C 0.2% % % < % D % 22% 3% % % E % % 4 7% Concentration Colour Legend Major 1 99% Minor 0.1 1% Trace ppm Low Trace < 500 ppb Not Detected 10 Session 5 (Continued) - Hinds 5

6 COMPLETE RESULTS 5 DIFFERENT AU DORE MINES Mine Li Be B C N O F Na Mg Al Si P S Cl K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Rb Sr Y Zr Nb Mo Ru Rh Pd Ag Cd In Sn Sb Te I Cs Ba La Ce Hf W Re Os Ir Pt Au Hg Tl Pb Bi Th U A B C D E Concentration Colour Legend Major 1 99% Minor 0.1 1% Trace ppm Low Trace < 500 ppb Not Detected 11 GD-MS FINGERPRINTING Potential to discriminate between different mined dore 60 elements x 5 different concentration bands = 3000 variables More variables with more concentration bands Advantages Very low detection levels (< ppb in solid) Reproducible Accurate Determines 60 elements Solid sample method no dissolution issues Disadvantages Destructive Must cut pin from the piece Material removed by glow discharge Expensive Low sample throughput 12 Session 5 (Continued) - Hinds 6

7 ELEMENTS NOT DETECTED Several elements not detected (or observed below detection limits) Be, F, Sc, Rb, Y, Nb, I, Cs, Ba, La, Ce, Hf, Re, Th, U Could some of these be added to fine gold as markers? Single element vs multiple elements Single element may be present by chance Several elements make a stronger element pattern Must be at very low concentrations to maintain gold purity 13 CONDITIONS FOR MARKER ELEMENTS Non-volatile (not Be, F, Rb, & I) Not dangerous (not Be, Th, & U) Melting temperature should be close to Au (1064 o C) Concentration must very low to maintain gold purity GD-MS determination < 3000 ppb or 3 ppm Must survive multiple melting and casting cycles 14 Session 5 (Continued) - Hinds 7

8 POTENTIAL MARKER ELEMENTS Element Symbol Melting Point, o C Comments Yttrium Y Scandium Sc 1541 Discolours on exposure to air Lanthanum La 920 Oxidizes rapidly in air Cerium Ce 798 Oxidizes rapidly in air Barium Ba 727 Easily oxidizes 15 FIRST SMALL SCALE MELT IN GOLD Concentration in ppm Element Target Determined Ce 20 9 La Sc Y Sc may not have mixed well in the melt Ba introduced as BaCl 2 lost during melt 16 Session 5 (Continued) - Hinds 8

9 SECOND LARGE SCALE MELT TRIALS Tracking Element Concentration in ppb Element Target Concentration 1 st Melt & Cast 2 nd Melt & Cast Ce La Sc Y CLOSING REMARKS Element Fingerprinting Potential of element fingerprinting of gold doré by GD-MS has been demonstrated Further study is required on a larger number of samples RCM has no plans to develop this further Marker Elements Elements not observed lead to the idea of trying to uniquely identifying refined gold Several elements were tested as marker elements Distributed at very low concentrations < 3 ppm GD-MS useful at determining these elements at ppb levels Still observed after two melt & cast sequences 18 Session 5 (Continued) - Hinds 9

10 ACKNOWLEDGEMENTS Dr. Ralph Sturgeon, NRCC, Ottawa Dr. Brad Methven, NRCC, Ottawa Rob Sargent, RCM 19 Session 5 (Continued) - Hinds 10