Gold fingerprinting by laser ablation inductively coupled plasma mass spectrometry

Spectrochlmrca

Pergamon

Gold fingerprinting

by laser ablation inductively spectrometry

Arm. Vol 499. No 2. pp. 205-219. 1994 Copyright 0 1994 Elsevter Sctence Ltd Prtnted tn Great Bntatn All rights reserved 056&8547/94 56 00 + Ml

coupled plasma mass

R. JOHN WATLING,* HUGH K. HERBERT Mineral Science Laboratory,

Chemistry Centre (WA), 125 Hay Street, East Perth, Western Australia 6004

and DIANNE

DELEV

Research Centre for Advanced Mineral and Materials Processing, University of Western Australia, Nedlands, Western Australia 6009

and IAN D. ABELL Fisons Instruments,

Ion Path, Road Three, Winsford, Cheshire CW7 3BX. U.K.

(Received

14 May 1993; accepted

30 September

1993)

Abstract-Laser ablation inductively coupled plasma mass specrrometry (LA-ICP-MS) has been applied to the characterization of the trace element composition “fingerprint” of selected gold samples from Western Australia and South Africa. By comparison of the elemental associations it is possible to relate goid to a specific mineralizing event, mine or bullion sample. This methodology facilitates identification of the provenance of stolen gold or gold used in salting activities. In this latter case, it is common for gold from a number of sources to be used in the salting process. Consequently, gold in the prospect being salted will not come from a single source and identification of multiple sources for this gold will establish that salting has occurred. Preliminary results also indicate that specific elemental associations could be used to identify the country of origin of gold. The technique has already been applied in 17 cases involvinggold theft in Western Australia, where it is estimated that up to 2% of gold production is “relocated” each year as a result of criminal activities.

1. INTRODUCTION GOLD mining in Australia dates from the middle of the last century with the discovery of gold near Bathurst in 1851. In the following decade, Australia produced, on average, 40% of the World’s gold. The economic importance of the metal, to the Australian economy, decreased from the early 1900s with the industry at its lowest ebb in the 1970s. However, since the 1980s gold production has increased to 5.4 million ounces (1991), with an average return of approximately $A447 per ounce. This represents an annual income of approximately $A2.5 billion. Associated with this increase in gold production, an increase in gold theft has occurred and it is now estimated by the West Australian Police Gold Stealing Detection Branch and the Perth Mint, that up to 2% of this gold “goes missing” in direct theft alone. Direct theft of this nature is not restricted to Western Austalia, but is a worldwide phenomenon. In addition, traumatic theft from banks, bullion repositories, mines and private individuals constitutes a significant world-wide criminal activity. Unlike precious metal artefacts, the provenance of which can be traced and documented, gold itself can be melted down and transported without losing its value and as such becomes both a national and international “ghost” currency. Faced with this ever increasing problem, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was investigated as a technique to fingerprint gold on the basis of its trace element signature. This technique facilitates the identification of element assemblages and isotopic abundance changes at analyte concentrations in * Author to whom correspondence

should be addressed. 205

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the pg/kg region. LA-ICP-MS has already been used successfully for the analysis of minerals [l-5], geological materials [6-81 and a variety of other solids [9]. Because the detection limits associated with the technique are so low, the number of elements that can usefully be employed to provide data to establish the fingerprint is much higher than any method previously available. This aspect increases the number of useful inter-element combinations and associations possible and thereby increases the probability of providing a unique fingerprint. In addition, the laser spot size is approximately 100 km in diameter and approximately 150 Frn deep which means that it is usually possible to retain a significant proportion of the sample for subsequent forensic examination and corroborative analyses. The concept that trace element associations represent unique mineralizing events is not new; in fact, it is the basis of exploration geochemistry. However, detailed forensic geochemical investigations of gold and other minerals have not previously found application. In Western Australia, there is a wide diversity of Archaen gold deposits hosted by an equivalent spectrum of rock types in differing structural settings. The host rocks largely control the precise geometrical and mineralogical form of wallrock alteration and mineralization. The deposits can be classified broadly on the basis of structural style into four main groups: (a) alteration haloes, commonly with quartz vein systems in brittle ductile shear zones; (b) quartz vein systems or quartz stockworks in extension fractures or hydraulic fracture arrays; (c) persistent, large discrete quartz veins, commonly laminated, in fractures and/or shear zones; and (d) deposits confined to specific sedimentary horizons. Petrological, isotopic, geochemical and fluid inclusion data are compatible with a metamorphic replacement origin of the Western Australian Archaen gold deposits [lo]. The metamorphic fluids are considered to form deep in the greenstone pile during regional metamorphism as a result of dehydration, decarbonization and desulphidation of predominantly mafic and ultra-mafic volcanics. The transient fluids generated at the boundaries of reacting phases are particularly effective at extracting low concentration species such as gold if the fluid chemistry is appropriate [ll]. Based on a knowledge of the ore emplacement chemistry of the gold in Western Australia, it was logical to assume that the elemental composition of gold from a specific mineralizing event would be unique and would reflect the geochemical dynamics of the ore fluid generation, migration and depositional characteristics that led to the production of the deposit. 2.

APPARATUS AND

TECHNIQUE

2.1. Instrumentation A VG Turbo Plus Inductively Coupled Plasma Mass Spectrometer with VG LaserLab accessory was used for the experimental work. The LaserLab is built around a 500 mJ pulsed Nd:YAG laser operating at a wavelength of 1064 nm. The system was used in the free running mode for all experiments as this allowed the smallest crater size to be formed. 2.2. Laser optimization The use of LA-ICP-MS could imply a degradation in precision of results, when compared with equivalent solution ICP-MS data, simply because of variations in volatilized sample mass between samples. This would be of serious concern if the fingerprinting technique required the production of quantitative data. However, it must be remembered that the technique of fingerprinting gold does not rely on producing classical quantitative data but on the presence, association or lack of association, of specific suites of analyte elements. Simple dilution of a gold sample in pure gold would reduce element concentrations but would not remove or change the fingerprinting element association. Consequently, quantitative data are not as important as they are in more conventional analytical methodologies.

Gold fingerprinting

Fig. 1. Photomicrographs

by LA-ICP-MS

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of laser craters in (a) gold bullion and (b) crystalline natural gold.

Significant variation in the characteristics of the physical matrix exists between gold samples. Samples of gold bullion (Fig. l(a)) facilitate laser coupling far more readily than samples of more porous crystalline gold (Fig. l(b)) and some other naturally occurring golds. Because of this, laser coupling to the surface of the sample, and hence the quantity of material ablated, will vary. Consequently, it is again stressed that quantitative analysis, of a wide variety of gold types, is not practical at this stage and is not required for fingerprinting gold because the association of elements, and not their actual concentrations, is used to establish the fingerprint. The concentration of gold in nugget and bullion samples submitted for fingerprinting can vary between 50 and 99.99%. Consequently, even if the mass of material removed under a uniform set of ablation conditions was the same for different gold samples, the magnitude of

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the gold argide peak would vary significantly and invalidate its use as an internal standard. The amount of material removed also varies depending upon the major contaminating elements. Under the same ablation conditions, a nugget rich in mercury will be significantly more volatilized than one containing nickel or iron. Detailed studies have also established that there can be selective volatilization of elements. Therefore it is not practical to use an internal standard for this type of analysis. Nonetheless, it is necessary to establish an optimum compromise series of parameters for laser ablation which ensure that a robust comparison mechanism for different samples of gold is achieved. The two variables of major significance, which were chosen for this investigation, were laser power and repetition rate of laser shots. Four gold nuggets, with significantly different concentrations of zinc, copper, iron and silver, were used to test the effect of varying these parameters on analyte signal. Laser power was varied between 600 and 1000 V and repetition rate between one and five shots per second for one second ablation at each specific site. A total of ten sites, across the surface of each nugget, was used for each experiment. The study was repeated on six separate occasions and results averaged to produce response intensities for each analyte. The response characteristics of 40 analytes were investigated. Results indicate a trend in the response that is considered to be related to differences in the melting and boiling characteristics of the analyte element. Increasing the frequency of laser shots causes progressive ablation of an increasingly liquid pool of gold and a corresponding increase in the size of particles being removed. Removal of these large particles increases the number of loci for attachment of smaller particles that would otherwise be transported to the plasma for ionization. This recombination phenomenon occurs within the laser cell and the 2 m of interface tubing between the laser cell and the plasma. In this way material is removed from the gas stream and deposited more effectively on the interface tubing. This artificially reduces the apparent concentration of the analyte under investigation. The relationship between laser pulse voltage and shot repetition rate is shown in a representative group of elements in the histogram plots in Fig. 2. In general, the lower boiling elements, exemplified by germanium, tellurium and copper (Fig. 2(a)-(c)), all show peaks at a laser shot frequency of between 2 and 3 Hz for each laser voltage used. However, the higher boiling elements, exemplified by osmium (Fig. 2(d)), d o not show a response peak even at loo0 V and 5 Hz. Additional experiments using a laser shot frequency of 10 Hz and 1000 V power, still did not cause the osmium response curve to peak. When establishing the optimum compromise set of ablation parameters, it is necessary to ensure that variations in response, using changed conditions, are as small as possible. This will increase the robustness of the methodology and ensure that variations in the fingerprint are minimal when the analysis is undertaken on different instruments. Spot size should be as small as possible, while still providing enough material for analysis. In this way small samples can be ablated under the same conditions as large ones thereby maintaining the relevance of inter-comparison of the fingerprint. Physical recombination of material in the vapour cloud, generated by the laser ablation procedure, should be kept to a minimum. Consequently lower laser power and reduced shot numbers are to be preferred. From the results obtained it was decided to adopt an experimental protocol of two laser shots per second per sampling site with a laser power setting of 900 V. Where possible, samples may be ablated at a number of sites per sample; however, it may only be possible to use one site when dealing with very small samples. Acquisition time for each fingerprint is usually set at a minimum of 120 s. In this way it is possible to use the multichannel analyser to watch the fingerprint accumulate and to move the sample and ablate different spots, thereby ensuring better precision and providing a more intense spectrum for comparison. In all samples analysed to date it has not been found necessary to ablate more than ten sites to obtain a suitable fingerprint for identification purposes. During these preliminary experiments the AuAr+ molecular ion peak at 237 amu was always observed. The use of this molecular

Gold

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Fig. 3. Varieties of sample preparation: (a) drilled gold bar; (b) gold drillings; (c) drillings mounted on plastic support.

ion, to provide internal standardization, was considered. However, because of the varying amount of coupling of the laser with different types of free gold and the lack of relationship between the volatilization characteristics of volatile and non-volatile elements, its use added nothing to the data and internal standardization was not considered further. 2.3. Sample preparation Sample preparation for this type of analysis can be as simple or as complicated as the analyst requires. In general, provided the sample fits conveniently into the sample cell, an analysis can be performed directly on the original material with no preparation. However, samples are generally either very small (grains) or too big (gold bars) for direct analysis. Larger samples are usually drilled (Fig. 3(a)), using a cobalt steel drill bit, and a suitable sample (Fig. 3(b)) mounted on a plastic block using cyanoacrylate glue (Fig. 3(c)). In the case of small samples they can be mounted directly on the

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plastic block (Fig. 3(c)). Up to 20 samples are mounted in a clockwise manner on a single plastic base. In this way it is often possible to mount all the samples from a single case on one sample mount and to undertake their analysis without removing the sample from the cell, thus saving time and maintaining identical cell dynamics. There is no need to prepare optically flat samples, as is the case with glow discharge techniques. This saves time, reduces the equipment needed for sample preparation and minimizes contamination. In cases of gold salting, where small gold grains can be panned out of the sample, such grains are mounted in a small hole in the plastic block, which is subsequently back-filled with more quick-setting plastic. The block is then polished to expose a surface of the grains of interest. Samples prepared in this way are also suitable for additional investigation using scanning electron microscopic energy dispersive X-ray and/or electron-microprobe analysis. 2.4. Optimization The instrument operating parameters were optimized using NBS Glasses 610, 612 and 614. The choice of this material and not gold is because optimization requires the production of a steady state signal and continuous ablation of gold coats the inside of the laser cell and contaminates the system for a considerable time. The fact that glass and gold are not similar matrices and that plasma loading will be different does not matter as the technique detailed in this paper is not based on the quantitation of analytes, merely their association. The use of NBS glasses has proved to be robust and convenient and facilitates reproducible instrument optimization. Once optimization had been achieved, two in-house reference gold samples were also analysed to provide a cross-reference comparison with previously obtained data. The data thus produced facilitated maintenance of long-term quality control and ensured inter-comparability of fingerprint data. Because the technique was not designed to provide quantitative data, it is only necessary to confirm comparability of the element associations within the sample with data from previous analyses of the same sample prior to undertaking the analysis of a new series of samples. This cross-calibration procedure usually takes approximately 30 min.

3. RESULTS 3.1. Comparison of the fingerprint It must be remembered that these data are often required for presentation to a jury composed of lay members of the public. Members of the jury usually have no scientific training and often cannot comprehend the subtleties of scientific evidence. The form of evidence presented by using LA-ICP-MS scans requires them only to visually compare patterns, an exercise that the human brain excels at with no training. Consequently, the implication of obvious differences and similarities between samples are easily recognized and assessed by jurors. Individual sections of the mass scan can then be selected for detailed comparison. An indication of the variability of the trace metal signature for gold, and hence its uniqueness, is shown by the comparison of spectra in Fig. 4(a)-(c). Here, the elemental associations of native gold from three Australian mines are compared. The vertical scale on these spectra, which is a rough indication of the concentration of each element in the sample, is set to 1500 counts to facilitate visual comparison. Providing that the spectra are graphically produced on the same vertical and horizontal scales, visual comparison of the figures is totally adequate and valid to distinguish between gold samples. Detailed elemental comparisons are unnecessary until the patterns of element distribution are similar. There are quite obvious and considerable differences in the associations of the elements with gold from Mine 1 containing high levels of cadmium (106, 108, 110, 111, 112, 113, 114 and 116 amu), gold from Mine 2 containing zinc (66, 67, 68 and 70 amu), tellurium (122, 123, 124, 125, 126, 128 and 130 amu), some lead (204, 206. 207 and 208 amu) and bismuth (209 amu) and gold from Mine 3

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containing chromium (50, 52, 53 and 54 amu), high lead (204, 206, 207 and 208 amu) and no bismuth (209 amu). The mass scan region 62-65 amu inclusive has been skipped because the excessively high levels of copper in all samples would produce an ion beam that would saturate the detectors and result in non-acquisition of data. It may be expected that native gold would have a much more easily identifiable elemental signature than gold bullion. However, a recent criminal investigation highlighted the potential of the technique also to distinguish between bullion samples. A large number of gold samples were submitted for analysis by police after an investigation. After establishing their elemental fingerprint, it was apparent that the samples could be subdivided into two groups (Fig. 5(a)l-5 and 5(b)l-5). The gold from both groups contained significant levels of copper, silver, lead and bismuth, and

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Fig. 5. Comparison of the elemental fingerprint of two groups of bullion samples confiscated during the same police investigation.

the mass regions containing these elements had to be removed from the scan program to avoid ion saturation of the analogue detector. However, Group A gold contained significantly higher zinc (Fig. 5(a)2) and had significantly higher levels of palladium (102, 104, 105, 106, 108 and 110 amu) and cadmium (106, 108, 110, 111, 112, 113, 114 and 116 amu), (Fig. 5(a)3). Group B gold had elevated levels of tin (112, 114, 115, 116, 117, 118, 119, 120, 122 and 124 amu) and barium (130, 132, 135, 136, 137 and 138 amu), (Fig. 5(b)3), the rare earth elements (138 to 176 amu), (Fig. 5(b)3 and 5(b)4), tungsten (180, 192, 183, 184 and 186 amu), (Fig. 5(b)4), mercury (196, 198, 199, 200, 201, 202 and 204 amu) (Fig. 5(b)4 and 5(b)5), uranium (235 and 238 amu), and thorium (232 amu), (Fig. 5(b)5). Simple visual inspection of the samples gave no

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Scale 1000 Fig. 6. Location

map of Western

Km

Australian gold mining areas: (A) Murchison Eastern Goldfields.

Goldfields;

(B)

indication of the underlying difference in the provenance of the gold. However, by reference to an in-house database it has been possible to identify the approximate area of origin of the gold and to provide additional information that was invaluable in obtaining a conviction. Trace element associations indicated that the group of gold samples, represented by spectra in Fig. 5(a)l-5, came from the Eastern Goldfields (Fig. 6), while those gold samples, represented by spectra in Fig. 5(b)l-5, came from the Murchison Goldfield (Fig. 6). However, the differences between gold from different mines are not always as marked. Elemental spectra of free gold from two mines (Mines 4 and 5), that have relatively similar profiles are shown in Fig. 7. From their gross spectra (Fig. 7(a) and (b)), it would be extremely difficult to distinguish between the two mines as they both have a mass spectrum containing elements of low, medium and high mass. However, if specific regions of their detailed spectra are compared (Fig. 7(c)-(f)) it is clear that Mine 4 gold has significant antimony (121 and 123 amu), high concentrations of thallium (203 and 205 amu), and bismuth (209 amu), while the gold from Mine 5 has insignificant levels of all these elements (Fig. 7(d) and (f)). This is an example where use of detailed spectral material can confirm differences in gold samples that are not apparent from gross mass spectral information. An indication of the potential of the technique to distinguish between gold from different continents can be illustrated by comparing the mass spectra of a naturally occurring gold sample from South Africa and an equivalent Australian gold (Fig. 8).

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and South African gold

The South African gold has significant concentrations of palladium (102, 104, 105, 106, 108 and 110 amu), ruthenium (96, 98, 99, 100, 101, 102 and 104 amu) (Fig. 8(d)), platinum (190, 192, 194, 195, 196 and 198 amu), osmium (184, 186, 187, 188, 190 and 192 amu), and iridium (191 and 193 amu) (Fig. 8(f)). The Australian gold has none of these elements (Fig. 8(c) and (e)). While recent studies have found one area in Western Australia, which has both palladium and platinum in gold, to date no ruthenium, osmium or iridium have been found in gold from Australia. Further studies of the South African gold, using a laser power setting of 1000 V and a shot frequency of 10 Hz per site, have been able to improve both osmium and iridium intensities while equivalent studies in Australian golds have not been able to find any that have significant concentrations of either of these two elements. 3.2. Reproducibility of the fingerprint with time Initial experiments were undertaken to establish the reproducibility of the technique. Four separate gold samples, taken from bars produced from the same bullion pour, were analysed and the results compared (Fig. 9(a)-(d)). As can be seen from the replicate results, there is excellent agreement for trace element distribution in the four gold samples. These results give clear evidence of the reproducibility of the technique

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and the acceptability of the technique in acquiring accurate data for comparative purposes. For the gold fingerprinting technique to provide robust analytical data, it is essential that the fingerprint of a particular source of gold varies little with time. To test this requirement, two samples of gold bars from a specific Western Australian mine, poured three weeks apart, were analysed. Detailed comparison of the mass spectra from these bars confirms beyond doubt that the fingerprint has been maintained, even on the detailed level, throughout this time. An additional two reference samples of gold held in the State’s Mineral Collection, which were taken from bars poured at the same mine two years apart, were also obtained and analysed. The spectra obtained are almost identical and strongly indicate that the signature of the two samples remained almost identical over this period. Obviously, element associations can and will change if the mine is mining a different ore body or if it is blending ore material from other mines. In these cases, some form of multi-component computer chemometrics package will be required to unravel the

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mix. Work on developing a package to undertake continuing in the Mineral Science Laboratory.

this statistical deconvolution

is

4. DISCUSSION In Australia, as in other gold producing countries, stolen gold represents a major loss of revenue to producing companies, dividends to investors, tax to Government, loss of bullion sales and hence loss of foreign revenue. More dramatically, gold theft is illegal and can result in death or injury to innocent bystanders and consequently requires discouragement. However, until now, identification of the provenance of gold, and thereby assistance to the authorities in criminal investigations, has remained largely impossible. Studies so far have been able to conclude that it is possible to identify the provenance of different gold samples on the basis of their unique trace element associations or fingerprint. The use of LA-ICP-MS facilitates collection of trace element data and identification of a particular piece of gold. The method is quick and relatively simple although interpretation of spectra requires significant experience. Ultimately, the identification of a particular piece of gold will make use of a computerized database of analysed gold and computer based comparison of mass spectra. To date, in Western Australia, there is only a graphical hard copy database of gold from approximately 60 mines and 40 bullion samples. At present, the only way to compare spectra is visually and while this is still possible for the limited number of samples available, it is becoming more and more difficult as new spectra are added. At present this work indicates that a fingerprint from gold samples is representative of both gold from a specific mine and of gold from a specific mineralizing event which is being mined at the mine. This aspect is of considerable importance since, if the fingerprint is representative of a mineralizing event, it could be identical for closely associated mines. However, it is likely that minor differences in trace element association will provide definitive identification of gold from individual closely associated mines and this will be the subject of future detailed investigations. To date some 17 cases involving possible gold theft have been handled by the Mineral Science Laboratory and the case load is increasing as positive results are obtained. One further aspect not dealt with in this paper is the impact such an analytical technique will have on the fingerprinting of artefacts. Initial investigations, using antique silver, gold and platinum objects, confirm that they have unique fingerprints, and that these are, in many cases, extremely different from modern equivalents. It will be possible, in association with recognized dealers and auctioneers and private owners, to provide a fingerprint of an artefact that, in the event of the item being stolen, will enable police to positively identify it. Furthermore, present day fabrication and sale of “antique artefacts and coinage”, an activity involving many millions of dollars annually, could also be curtailed.

5. CONCLUSIONS The methodology described in this paper provides reproducible data for use in the fingerprinting and provenance identification of gold samples. It further demonstrates the application of LA-ICP-MS to this area of forensic mineralogy and provides the experimental design to transfer this technology to other aspects of the analysis of solid samples. The methodology is not designed to provide quantitation of analyte concentrations in a sample, as this will vary from sample to sample. Rather it is designed to produce a distribution pattern of elements, present within a particular sample, and on the basis of their association relate the sample to reference samples or to samples in a database. Application of this technique to real crime situations has resulted in definitive evidence being provided.

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authors wish to thank the Western Australian Police Gold Stealing Detection Branch and the Western Australian Gold Mining Industry for active support of these investigations. Thanks are extended to the Director of the Research Centre for Advanced Mineral and Materials Processing, University of Western Australia, for financial support of one of the authors (DD) and to the Director of the Chemistry Centre (WA) for permission to publish this work.

AcknowZedgernenrs-The

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[9] [lo]

[ll]

W. T. Perkins, N. J. G. Pearce and R. Fuge, J. Anal. At. Spectrom. 7, 611 (1992). J. S. Crain and D. L. Gallimore, J. Anal. At. Specrrom. 7, 605 (1992). S. Chenery, A. Hunt and M. Thompson, J. Anal. At. Spectrom. 7, 647 (1992). W. T. Perkins, R. Fuge and N. J. G. Pearce, J. Anal. At. Spectrom. 6, 445 (1991). N. J. G. Pearce, W. T. Perkins, I. Abell, G. A. T. Duller and R. Fuge, J. Anal. At. Spectrom. 7, 53 (1992). N. Imai, Anal. Chim. Actu 235, 381 (1990). W. T. Perkins, N. J. G. Pearce and T. E. Jeffries, Geochim. Cosmochim. Acfa 57, 475 (1993). M. Broadhead, R. Broadhead and J. W. Hager, At. Specrrosc. 11, 205 (1990). P. van de Weijer, L. M. Baeten, M. H. J. Bekkers and P. J. M. G. Vullings, J. Anal. At. Spectrom. 7. 599 (1992). S. E. Ho, D. I. Groves and G. N. Phillips, in Stable Isotopes and Fluid Processes in Mineralization, Eds H. K. Herbert and S. E. Ho, p. 35. University of Western Australia Geology Department and University Extension Publication 23, Perth (1990). W. S. Fyfe and R. Kerrich, in The Geology, Geochemistry and Genesis of Gold Deposits, Ed. R.P. Foster, p. 99. A. A. Balkema, Rotterdam (1984).