Gold fingerprinting is the identification of different origins of gold through specific element associations, resulting in a characteristic chemical signature. Fingerprinting is used to distinguish between the geological sources of gold, for example lode versus alluvial gold, as well as to determine its geographical origin, with Colombian gold, for example, enriched in Pt and Brazilian gold higher in Pd. Fingerprinting is also an essential tool in the global battle against illicit precious metals trafficking, with the source of the illegally mined gold as well as the elements associated with the different extraction processes providing a unique signature.
Tracing illicitly mined gold
The illicit trafficking in precious metals creates a market of billions of USD per year, threatening global economies. Along with the impact on precious metal producers through theft, communities are also affected by the growing corruption, violence, and health and safety issues surrounding illicit mining activities. The challenge to law enforcement is proving that any seized material has been stolen. The value of fingerprinting therefore lies in the chemical characterisation of the seized material and comparing it against a reference databank containing fingerprint information of gold, either natural or processed. This databank is unfortunately not yet very extensive but is continually updated due to new legislation.
The elements of interest in fingerprint studies typically include As, Bi, Cr, V, Cu, Fe, Mn, Ni, Pb, Pd, Sn, Zn, Ag, Cd, Rh, Ru, Sb, W and Hg. Except for Cu and Ag, most element concentrations vary at the ppm or lower levels. Laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) has long been used as the preferred analytical tool in fingerprint studies for several reasons:
ICP-MS instruments have detection limits in the ppb range, enabling a large number of elements to be measured in a short time.
Minimal destruction of samples that are small or precious occurs, with spot sizes typically ranging from 15 to 100 µm.
Minimal sample preparation is required: Au grains or metal shavings are resin mounted and polished (Fig. 13).
Analysis of Pb isotopes is useful to further distinguish gold provenance using laser ablation triple quadrupole ICP-MS, whereby 204Hg interference on 204Pb can be removed.
Laser ablation method development
Since 2019, the LA ICP-MS Laboratory has been collaborating with an institution in Colombia to analyse samples from various sources within the country, including alluvial gold, gold seized from illegal mining activities and gold extracted through different process in order to identify fingerprinting elements unique to their environment as well as to optimise the analytical method. The equipment available at CAF includes an Applied Spectra Resolution 193 nm excimer laser connected to an Agilent 8800 triple quadrupole (QQQ) ICP-MS (Fig. 14).
There were several objectives in the method setup in order to obtain quantitative and reproducible results from gold material:
A challenge in LA ICP-MS is the availability of matrix-matched standards required for quantitative analysis. National Institute of Standards and Technology (NIST) glass reference materials are the most common calibration standards, but their ablation properties are completely different from gold (Fig. 15), with more gold material volatilised compared to the NIST glass under the same conditions, resulting in higher reported concentrations for the elements in the gold sample. Gold matrix standards spiked with all the trace elements of interest are scarce and very expensive, but a custom standard was procured from Rand Refinery (Pty) Ltd South Africa, with elements distributed homogeneously at least at the 60 µm scale.
The resolution of sample homogeneity had to be identified by a separate technique such as scanning electron microscopy (SEM) as it affects the major element composition used for LA ICP-MS quantification (Fig. 16) and determines the spot size (Fig. 17) and number of replicates for representative analysis (Fig. 18).
Laser parameters were optimised to ablate the gold material with enough energy to volatilise the gold but not too high to cause excessive sputtering and generating ablated material of varying size fractions (Fig. 19).
The analytical method developed for the ongoing Colombian study can also be used in the chemical characterisation of gold from various geological environments and unravelling its formation history.