Gold Fingerprinting by Laser Ablation Inductively Coupled

Plasma-Mass Spectometry

So what exactly is Gold Fingerprinting???

• It is a process to generate a design of profiles corresponding to trace elements distribution or the “fingerprint” in gold bullions which are unique to a given source.

• The procedure adopts a qualitative approach based on the singular patterns produced by the minor and trace element impurities present in gold.

• It is used to identify stolen gold or gold used in salting activities.

• The generation of a Gold Bullion Databank, GBD, has provided an effective means of ascertaining gold provenance in cases where the origin is unknown. The AARL fingerprinting technique is described, in particular the role of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in profile generation, together with the use of the AARL unique statistical model specifically designed to provide % similarity information between an unknown and members of the GBD.

So what is Laser Ablation Inductively Coupled Plasma Mass-Spectometry ?

• It is a type of mass spectometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one partin (part per trillion). It is based on coupling together an inductively coupled plasma as a method of producing ions with a mass spectrometer as a method of separating and detecting the ions. ICP-MS is also capable of monitoring isotopic speciation for the ions of choice.

H ow does it work?

• In Laser Ablation Inductively Coupled Plasma Mass Spectrometry the sample is directly analyzed by ablating with a pulsed laser beam. The created aerosols are transported into the core of inductively coupled argon plasma ( ICP) , which generates temperature of approximately 8 0 0 0 C˚.

• The plasma in ICP-MS is used to generate ions that are then introduced to the mass analyz er. These ions are then separated and collected according to their mass to charge ratios. The constituents of an unknown sample can then be identified and measured. ICP-MS offers extremely high sensitivity to a wide range of elements.

• For laser ablation, any type of solid sample can be ablated for analysis; there are no sample-si z e requirements and no sample preparation procedures. Chemical analysis using laser ablation requires a smaller amount of sample ( micrograms ) than that required for solution nebuli z ation ( milligrams) .

• D epending on the analytical measurement system, very small amount of sample quantities may be sufficient for this technique. In addition, a focused laser beam permits spatial characterization of heterogeneity in solid samples, with typically micron resolution both in terms of lateral and depth conditions.

• At the start of the laser pulse until one nanosecond later, violent evaporation takes place at the material's surface. From one nanosecond to one microsecond ( 1 , 0 0 0 ns ) after the end of the pulse, the high-temperature plume expands outward.

From one microsecond to 1 , 0 0 0 microseconds ( 1 millisecond ) after the end of the pulse, the heat leaks away through radiative heating and the plume cools.

• In laser induced breakdown spectroscopy, the plasma emission from the ablated sample is gathered using special optics. A spectrometer analyzes the white light emitted from the plasma, separating the light into its colors ( wavelengths ) .

• B ox and whisker plot showing the average proportion of silver bullion content of denarius coins during the reign of N ero ( hori z ontal band) and the degree of variability ( percentage range).

Ideal Uses for LA-ICP-MS Analysis

Major, minor and trace level compositional analysis of conductive, semi-conductive and nonconductive materials

Contamination of plastics, pharmaceuticals organics or biological materials

Failure, contamination and inclusion analysis

Forensics analysis E nvironmental and

mineral sample analysis

E lemental distribution analysis

Glass and ceramics Forensics Semiconductor

manufacturing Geological Failure Analysis

• Relevant Industries for LA-ICP-MS Analysis

Strengths of LA-ICP-MS Analysis

D irect sampling multielement quantitative or semi-quantification at the surface and bulk for elemental composition of solids

N o chemical procedures for dissolution

R educed risks of analyte loss or cross-contaminations

Independent of sample geometry

Analysis of very small samples

D etermination of spatial distribution of elemental compositions

The common matrix elements and other molecular species can interfere with the determination of some elements. Some doubly charged or molecular ionic species create difficulties in quantifications.

Limitations of LA-ICP-MS Analysis

Sources:

• http://www.goldbulletin.org/assets/file/goldbulletin/downloads/Aarl.pdf• http://www.cea.com/techniques/analytical_techniques/la_icp_ms.php• http://www.nature.com/nprot/journal/v2/n5/fig_tab/nprot.20

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