[2008-02-16] Defeating gold fingerprinting

"Gold fingerprinting" (GF) is an analytical chemical technique that uses mass spectrometry to identify the origin of a gold sample. "Origin," in this case, refers to geological origin, or, in simpler terms, the mine from which a particular piece of gold originated. The technique is really not limited to gold, and could be used on just about any material that could be expected to have a unique isotopic profile which could be correlated with geography. Any kind of matter--be it animal, vegetable, or mineral--can theoretically be "fingerprinted" in this way. The primary use of the technique is in combatting precious metal thefts, but it has nonforensic applications in the earth sciences as well.

The preferred technique for GF is laser-ablation inductively-coupled-plasma mass spectrometry, or LA-ICP-MS, as it is commonly abbreviated. Attacking the concept piecewise, from the root: Mass spectrometry (MS) is an analytical technique that fragments samples at the molecular or atomic level and produces a graph of the number of charged fragments which are detected at each mass. On a statistical level, this fragmentation process is non-random and patterns of these charged mass fragment counts can be correlated with particular atomic or molecular constituents. In the case of ICP-MS, the temperature at which fragmentation occurs is high enough to reduce the sample to its component atoms, so there are no molecular fragments, only individual atoms. This temperature is provided by an inductively coupled plasma (ICP), which works by the same principle on which a hand-held plasma cutting tool operates. An inductively coupled plasma can easily maintain temperatures of 7000K. Owing to the great sensitivity of the detector, only a very small sample of material is required and, indeed, too large a sample will overload the instrument. "Laser ablation" (LA) refers to the method whereby a sufficiently tiny sample is removed from a sample which is large enough for human beings to handle. Basically, the sample is blasted with a laser and, by a spalling process which is not completely understood, very tiny amounts of material are ejected from the surface, from whence they are drawn by a flow of inert gas into the ICP ionization chamber. LA is by far the quickest way to go from field-sample to instrument-sample.

The statistical method of the technique involves comparing the mass spectrum of a particular sample against a database of samples of known provenance. Because the exact amounts of each isotope present will vary with the purity of the sample and the method which has been used to purify it, the quantities of various isotopes are not considered diagnostic. Rather, it is the presence or absence of particular isotopes and the pattern created by the presences and absences of each isotope in the typical mass range that is considered the "fingerprint." Isotopic ratios of particular elements, notably lead, are also usefully diagnostic, as these will remain largely unchanged regardless of refinement chemistry and are often unique to particular geographic origins.

The obvious method for foiling the fingerprinting technique is to spike one's gold with unnatural isotopes. Because of the extreme sensitivity of LA-ICP-MS, only infinitesimally small amounts of adulterants are required. There are a large number of conceivable methods to effect this isotopic adulteration, but probably the most straightforward is to mix gold from multiple geographic sources in the melt. However, deconvolution of heterogeneous samples is relatively straightforward in the case of simple mixtures, so ideally, the number of isotopically unique samples mixed in this way will be large, and their origins will be as geographically diverse as possible. A second alternative is to alloy the gold with some other metal with its own unique isotopic profile. Lead is an ideal choice in this respect. Mercury is another viable option, but hazardous to handle and dispose of. For maximum obfuscation, gold might be alloyed with an equal or even greater quantity of base metal at the crude stage, prior to refinement, so that the resulting isotopic profile will be as much or more that of the base metal as of the gold itself.

B Grigorova, S Anderson, J de Bruyn, W Smith, K Stülpner and A Barzev. "The AARL Gold Fingerprinting Technology." Gold Bulletin, 1998, 31(1).

last modified 2008-02-16