Application of energy-dispersive X-ray fluorescence to jewellery samples determining gold and silver

Abstract

Cupellation is the most usual reference technique for determining the gold content of jewellery alloys in Assay Offices. This technique, based on the refining of gold, have been known to humanity a long time ago -it was first described by the Egyptians in a Cuneiform tablet dating back to around 1360 BC -. It consists on the removal of all base metals from a sample by oxidation in a cupel (a porous crucible made of magnesium oxide which absorbs the oxidised metals) placed in an oven under conditions as described by the ISO Standard 11426. The result is a pure gold-silver alloy. The removal of the silver is carried out by dissolution in nitric acid to leave a residue of pure gold which is then weighed. The ratio between the sample weights before and after the process yields the gold content of the former alloy. This technique is cheap and accurate (its precision can reach 0.02%) (1). However, it is affected by a series of drawbacks, namely:• It is destructive.• It determines only the gold content.• Platinum group metals (insoluble in nitric acid) present in the samples lead to errors if they are not detected and conveniently removed. There are some other techniques suitable for gold analysis such as touchstone, parting, atomic absorption spectroscopy (AAS), inductively-coupled plasma spectrometry (ICP). They are destructive techniques, although some of them need minute sample amounts, and most require sample preparation. Furthermore, with the exception of ICP, these methods lack sufficient accuracy.The most common official method for the determination of silver is potentiometric titration. It is a destructive technique based on the potentiometric monitoring of the precipitation of silver chloride (ISO Standard 11427). Silver can also be determined by spectroscopic techniques, but its aqueous solutions are quite unstable and the results are not as accurate as required.X-ray fluorescence (XRF) is a technique based on the effect that high energetic X-radiation excites at a specimen of an elemental mixture lower enegetic X-lines specific for the present elements. The source that provides photons of sufficient energy to carry this out -known as primary radiation -is, most times, an X-ray tube in which electrons are accelerated from the cathode to the anode by a high voltage. A high-energy quantum radiation is generated by interaction with the anode material. This radiation is led to the sample, in which the interaction generates a second X-ray beam, called secondary radiation, that is detected and conveniently quantified. Each element emits its unique characteristic secondary X-rays, the energy of which is related to the analyte concentration, the sample matrix, and excitation and detection conditions (2). The main advantages of the technique are its non-destructive character and the capability of determining, not only the gold content, but the whole composition of an alloy. AbstractThe results from a study of the real scope of energydispersive X-ray fluorescence analysis in an Assay Office, in which precious metal s...

Improving a procedure for determining the assay of gold in a precious alloy of different composition using a touchstone

“…In [17], authors point a series of advantages in using XFA at analytical laboratories as a method that can replace testing on a touchstone. However, they did not analyze the effect of alloying components on the accuracy of obtained results, did not define the intercorrelation between methods, nor other indicators that characterize it.…”

“…Samples of articles made of precious metals based on gold containing zinc, submitted for testing (samples No [15][16][17][18]. …”

X-Ray Fluorescence Analysis of Valuable Metals

“…Thus, in the determination of gold in an alloy containing copper, an overestimation (0.5-1%) will take place due to the overlapping of some emission bands. On the other hand, the analysis of silver-copper alloys is very accurate because their spectral bands are clearly separated [18].…”

Objetos de oro y epicampaniforme en la Cova del Gegant. Relaciones en la costa mediterránea de la Península Ibérica durante la Edad del Bronce

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