Introduction

The traditional methods determining Gold in precious metal alloys, preferable jewellery alloys, are carried out by the Touchstone test or by the Cuppellation. But these methods are destructive and time consuming. There since many years the X-ray Fluorescence Spectrometry has been introduced to solve this problem.

In absence of certified reference samples the no standard analysis based on fundamental parameter model is the power means for sufficiently precise determination of alloy composition and concentrations of Ag.

Alloy system       Au-Ag-Cu-Zn-Pd-Ni

The analytical instrument

The instrument Xray ComPact 5/PIN offered by Roentgenanalytik Messtechnik GmbH has been especially developed for the analysis of jewellery alloys made of gold, silver or platinum.

A low power tube is used in either instrument in excitation mode. A special collimator reduces the exciting beam down to 0.6 mm in diameter. In this way a local analysis of several parts of the jewellery object and its filigree zones with irregular geometry, as usual for jewellery manufacturing, can be achieved.

Energy dispersive measurement is made with a PIN-diode. Thanks to the high resolution of this type of detector, the small amount of elements in complex composition can be determined. Good statistical measurement guarantees high reproducibility.

The following figure 1 shows a typical spectrum of AuAgCuCd alloy.

Figure 1: spectrum of AgCu alloy

Sample Preparation

The surface of the samples should be thoroughly cleaned free of contaminants. This will enable the irradiated area to be representative of the whole.

Further preparation is not necessary as the very small excited area and the use of normalized intensities reduce the geometric influence of the signal.

Excitation conditions

Program Parameters

Analytical Features

Limits of detection

Traces in alloys higher than 0.1% can be analysed. In most cases this sensitivity is more than sufficient for jewellery analysis.

Accuracy of analysis

The accuracy of the X-ray spectrometric determination of gold and silver shall be validated by certified standard jewellery alloy samples. The following table enables to compare X-ray measured values with these of Gold and Silver standards.

Reproducibility

For determination of reproducibility repeated measurements with a measuring time of 60 s have been realised within 25 min on a coin with 22 carat gold. The following table present the measurement results, the figure 2 shows the corresponding spectrum.

Figure 2: spectrum of the 22 carat gold coin.

These results are showing the performance of the spectrometer delivering sufficiently accurate values of Gold.

Conclusion

The analytical results demonstrate that the standardless X-Ray fluorescence analysis is a most important alternative method for the analysis of jewellery alloys. The X-ray spectrometer Compact 5/PIN allows the determination of Gold in jewellery alloys with high accuracy and high precision. Other present precious and non-precious metals can be determined as well. Also other technical alloys can be analysed such as Cu-alloys (brass, bronze), thermo couple materials and steel. Analytical performance is comparable to that for jewellery alloys. The elemental range is limited for elements with Z > 17