Introduction

This application note gives all recommended data for the described analytical task, including the list of measured values based on certified reference samples.

Alloy system       Au-Ag-Cu

The Analytical problem

Analysis of gold alloys is an important task in the jewellery industry for determination of alloy composition and monetary value. The alloy system examined here can be seen as an basic gold jewellery alloy.

From the analytical point of view, the resolution of gold „L“ and Copper „K“ lines is the main problem.

The Analytical instrument

Measurements are made with the benchtop spectrometers

A low power tube is used in either instrument in excitation mode. A special collimator reduces the exciting beam down to 0.5 mm diameter. In this way, analysis of irregular or filigree samples will be possible. This is very important for jewellery manufacturers.

Energy dispersive measurement is made with a proportional counter. Despite the limited resolution of this type detector, highly precise analysis is possible if the qualitative composition of the sample is known. Good statistical measurement guarantees high reproducibility.

Max. Layer Thickness: approx. 12 µm

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 normalised intensities reduce the geometric influence on the signal.

The irradiated area must be homogeneous. Non-homogenities can produce unsufficient reproducibility of measurement.

Excitation conditions

Program Parameters

Calibration

The relatively wide concentration range for Gold will be divided into four calibration ranges, each of them includes a typical concentration for jewellery alloys. The following ranges were prepared: 8 - 10, 14, 18 and 22 - 24 Karat.

For every range a few reference samples are necessary. A total of more than 25 samples will be used for this calibration.

Analytical results

Accuracy

Calibration curve

In the following figure the gold calibration curve is shown.

Error of fit

The following values for error of fit are reached for the whole calibration range of the different elements. [Wt %]

This means, that for the alloy system described here the analytical error is in the range of + 0.10 Wt.% for gold.

The accuracy is a little lower for the other elements but these elements do not determine the value of the alloy.

Reproducibility

For repeated measurements the following results were obtained:

Dependence of Reproducibility on gold concentration:

(Measuring time: 120 s]:

Precision of analytical results depends only a little on concentration. On the other hand the measured intensity is affected by concentration

Dependence of Reproducibility of Measuring time:

Measuring time should be at least 120 s. For higher accuracy longer measuring times of up to 300 s should be used.