It has now become possible to combine X-ray fluorescence and energy dispersive spectrometers to produce an instrument with significant advantages to both techniques
The importance of X-ray fluorescence for the measurement of coating thickness and analysis has increased in recent years, but until quite recently it has remained empirical, requiring standards to calibrate the instrument and being restrictive in the number of elements analysed at any one time.
A new approach using the basic characteristics of the substances being irradiated has brought significant improvements to the technique, reduced the requirement for expensive and complex sets of standards, and expanded the applications and range of measurement, says Fischer Instrumentation.
The new process, called fundamental parameter analysis, is also applicable to energy dispersive spectrometers, so it has now become possible to combine the two and produce an instrument with significant advantages to both techniques.
For assay work this gives the ability to analyse bulk materials to high accuracy and also measure and analyse precious metal coatings, even many layers of different coatings.
The Fischer Goldline Assay system has also been used for even more complex applications in the jewellery industry.
Typically in assay applications, the objective is to confirm coating thickness and to prove the gold's purity.
Often, these coatings consist of alloys with various compositions of silver, gold, copper and possibly iron, cobalt and cadmium.
It is possible to make a complete analysis of such materials and confirm the true value of the item.
In discussion with assay offices it has become apparent that they have differing requirements depending on the type of work they most often undertake.
Accuracy is always important, but analysis of unknown samples is key if many samples are of unknown origin, whereas if the samples are identified, the resolution of the instrument may be less important than its absolute accuracy.
The new instruments are said to be capable of analysing alloys to levels acceptable for assay work, and the instruments can also identify unknown samples.
Accuracies of 0.1 or 0.2% are desirable and achievable from fire assay techniques, in ideal conditions.
For production measurements, however, accuracies of around 0.5% may be the norm and also acceptable, especially if objective tests are conducted on the accuracy of cupolation in a production environment.
For alloys such as silver-copper-cadmium, repeatability can sometimes be compromised to around 1.0%.
This level of repeatability can be achieved with the new XRF instruments, using measurement times of less than 60 seconds, and even better accuracies with longer measurement times, says Fischer.
This new generation of instruments is now operating in assay offices with greatly improved efficiencies over traditional methods.
Sample handling is also important and the addition of a programmable XY table to the instrument automates multiple measurements and a motorised Z-axis will allow for the measurement of larger items.
These additions improve the efficiency of operation even further.
XRF systems can only analyse the near surface of a sample, but exhaustive comparative tests with cupolation have shown very good correlation between the methods and XRF is now an accepted assaying technique, offering the major advantages of being low cost, non-destructive and being very much quicker and less labour intensive.
