Analysis of Gold Alloys Using XRF for Accurate and Precise Determination of Gold

Gold is one of most sought after and hence a valuable and precious metal in the world. It has been used for manufacturing jewellery, coins, decorations and ornaments for thousands of years. Lately there has been a dramatic increase in the market value of gold as it is a stable investment. All over the world several new companies and industries for refinement and recycling have sprung up.

The manufacture of gold alloyed products is based on how much gold is used in the alloy process. This is because gold is the most valuable element in these products. The accuracy and precision in gold analysis is highly demanding because of its high price. Hence high power wavelength dispersive X-ray fluorescence (WDXRF) is one of the primary methods for quality control in gold analysis.

Instrument

The Thermo Scientific ARL PERFORM’X series spectrometer used in this analysis was a 4200W system. The system is configured with 4 collimators, 6 primary beam filters, 9 crystals, 2 detectors, helium purge and a 5GN+ Rh X-ray tube for best performance from ultra-light to heavier elements thanks to its 50µm window. This novel X-ray tube with a low current filament ensures analytical stability month after month.

The ARL PERFORM’X offers excellent performance and sample analysis safety. Its unique LoadSafe design includes a series of features that prevent damage during sample pumping and loading. The feature of liquid cassette recognition prevents exposure of any liquid sample to a vacuum even by mistake. The primary chamber is protected by the Secutainer system by collecting any loose powders in a specially designed container using vacuum and the powders can be easily removed and cleaned by an operator. The ARL PERFORM’X uses a helium shutter designed for absolute protection of the goniometer during liquid analysis under helium operation for spectral chamber protection. In the “LoadSafe Ultra” optional configuration, a special X-ray tube shield offers total protection against sample breakage or liquid cell rupture.

Calibration

The elemental regression plots were created using a set of certified reference standards. These graphs are linear regressions of the known concentrations plotted against the measured intensities. One set of regression calibrations were made using the ARL PERFORM’X’s small spot aperture at 0.5 mm.

Figure 1. Au calibration at 0.5 mm aperture.

Figure 2. Ag calibration at 0.5 mm aperture.

Figure 3. Cu calibration at 0.5 mm aperture.

Figure 4. Zn calibration at 0.5 mm aperture.

Table 1. Typical gold alloy calibration at 0.5 mm aperture.

Elements Conc. (%) SEE (%) SD (%) RSD (%)
Au 75.5 0.0012 0.58 0.77
Ag 14.1 0.0046 0.14 0.99
Cu 8.8 0.0014 0.12 1.41
Zn 0.8 0.0067 0.03 3.78

Results

The sample was run for 10 repeat analyses at 100s counting time for conducting the precision tests for gold alloys. Table 1 summarizes the results. The standard estimates of error (SEE), the standard deviations(SD) and the relative standard deviations (RSD) are the typical deviations achieved from the above regressions. Table 2 shows the precision results using the 0.5mm small spot feature for this sample. This feature enables analysis of very small and/or oddly shaped samples or small defects in samples.

Small Spot Analysis

The small spot feature of the ARL PERFORM’X was used to analyze a South African Krugerrand coin and the analytical results in Figures 5 and 6 shows the precision of the small spot capability achievable by the ARL PERFORM’X.

Figure 5. Photo of spot #1.

Figure 6. hoto of spot #2.

Table 2. Small spot results on Krugerrand.

Samples Au (%) Ag (%) Cu (%) Zn (%)
Spot #1 97.5616 0.1314 2.3382 LoD
Spot #2 97.4032 0.1011 2.3148 LoD

Elemental Mapping Analysis

The ARL PERFORM’X can add a new dimension of sample analysis with its elemental mapping functionality. The mapping capability enables sample inhomogeneity, contamination, gradient segregation and inclusion analysis. Detailed composite maps of elemental distributions within a sample can be constructed using the ARL PERFORM’X mapping. The cartography control and overlay has an ultra fine resolution of 0.1mm steps providing superior analysis allowing for process improvement and problem solving applications.

The four primary elements (Au, Ag, Cu and Zn) were mapped over the entire sample surface. A different color is used to represent each element (Au–green, Ag–red, Cu– blue, Zn–yellow). The overlay shows a distinct band of high concentrations for Cu and Zn and a low concentration of Au. The naked eye will not be able to see the segregation. Each element color can be shown separately from the overlay. Here elemental concentration differences in the sample can be seen.

The differences can also be shown in 2D with the highest and lowest concentration being the brightest and darkest respectively. The results can also be illustrated in a 3D representation with highest and lowest concentration being the tallest and shortest respectively.

Conclusion

According to the results, it can be shown that gold alloy analysis can easily be performed with the ARL PERFORM’X sequential XRF spectrometer. The accuracy and precision are shown to be incredibly high in this matrix type for routine and non-standard analysis. The ARL PERFORM’X is not just an ideal analytical instrument for quality control, but also for identifying inhomogeneity, contamination, gradient segregation and inclusions in gold samples. Also operation is made easy through the latest and sophisticated advanced Thermo Scientific OXSAS WDXRF software which can operate with the latest Microsoft Windows,sup>® 7 packages.

About Thermo Fisher Scientific - Elemental Analyzers

Thermo Fisher Scientific Inc. is the world leader in serving science. Their mission is to enable their customers to make the world healthier, cleaner and safer. With revenues of $17 billion, they have approximately 50,000 employees and serve customers within pharmaceutical and biotech companies, hospitals and clinical diagnostic labs, universities, research institutions and government agencies, as well as in environmental and process control industries.

They create value for their key stakeholders through four premier brands, Thermo Scientific, Life Technologies, Fisher Scientific and Unity™ Lab Services, which offer a unique combination of innovative technologies, convenient purchasing options and a single solution for laboratory operations management. their products and services help our customers solve complex analytical challenges, improve patient diagnostics and increase laboratory productivity.

For over 80 years, their Ecublens site in Switzerland has been a worldwide supplier of spectrochemical instrumentation to such major industries as steel, transportation, cement, construction, food, pharmaceuticals, chemicals, academic research, petroleum and electronics. They offer unsurpassed capabilities in the areas of optical emission (OE), X-ray fluorescence (XRF), X-ray diffraction (XRD) and automation of spectrometers.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.

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