Analyzing Gemstones with Energy Dispersive X-Ray Fluorescence (EDXRF)

To determine the authenticity of colored gemstones and their geographical origin, Energy Dispersive X-Ray Fluorescence (EDXRF) is an essential tool. Precious gemstones like rubies, emeralds, or sapphires from different origins often display a characteristic combination of trace elements at varying concentrations based on the geological setting.

Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

For instance, tracking a precious emerald down to its location of origin, such as Zambia, Brazil, Colombia, Afghanistan, or Zimbabwe, can be assisted by the quantification and identification of such elements.

Likewise, differentiating between a valuable naturally formed gemstone such as ruby and a quasi-worthless synthetic crystal such as synthetic ruby can also be helped by the presence of specific trace elements.

Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Instrumentation

When it comes to the non-destructive analysis of gemstones, the Thermo Scientific™ ARL™ QUANT’X EDXRF Spectrometer is perfect. The Silicon Drift Detector (SDD) also provides exceptional detection efficiencies for characteristic elements such as gallium (Ga), a key trace element of rubies, thanks to its large area (30 mm²).

Moreover, the direct excitation geometry and adjustable X-Ray beam collimation enable analysis with small analysis spots while preserving most of the analytical sensitivity. To modify the spot size, X-Ray beam collimators of different sizes are available. For effective excitation and analysis, the sample imaging CCD camera permits the placing of small gemstones.

Excitation Conditions

One of the many features of the ARL QUANT’X Spectrometer is a 50-Watt X-Ray tube that offers a huge range of excitation voltages (4–50 kV) which are well regulated in steps of 1 kV. Elemental sensitivity is enhanced while decreasing the background, along with nine primary beam filters for optimum background control, ultimately providing better peak-to-background ratios and enhanced performance.

The set of excitation settings used for the analysis of emeralds and rubies is shown in Table 1. The tube current is automatically tweaked to enhance the detector’s dead time. The total counting time for each analysis is under 10 minutes, and all the measurements are performed in vacuum. 

Table 1. Analytical settings. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Sample Preparation and Presentation

To prevent any damage to the gemstones, they are examined carefully and with a number of protective steps so as to prevent any sample damage. A custom-built sample holder is used on which the small gemstones are mounted on, or they are positioned in an XRF cup sealed with a 4 μm thick polypropylene film.

Calibration

To carry out the calibrations, a Fundamental Parameter (FP) method was used. This technique is included in the regular quantitative package of the ARL QUANT’X Spectrometer.

A total of 20 easily-available pure element and compound standards are employed to calibrate the spectrometer. These standards do not exhibit any diffraction peaks in the spectrum, as these are amorphous materials, and they are therefore favored over pure gemstones or minerals of identified composition but of crystalline nature.

Thermo Fisher currently has a dedicated set of standards along with the calibration techniques specifically formulated for gemstone analysis.

Analysis Results

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been used to examine a range of natural and synthetic rubies, emeralds, and sapphires as it is a reliable, semi-non-destructive analysis method that is frequently used as a reference method for gemstone analysis.

Results acquired on the ARL QUANT’X Spectrometer are compared with the results of these analyses. Table 2 compares the concentrations found for several rubies and sapphires. Table 3 shows the results from the analysis of emeralds. All analyses were performed using a 2 mm X-Ray beam collimator. 

A strong agreement is revealed between the LA-ICP-MS and EDXRF results. The difference in concentrations falls within the uncertainty interval, as fixed by the standard deviation in the majority of cases.

Table 2. Analysis results for rubies and sapphires (conc. expressed as % w/w). Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Table 3. Analysis results for emeralds (conc. expressed as % w/w). Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Conclusion

This article has outlined how gemstone analysis can be simplified with the ARL QUANT’X EDXRF Spectrometer. Outcomes that show excellent agreement with LA-ICP-MS data are yielded by a direct calibration using pure compounds or elements.

The ARL QUANT’X Spectrometer, when used alone, proves to be a truly non-destructive and highly cost-effective analysis tool for the gemological lab. A similar approach can be used to examine other precious stones, such as spinels, chrysoberyls, and even pearls, as well as rubies, sapphires, and emeralds.

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

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