Analysis of Gemstones Using EDXRF

Energy Dispersive X-Ray Fluorescence (EDXRF) is a significant tool used to determine the authenticity of colored gemstones and their geographical origin. Based on the geological setting, precious gemstones like emeralds, rubies, or sapphires from different origins frequently display a characteristic combination of trace elements at varying concentrations.

For instance, quantification and identification of such elements may enable tracking an emerald down to its location of origin such as Brazil, Zambia, Afghanistan, Colombia, or Zimbabwe. Likewise, the presence of specific trace elements also helps to differentiate between a valuable naturally formed gemstone such as ruby and a quasi-worthless synthetic crystal such as synthetic ruby.

Instrumentation

The Thermo Scientific™ ARL™ QUANT’X EDXRF Spectrometer is ideally suited for the non-destructive analysis of gemstones. Its large area (30 mm²) Silicon Drift Detector (SDD) provides exceptional detection efficiencies for characteristic elements such as gallium (Ga), a key trace element of rubies. Furthermore, the adjustable X-ray beam collimation and direct excitation geometry enable analysis with small analysis spots while preserving most of the analytical sensitivity. X-ray beam collimators of different sizes are available to modify the spot size. The sample imaging CCD camera permits placing of small gemstones for effective excitation and analysis.

Excitation Conditions

The ARL QUANT’X Spectrometer is fitted with a 50-Watt X-ray tube offering a huge range of excitation voltages (4–50 kV) which are well regulated in steps of 1 kV. Along with numerous primary beam filters (nine) for ideal background control, elemental sensitivity is enhanced while decreasing the background, providing better peak-to-background ratios and enhanced performance.

Table 1 provides the set of excitation settings used for the analysis of emeralds and rubies. The tube current is automatically tweaked to enhance the detector’s dead time. The total counting time for each analysis is less than 10 minutes. All the measurements are carried out in vacuum.

Table 1. Analytical settings

Sample Preparation and Presentation

Gemstones are examined as such so as to prevent any damage to the sample. Small gemstones are mounted on a custom-built sample holder or positioned in an XRF cup sealed with a 4 μm thick polypropylene film.

Calibration

A Fundamental Parameter (FP) method was used to carry out the calibrations. This technique is included in the regular quantitative package of the ARL QUANT’X Spectrometer. To calibrate the spectrometer, a total of 20 easily available pure element and compound standards are employed. Since these are amorphous materials, such standards do not exhibit any diffraction peaks in the spectrum and are thus favored over pure gemstones or minerals of identified composition, but of crystalline nature. A dedicated set of standards along with the calibration techniques specially formulated for gemstone analysis is currently available from Thermo Fisher.

Analysis Results

Many natural and synthetic rubies, emeralds, and sapphires have been examined with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), a semi non-destructive and reliable analysis method frequently used as a reference method for gemstone analysis. The results of these analyses are compared with those acquired on the ARL QUANT’X Spectrometer. The concentrations found for several sapphires and rubies are compared in Table 2. The results from the analysis of emeralds can be seen in Table 3. All analyses have been done using a 2 mm X-ray beam collimator.

Table 2. Analysis results for rubies and sapphires (conc. expressed as % w/w)

Table 3. Analysis results for emeralds (conc. expressed as % w/w)

The data reveals a good agreement between EDXRF and LA-ICP-MS results. In a majority of cases, the difference in concentrations comes within the uncertainty interval as fixed by the standard deviation.

Conclusion

This article has covered how the ARL QUANT’X EDXRF Spectrometer is used for gemstone analysis. A direct calibration using pure compounds or elements yields outcomes that show excellent agreement with LA-ICP-MS data. By itself, the ARL QUANT’X Spectrometer proves to be an economical and a truly non-destructive analysis tool for the gemological lab. Next to sapphires, rubies, and emeralds, a similar approach can be applied for the examination of other precious stones such as chrysoberyls, spinels, and even pearls.

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

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