Analysis of Gemstones Using EDXRF

Energy Dispersive X-Ray Fluorescence (EDXRF) is a crucial tool in ascertaining the legitimacy and geographical origin of colored gemstones.

Precious gemstones like rubies, emeralds, or sapphires from diverse sources display characteristic combinations of trace elements at varying concentrations, contingent on the geological context.  

By identifying and quantifying these elements, it becomes possible to trace the origin of an emerald, pinpointing its location, be it Colombia, Brazil, Afghanistan, Zambia, or Zimbabwe.  

The presence of specific trace elements aids in distinguishing between valuable naturally formed gemstones (e.g. ruby) and relatively worthless synthetic crystals (e.g. synthetic ruby).

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

Instrumentation

The Thermo Scientific^™ ARL^™ QUANT’X EDXRF Spectrometer proves highly suitable for non-destructive gemstone analysis. The instrument’s Silicon Drift Detector (SDD) boasts a generous area of 30 mm², ensuring excellent detection efficiencies for crucial elements like gallium (Ga) in rubies.  

The instrument's direct excitation geometry and adjustable X-Ray beam collimation allow analysis with small yet precise spots while retaining exceptional analytical sensitivity. Various X-Ray beam collimators are available to fine-tune the spot size.  Including a sample imaging CCD camera further facilitates the positioning of small gemstones, optimizing the efficiency of excitation and analysis processes.

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

Excitation Conditions

The ARL QUANT’X Spectrometer features a 50 Watt X-Ray tube, providing a wide range of excitation voltages (4-50 kV) that can be adjusted in increments of 1 kV.

The spectrometer has nine primary beam filters to optimize background control and enhance elemental sensitivity, improving peak-to-background ratios and overall performance.  

Table 1 outlines the excitation conditions utilized for analyzing rubies and emeralds. The tube current is automatically optimized to reduce the detector's dead time, and each analysis takes less than 10 minutes, all performed under vacuum conditions.

Sample Preparation and Presentation

To preserve the integrity of the gemstone samples, they undergo analysis without any damage. Small gemstones are mounted on a custom-made sample holder or placed in an XRF cup, sealed with a 4 μm thick polypropylene film.

Calibration

For calibrations, a Fundamental Parameter (FP) approach is employed and included in the standard quantitative package of the ARL QUANT’X Spectrometer. The calibration process involves using a set of 20 easily accessible pure element and compound standards.  

These standards are preferred over pure minerals or crystalline gemstones, as they are amorphous and do not exhibit any diffraction peaks in the spectrum.  

Thermo Scientific offers dedicated standards and calibration methods specifically designed for gemstone analysis.

Analysis Results

Multiple synthetic and natural rubies, sapphires, and emeralds underwent examination using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), a reliable and semi-non-destructive analysis technique commonly utilized as a reference for gemstone analysis.  

The results were then compared to those obtained from the ARL QUANT’X Spectrometer. Table 2 compares concentrations found for various rubies and sapphires, while Table 3 showcases the analysis results for emeralds. All analyses were conducted using a 2 mm X-Ray beam collimator.

The data indicates a strong agreement between the EDXRF and LA-ICP-MS results, with most concentration differences falling within the uncertainty interval determined by the standard deviation.

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

LA-ICP-MS data generated by and property of Dr. M.S. Krzemnicki, Swiss Gemmological Institute SSEF.

Conclusion

This article demonstrates the application of the ARL QUANT’X EDXRF Spectrometer in gemstone analysis. Employing a simple calibration method with pure elements or compounds yielded results closely aligned with LA-ICP-MS data.  

Consequently, the ARL QUANT’X Spectrometer is a cost-effective and genuinely non-destructive analytical tool for gemological laboratories. Beyond rubies, sapphires, and emeralds, this approach can be applied to the analysis of other precious stones like spinels, chrysoberyls, and even pearls.

Acknowledgments

Thermo Fisher Scientific would like to thank Dr. F. Herzog and Dr. M.S. Krzemnicki of the Swiss Gemmological Institute SSEF, Basel, Switzerland for sharing the LA-ICP-MS data as well as offering their valuable expertise regarding gemstone authenticity, origin and chemical fingerprinting.

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

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