Analysis of Gemstones Using EDXRF for Determination of Origin and Authenticity

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.

EDXRF

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

Voltage (kV) Tube filter Atmosphere Live time (s) Elements
4 No Filter Vacuum 120 Na, Mg, Al, Si
8 C Vacuum 60 Ca
12 Al Vacuum 60 Ti, V, Cr, Mn
16 Pd Thin Vacuum 60 Fe, Ni
20 Pd Medium Vacuum 30 Cu, Zn, Ga, W, Ir, Pt, Au
28 Pd Thick Vacuum 30 Pb, Zr, Mo
40 Cu Thin Vacuum 30 Ag, Pd, Sn

 

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)

Synthetic ruby Douros, 4.80 ct
  Al2O3 TiO2 V2O3 Cr2O3 Fe2O3 Ga2O3
Conc. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev.
LA-ICP-MS 99.5 0.0015 0.001 0.0000 0.0001 0.883 0.428 0.048 0.006 0.043 0.003
ARL QUANT’X Diff. 0.0029 0.0016 0.000 - 0.792 0.004 0.024 0.001 0.032 0.001
Synthetic pink sapphire, 1.405 ct
  Al2O3 TiO2 V2O3 Cr2O3 Fe2O3 Ga2O3
Conc. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev.
LA-ICP-MS 99.5 0.0027 0.0002 0.0000 0.0001 0.0299 0.0002 <DL - 0.0000 0.0001
ARL QUANT’X Diff. 0.0029 0.0008 0.000 - 0.0334 0.001 0.000 - 0.002 0.001
Natural Shadong sapphire, 1.784 ct
  Al2O3 TiO2 V2O3 Cr2O3 Fe2O3 Ga2O3
Conc. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev.
LA-ICP-MS 99.5 0.0208 0.0014 0.0030 0.0003 0.0045 0.0032 1.182 0.043 0.029 0.002
ARL QUANT’X Diff. 0.0195 0.0015 0.0028 0.0008 0.0034 0.0006 1.043 0.006 0.027 0.001
Synthetic brown star sapphire, 3.935 ct
  Al2O3 TiO2 V2O3 Cr2O3 Fe2O3 Ga2O3
Conc. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev.
LA-ICP-MS 99.5 0.096 0.006 0.398 0.015 0.0112 0.0004 < DL - 0.0000 0.0001
ARL QUANT’X Diff. 0.110 0.003 0.349 0.004 0.0130 0.0012 0.000 - 0.000 -

 

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

Natural emerald, Pakistan, 1.022 ct
  Na2O MgO Al2O3 SiO2 Sc2O3 V2O3 Cr2O3 Fe2O3
Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev.
LA-ICP-MS 1.95 0.04 2.37 0.03 14.25 0.36 65.22 0.40 0.461 0.032 0.074 0.005 1.40 0.27 0.256 0.007
ARL QUANT’X 2.04 0.16 2.33 0.06 12.86 0.10 65.74 0.10 0.500 0.005 0.080 0.002 2.00 0.01 0.318 0.004
Synthetic emerald, Gilson flux grown, 1.43 ct
  Na2O MgO Al2O3 SiO2 Sc2O3 V2O3 Cr2O3 Fe2O3
Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev. Conc. Std Dev.
LA-ICP-MS 0.104 0.002 0.0033 0.0001 19.35 0.22 67.12 0.40 0.00023 0.00001 0.072 0.004 0.363 0.007 0.046 0.001
ARL QUANT’X <DL - <DL - 19.29 10.08 67.22 0.11 <DL - 0.092 0.002 0.436 0.001 0.062 0.002

 

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 and Phase Analyzers.

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

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit