Analysis of Gemstones Using EDXRF for Determination of Origin and Authenticity

Energy Dispersive X-Ray Fluorescence (EDXRF) is a key instrument to determine the authenticity of precious gemstones and their geographical origin (Figure 1). Based on the geological setting, gemstones such as sapphires, emeralds, and rubies from different origins often have a typical combination of trace elements at different concentrations.

Identifying and quantifying such elements may allow for identification of the origin of gemstones. Additionally, the presence of certain trace elements also facilitates differentiating a valuable naturally formed gemstone from a quasi-worthless synthetic crystal.

Figure 1. Gemstones.

Instrumentation

The Thermo Scientific ARL QUANT'X EDXRF spectrometer (Figure 2) is ideal to perform non-destructive analysis of gemstones. The integrated lithium drifted silicon Si(Li) detector has 3mm thickness, which provides better detection efficiencies for characteristic higher energy X-rays of heavier elements like gallium (Ga), a key trace element of rubies.

Figure 2. The Thermo Scientific ARL QUANT'X EDXRF spectrometer.

Additionally, the adjustable X-ray beam collimation and direct excitation geometry facilitates analysis with small analysis spots without significant drop in the analytical sensitivity. There are different sizes of X-ray beam collimators available to refine the spot size. The sample imaging CCD camera facilitates positioning of small gemstones to achieve efficient excitation and analysis.

Excitation Conditions

The ARL QUANT'X spectrometer is coupled with a 50W X-ray tube that offer a wide excitation range of 4-50 kV, which can be controlled in steps of 1kV. The presence of multiple primary beam filters (eight) provides optimal background control and improved elemental sensitivity, while improved peak-to-background ratios and better performance are achieved with reduced background.

The excitation conditions applied for the analysis of emeralds and rubies are summarized in Table 1. The tube current is fine tuned automatically to optimize the dead time of the detector. All measurements are carried out in vacuum and the total counting time per analysis is below 10 minutes.

Table 1. Analytical settings.

Voltage (kV) Tube Filter Atmosphere Live Time (s) Elements
4 No Filter Vacuum 120 Na, Mg, Al, Si
8 Cellulose Vacuum 60 Ca
12 Aluminium 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 Calibration

Gemstones are analyzed as such without causing damage to the sample in any way. Small gemstones are positioned on a bespoke sample holder or in an XRF cup sealed with a 4µm thick polypropylene film. A Fundamental Parameter (FP) approach is applied for calibrations. This approach is incorporated in the standard quantitative package of the ARL QUANT'X.

The calibration of the spectrometer involves the use of 20 readily available pure element and compound standards. As these are amorphous materials, diffraction peaks are not shown in the spectrum by such standards. Hence, they are preferred over pure minerals or gemstones of prescribed composition, but of crystalline nature. Thermo Scientific offers a dedicated set of standards along with the calibration methods specially designed for gemstone analysis.

Analysis Results

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is a semi non-destructive and reliable analysis method. It is often utilized as a reference gemstone analysis technique for a number of natural and synthetic emeralds, sapphires, and rubies. The results of these analyses are then compared with the results obtained on the ARL QUANT'X spectrometer.

The comparison of the concentrations determined for several rubies and sapphires is shown in Table 2. The analysis of emeralds are summarized in Table 3. A 2 mm X-ray beam collimator is used in all analyses. The data shows that EDXRF results are in good agreement with the LA-ICP-MS results. In most cases, the variation in concentrations is within the uncertainty interval as stated by the standard deviation.

Table 2. Analysis results for rubies and sapphires (concentration 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 1 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 (concentration 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 0.08 67.22 0.11 0.092 0.002 0.436 0.001 0.062 0.002

Conclusion

This article illustrates the application of the ARL QUANT'X EDXRF spectrometer in gemstone analysis. A direct calibration utilizing pure elements or compound provides results, which are in line with the LA-ICP-MS data. The results clearly demonstrate that the ARL QUANT'X is a truly non-destructive and economical analytical instrument for the gemological laboratory. A similar method can be used to analyze other precious stones such as pearls, chrysoberyls and spinels.

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