Analysis of Gemstones Using EDXRF

In gemology, elemental measurement is critical to identify, characterize and classify both natural and synthetic gemstones. Minor and trace amounts of metals, particularly the transition metals, alkaline earth elements and alkali elements, help in establishing the geographic region and environmental conditions during the creation of a gemstone.

Other elements such as Pb and Au are useful to specify the processing requirements of synthetics. This article discusses the analysis of gemstones using low power 50W EDXRF with indirect excitation.

Instrumentation

EDXRF is a convenient, non-contact, non-destructive analysis method suitable for testing precious materials.

The technique can perform gemstone characterization or qualitative screening through elemental quantification. It can be used to quantify the elements found in many gem materials, as well as other materials that serve as evidence of certain treatment processes.

The X-ray source of the low power 50W EDXRF will not cause damage to the gems. Secondary targets and polarization also facilitate the analysis by eliminating most of the extraneous background scatter X-rays. The experimental setup is listed below:

Model : Rigaku NEX CG
X-ray tube : 50 W Pd-anode
Excitation : Indirect with polarization
Detector : High performance SDD
Analysis Time : 500 sec
Environment : Helium Purge
Standard : 15-position Sample Tray (32mm)
Options : Teflon sample cup insert

Rigaku NEX CG EDXRF spectrometer

Figure 1. Rigaku NEX CG EDXRF spectrometer

Rigaku NEX CG is shown in Figure 1. The Rigaku NEX CG EDXRF spectrometer integrates indirect excitation with polarization targets, secondary targets, and a high performance SDD to provide the user a robust, versatile analysis instrument with a user-friendly software interface.

Unlike traditional EDXRF systems, indirect excitation eliminates almost all the background noise, thereby providing spectra with a very high characteristic signal-to-noise ratio. This facilitates a higher degree of accuracy and much lower detection limits. A secondary target schematic is depicted in Figure 2.

Secondary Target Schematic

Figure 2. Secondary Target Schematic

Sample Preparation

A special Teflon insert was used to place each gemstone into a small plastic XRF sample cup (Figure 2). Besides centering the stone sample, the setup provides no extraneous fluorescence.

The gemstones were not subjected to any destructive sample preparation treatments or exposed to foreign substances.

A. Sample cup with film bottom and separate Teflon insert; B. Sample cup with Teflon insert placed inside sample cup for measurement; C. View from top and bottom of gemstone in cup with insert ready for analysis.

Figure 3. A. Sample cup with film bottom and separate Teflon insert; B. Sample cup with Teflon insert placed inside sample cup for measurement; C. View from top and bottom of gemstone in cup with insert ready for analysis.

Rigaku RPF-SQX Fundamental Parameters (FP)

Using Rigaku's RPF-SQX Metals Template, a FP method was developed. The RPF-SQX method utilizes a sophisticated FP program, which automatically deconvolutes spectral peaks and simulates the sample matrix by fundamental XRF equations.

This yields an approximation of the percentage of the sample that is not possible to measure, and provides precise analytical results for elements that are measureable, enabling semi-quantitative measurement of elemental concentrations without using an extensive set of known assayed calibration standards.

Additionally, it is possible to create a matrix-specific Matching Library utilizing one assayed sample of each gemstone in order to improve the analytical results further. The Matching Library is simple to create and is used in combination with the standard FP library to improve model of each matrix to optimize the estimation of concentration results.

Qualitative Analysis

The gemstone spectra are overlaid as an illustration of qualitative analysis, yielding the following results:

RX9 (HOPG polarizer) secondary target

Figure 4. RX9 (HOPG polarizer) secondary target

Cu secondary target

Figure 5. Cu secondary target

Mo secondary target

Figure 6. Mo secondary target

Al secondary target

Figure 7. Al secondary target

Results

The results clearly demonstrate the applicability of the Rigaku NEX CG as a valuable instrument in the characterization of gemstones. If it is a known gem, then it is possible to optimize the analysis method by defining the major component of the gem as the balance of the matrix.

For instance, Al2O3 is the major component for sapphire, Be3Al2 for emerald, and CaCO3 for pearl. The presence of trace Au and/or Pb can provide data about synthetics and processing in part. Other identifiers may lie in specific elemental ratios, such as the Sr/Mn ratio in sea and fresh water pearls.

Conclusion

The combination of the Rigaku NEX CG and the RPF-SQX FP method delivers unprecedented performance for the elemental analysis of gemstones. Indirect excitation and polarization provide the gemologist an outstanding low background instrument and the use of low 50W power causes no damage to the gemstone.

Calibration standards are not required for the RPF-SQX FP method, and it is possible to optimize the quantification with Matching Libraries based on one assayed sample of each gemstone. With these features and more, the Rigaku NEX CG is more suitable for the elemental determination and characterization of gemstones and similar materials.

This information has been sourced, reviewed and adapted from materials provided by Rigaku Corporation.

For more information on this source, please visit Rigaku Corporation.

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