Application Of Raman Spectrometers In Gemology

author: Joy

2022-01-06

Identification of gemstones is the core of gemological research and one of the important fields of gemological research.With the progress of synthetic gemstone technology and gem optimization processing technology, new gemstone synthetic products and substitutes (imitation gemstone materials) appear constantly, and the difference between some optimized gemstones and natural gemstones decreases day by day.The traditional gemstone identification methods are difficult to solve all the problems of gemstone identification.The introduction of Raman spectroscopy into gemological research has greatly promoted the improvement of gemstone identification technology and research level in China.

Fig 1Gem Figure

Principle: Raman Scattering Spectroscopy
Advantages of Raman spectroscopy for analysis raman spectroscopy analysis method does not require pretreatment of the sample, and there is no sample preparation process, avoiding some errors, and in the process of analysis, simple operation, short determination time, high sensitivity. Laser Raman spectroscopy is a kind of laser photon inelastic collision with gem molecules, which changes the original entry A molecular coscattering spectrum at the frequency of emission. The scattered light from such inelastic collisions is often referred to as raman light spectrum. In the process of laser photons colliding with molecules, they are not only absorbed by molecules, but also scattered.Due to the touch,there are various forms of scattering between photons and molecules in different collision modes:

1. Elastic Collision
There is no energy exchange between the photon and the molecule, only the direction of motion of the photon is changed, and its scattering frequency is equal to Incident frequency, this type of scattering is called Rayleigh scattering on the spectrum.

2.Inelastic Collision
Energy is exchanged between the photon and the molecule when they collide, which changes the direction of the photon's motion.The energy is changed so that the scattering frequency is different from the incident frequency. This kind of scattering is called Raman scattering in the spectrum.

3. Energy Difference OfTwo Transitions In Raman Scattering

When the frequency of the scattered light is lower than the frequency of the incident light, the molecular energy is lost. This type of scattering line is called stokes line;If the frequency of the scattered light is higher than that of the incident light, the molecular energy increases, such scattered lines are called antiStokes lines.The former is the molecule absorption energy transition to a higher energy level, the latter is the molecule release energy transition to a lower energy level. A beam of light shining on a substance is actually a probe of the substance, because the light reflected, refracted and scattered from the substance carries information about the substance.The scattered light, though very faint, contains deep information about the structure of matter. In addition to Rayleigh scattering light, there is another, fainter component of light called Raman scattering light.It is formed by inelastic collisions between photons and elemental excitations (such as phonons) in matter.According to the principle of energy and momentum conservation in the collision process, the direction and energy of scattered photons have changed, and this part of scattered light is called Raman scattered light.Any molecule, any crystal, because of its particular atomic composition, structure, symmetry, has a particular set of vibrations, called phonon spectrum or vibrational spectrum.This spectrum is actually the "fingerprint" of the corresponding molecule and crystal, and accurate measurement of the phonon spectrum of a substance provides a solid foundation for the identification of crystal and molecule.

Test Result:
1. Real And Fake Gems

Fig 2Raman Spectra OfBurmese Jade, False Burmese Jade And Yuhua Stone

In figure 2,a is the standard spectrum of Burmese jade, b is a sample ofBurmese jade from my personal collection, and d is Burmese jade sent for examination b it is a genuine Burmese jade,d it is not,c is our Raman spectrum of yuhua stone (a kind of fused quartz whose main chemical composition is SiO2), which is the same as d spectrum.This consistent, prove the socalled "Burmese jade" is nothing but the cheap stone such as yuhua stone dyed fake.The measurement of the gem body gives the corresponding gem crystal "fingerprint" vibration spectrum, while the fake treasure.The vibration spectrum of stone is completely different from that of real gems due to different materials, which is clear from the Raman spectrum obtained.

2. Inclusion Measurement
Inclusions are impurities existing in the gem, foreign matter captured during the mineral formation process, or a small amount of second phase removed from the body. Due to the different formation mechanism and geological environment of various gem minerals, the compositions of surrounding rocks and media are different, and the inclusions contained in them are also different. If we can directly analyze and measure these inclusions,we can provide important information for gem occurrence.And destroy the gem, which is often unacceptable.Confocal Raman microscopy shows the superiority, by focusing the incident light beam on the inclusion below the surface of the gem, the phonon spectrum of the inclusion can be directly obtained.We used Raman spectrometer to detect an emerald.When the laser shines on the normal position of the gem, the obtained Raman spectrum is shown in FIG. 3a, and then the laser beam is focused on an enveloping body with a size of only 6 λ m in the middle of the gem. The obtained Raman spectrum is shown in FIG. 3b, and the two spectra are normalized and differentiated by computer to get FIG. 3c.Figure 3c was analyzed and compared with the computer Internal Raman spectrogram database. Finally, it was determined that the inclusion was a mineral crystal with anatase structure.Until now, it had been assumed empirically that the unique inclusion was a rutile (TiO2) mineral.Raman has now provided more accurate information by providing characteristic parameters of the mineral's structure, revealing that it is in fact an anatase crystal.

Fig 3Raman Spectra OfEmeralds And Their Encapsulation

3. Filling Measurement
Defects and cracks in some natural gems seriously affect their value.Therefore, the manufacturer uses the principle of physical refraction to select some substances with the same refractive index as gems, such as oil, glue, resin and other fillers to fill these defects, so that people can not see the cracks, so that the defects are covered up, so that its "quality" is greatly improved. Using a microscopic Raman spectrometer to focus a laser beam on a small area of suspicion, a rapid and accurate Raman spectrum can be obtained by analyzing whether the gem has been treated and what the filling is.Figure 4 is the Raman spectrum of a common epoxy resin filler.

Fig 4Raman Spectra OfEpoxy Resin

4. Light Fluorescence Spectrum
Figure 5a shows the Raman spectrum of Myanmar spinel and Figure 5a shows the Raman spectrum of synthetic vermillion spinel.By comparison, it can be seen that there are five main bands and obvious companion lines in Figure 5a, while it is difficult to distinguish companion lines in the synthesized figure 5b.This makes it easy to tell a natural stone from a synthetic one.The inaccuracy of the conventional visual identification has been eliminated, and the fluorescence spectra obtained by Raman spectrometer can quickly, accurately and clearly indicate the origin of gemstones.

Fig 5Photoluminescence of Synthetic Spinel (b) From Myanmar Spinel (A) As Recorded From Raman Spectrum Measurements

Raman Shift

Fig 6Raman And Fluorescence SpectraOfA, B and C Cargoes ofBurmese Jade

Conclusion:

Compared with the traditional gemstone identification method, the identification information given by Raman spectroscopy belongs to the deep information of material,and is the comprehensive information of material composition and structure. Therefore, from the point of view of identification,ithas greater reliability and accuracy.Through the analysis of Raman spectrum, not only can the authenticity of gemstones be analyzed at a glance, but also the natural or artificial gemstones can be analyzed from the microscopic Raman measurement of the inclusion of gemstones, and even the origin of natural gemstones can be traced.