Advancements in Gemology: Harnessing Raman Spectroscopy for Non-destructive Analysis

Spectroscopy technique is an energy sensitive method and often employed in the characterization of molecules and chemical reactions. Vibrational spectroscopic methods use infrared or near infrared (the low energy end of the visible spectrum) to create vibrations (bond stretching or bending) in chemical species. Raman spectroscopy is a vibrational technique and one of the main spectroscopies employed to detect vibrations in molecules is Raman scattering which currently gaining popularity because recent technological advances have made the instrumentation more accessible.

Gemology is a science which is in special demand for non-distractive methods.

Classical gemology consisted of mainly taking the specific gravity of a gemstone, studying it in the microscope and taking refractive indicates. Test which are common in mineralogy, such as hardness test, etching the stones with acids and taking thin sections are not applicable for gemstone. Another quick and non-distractive method for identifying gemstone is the use of Raman spectroscopy. It is widely used to provide information on chemical structures and physical forms, to identify gemstones, and to determine quantitatively or semi-quantitatively the gemstone.

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The discovery of Raman scattering by Krishnan and Shankar in 1928 is well documented and the phenomenon has been the subject of thousands of research papers.

Raman spectroscopy exposes a sample to a single wavelength of light, usually from a laser in the visible, infrared, or ultraviolet range. The laser light is then scattered by the sample. Scattering simply means that the light particles (photons) are forced to deviate from their straight path by some interaction with the sample, different scattering processes can occur.

Two types of scattering are readily identified (Figure 1). The most intense form of scattering, Rayleigh scattering, occurs when the electron cloud relaxes without any nuclear movement. This is essentially an elastic process and there is no appreciable change in energy and consequently the light returns to the same energy state. Raman scattering on the other hand is a much weaker than the intensity of Rayleigh scattering. This occurs when the light and the electrons interact and the nuclei begin to move at the same time.

The Raman scattering process from the ground vibrational state m leads to absorption of energy by the molecule and its promotion to a higher energy excited vibrational state (n). This is called Stokes scattering. In the Stokes process the scattered light loses energy ν0- νm, due to the recover to a higher energetic state of the phonon than before. Scattering from these states to the ground state m is called anti-Stokes scattering and involves transfer of energy to the scattered photon. In the anti-Stokes process there is an energy gain of scattered light ν0+ νm (Leissner, 2014; Chalmers and Griffiths, 2011). The resulting scattered light has different wavelengths (because of the energy difference between the incident and scattered photons), corresponding to the energy of vibrational modes. This allows the identification of a variety of species based on their vibrational “fingerprints”.

We were measured using a the Thermo Scientific DXRRaman microscope 780 nm laser is designed to produce the results demanded by LabRamanAramis 633nm (Red) with automated calibration in the geology laboratory at Lyon University of France. The LabRAMARAMIS has computer controlled operation for quality control, analytical and research applications alike. The LabRAM series provide the most comprehensive analytical tool available; such as: True confocalRaman microscopy (below) and unique Line Scanning mode for fast Raman mapping with high spectral resolution.

The Raman system consists of four components:

1. an excitation source,
2. an optical sampling system,
3. a wavelength separator,
4. a detector.

A block diagram of the generic components making up a Raman system is displayed in Figure 2. Raman spectroscopy uses a single wavelength laser source in wavelength visible 633 nm to excite the electrons in a sample; the laser source is aimed at an optical sampling system includes the sample illumination system and light collection optics (a mirror that directs the beam to a polarizer, ensuring all photons are moving in the same direction for sample excitation).

The laser is then sent through a beam expander to ensure the spot size is a large enough to produce a strong enough response for measurement., Laser-line rejection devices have the role of preventing the weak scattered Raman light from being overlapped by the intense elastically scattered radiation and Then, The wavelength dispersion devices, separate the wavelengths that comprise the Raman signal and present them to the detector. The most common device used in dispersiveRaman spectroscopy is the spectrograph, which is a dispersivepolychromator, that contains a diffraction (or dispersion) grating responsible for the angularly separation of the Raman signal into their individual wavelength components, projecting them into the detector which is multi-channel detectors, the charge-coupled device (CCD).

The Thermo Scientific DXRRaman microscope with 780 nm was the methodology used in this thesis, where the Raman spectrometer is combined with a standard optical microscope allowing spatial resolution. The lasers, together with the appropriate filters and gratings, lock into place using SmartLock technology for precisely reproducible results. OMNIC™ software recognizes each component, records the serial numbers and checks for compatibility. A convenient storage container is provided for when the components are not in use.

The DXRRaman microscope Expand the capabilities by adding a second or third excitation laser. Minimize fluorescence interference, or enhance sensitivity by up to 10-fold. Components are automatically recognized by Smart technology and pre-aligned so there is no need for a service call.

The use of optical microscopes allows the excitation laser beam to be focused in the sample, creating a micro-spot with small diameters. In addition, it is possible to look at extremely small samples and therefore, despite the fact that Raman scattering is weak, to detect very small amounts of material and also, enable to look for evidence of inclusions or impurities and characterize multi-layer polymer films. The major advantage of the microscope is that any sample or part of a sample that can be aperture optically can also have the Raman spectrum recorded.

The use of an X-Y-Z scanning stage allows to perform different mapping techniques, such as X-Y optical sectioning (surface scan), X-Z cross-sectioning (depth profiling or multiple layers of the sample), Z-series (X-Y-Z scan), and surface profiling. Besides that, no preparation of the sample is required, small sample volume measurements are possible, and allows the acquisition of Raman chemical imaging (Chalmers and Griffiths, 2001; Das and Agrawal, 2011; Almeidaetal. 2015; Diogo, 2017).

According to Figure 3, the pathway of the Raman laser in the Thermo Scientific DXRRaman microscope can be described as follows: the laser beam is reflected in a first mirror and passes through the first pair of lenses and through the bandpass filter into the beamsplitter or dichroic mirror, where half of the beam is transmitted to a second mirror that reflects it into the microscope objective. The light strikes the sample, and is scattered by the sample.

The scattered light is collected by the microscope objective at 180º degrees, and passes through the second mirror and the beamsplitter or dichroic mirror, to the filter. The resulted filtered light passes through the second pair of lenses and is reflected by a third mirror to the spectrograph. This dispersive spectrometer contains a grating that diffracts the light and directs it to the cooled CCD detector, connected to a computer for data treatment. The pathway for the portable BWTEK i-Raman pro is similar, except for the fact that the laser needs to pass first through a fibre optic probe, connected to the confocal microscope (Diogo, 2017).

The others advantages and character could be mention for LabRamanAramis are:

• Full range gratings for complete spectra in one shot, 50 cm-1 to 3500 cm-1,
• 5 cm-1 spectral resolution (Spectral range for the 780 nm excitation laser is 50 cm-1 to 3300 cm-1),
• High-resolution gratings for difficult to resolve bands, 50 cm-1 to 1800 cm-1,
• Can be done Many organic and inorganic materials are suitable for Raman analysis (solids, liquids, polymers, vapors or gases) without preparation and no water interference – easy to measure aqueous samples,
• Fast multi-wavelength operation,
• SmartLock technology for reproducibly mounting lasers, gratings, filters, fiber port,
• Rapid automated calibration to ensure validity of results,
• Can be done also at high temperatures (e.g. 1000°C) because of detection in the optical regime;

And disadvantages of Raman are low intrinsic cross section, susceptible to fluorescence (can be up to 106 higher; especially coke has a high fluorescence yield), Quantification of Raman intensities is very difficult (even with reference samples, because of possible electronic effects of the substrate); however, there are Ways to improve Raman signal intensity by

1. Resonance Raman (RR) and
2. Surface-enhanced Raman scattering (SERS) (Smith and Dent, 2005).

References

1. Almeida, M. R., Correa, D. N., Zacca, J. J., Logrado, L. P. L., Poppi, R. J. (2015) Detection of explosives on the surface of banknotes by Ramanhyperspectral imaging and independent component analysis, Anal. Chim. Acta, vol. 860, 15–22.
2. Chalmers, J., Griffiths, P. R. (2001) Handbook of Vibrational Spectroscopy. Chichester: Wiley.
3. Das, R. S., Agrawal, Y. K. (2011) Raman spectroscopy: Recent advancements, techniques and applications, Vib. Spectrosc, vol. 57, no. 2, 163–176.
4. Diogo, D.M.V (2017) Spectroscopic approaches for forensic problems, Identification of pre-blast explosive residues and energetic materials by Raman spectroscopy, Master thesis UniversidadedeCoimbra.
5. Krishnan, R. S., Shankar, R. K. (1981) J. RamanSpectosc., 10, 1.
6. Leissner, L. (2014) Crystal chemistry of amphiboles studied by Raman spectroscopy. Master thesis in Geoscience, University of Hamburg, Germany.
7. Lewis, I.R. (2001) Handbook of Raman Spectroscopy, CRC Press.
8. Smith, E., Dent, G. (2005) Modern Raman Spectroscopy– A Practical Approach. John Wiley & Sons Ltd, West Sussex PO19 8SQ, England.