Raman spectroscopy is a spectroscopic technique based on inelastic scattering of monochromatic light, usually from a laser source. It relies on inelastic scattering — meaning that the frequency of photons in monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range, changes upon interaction with the sample. This change provides information about vibrational, rotational, and other low-frequency transitions in molecules. When a sample is illuminated with a laser beam, the laser light interacts with phonons of the molecules — primary modes of vibration or rotational motion within the chemical bonds of the molecule — or other excitations in molecular structure. Photons from the laser light can, through interactions, either lose energy (resulting in energy being transferred to molecules through phonons) or gain it from the molecules (resulting in loss of energy to molecules through phonon transitions). The energy shifts in the laser light provide a unique spectroscopic fingerprint by which molecules can be identified.
Advances in Raman Spectroscopy Instrumentation
Over time, several advancements in Raman spectroscopy instrumentation have occurred that have facilitated increased adoption and novel applications. Some key developments include improved laser sources, diffraction gratings for monochromators, charge-coupled device (CCD) detectors, fiber optic probes, and miniaturization.
Initially, Nd:YAG and argon-ion lasers were commonly used excitation sources but were bulky and limited in wavelength range. The development of diode lasers enabled compact Raman systems with variable wavelength selection. Now, continuous wave diode lasers and stabilized laser diodes provide monochromatic, tunable excitation with increased power.
The incorporation of holographic diffraction gratings improved monochromator performance compared to traditional reflective gratings. This facilitated narrower band-pass filtering and greater spectral resolution.
Additionally, CCD detectors replaced photomultiplier tubes for broad spectral range, high sensitivity, and digitization of spectra. CCDs gather full spectral information simultaneously compared to scanning detectors.
Fiber optic probes now allow collection of Raman signal from difficult to access samples. Miniaturization has resulted in portable Raman spectrometers, often employing solid-state diode lasers and CCD detectors.
Raman Spectroscopy Applications
Raman spectroscopy has diverse applications in several fields due to its non-destructive analysis and fingerprinting capabilities. Some key application areas include:
Pharmaceutical analysis — Identification of active pharmaceutical ingredients and other components, polymorph detection, and quality control.
Cultural heritage and art analysis — Authentication of artworks, identification of pigments and dyes in paintings and manuscripts.
Semiconductor characterization — Assessment of crystallinity, strain, and dopants levels in microelectronics materials.
Forensics — Analysis of illicit substances, match evidence, and identification of questioned materials.
Gemology and minerals — Identification and characterizations of gemstones, minerals, and ores.
Chemical industry — Analysis of catalysts, process monitoring, and identification of intermediates.
Biomedical analysis — Identification of microorganisms, tissue histopathology, and biofluid analyses.
Gradually, Raman spectroscopy has permeated many other new application domains as the technique and instrumentation have advanced and become more portable and affordable. Examples include archaeology, food science, and environmental analysis.
Global Engagement and Standardization
As Raman spectroscopy increased in adoption, scientists worldwide engaged in strengthening the technique and building inter-laboratory consistency. Several international groups and standards formed for this purpose.
The International Union of Pure and Applied Chemistry Commission on Raman Spectroscopy promotes exchange of information and coordinates analytical standards activities.
The European Society for Spectrochemical Analysis Working Group on Raman Spectroscopy aims to establish best practices and protocols within Europe.
ASTM International publishes standards on Raman instrumentation performance evaluation and spectroscopic libraries for applications like pharmaceuticals.
These groups work to develop reference materials, data quality guidelines, and globally recognized Raman spectral libraries to facilitate data sharing and method validations across borders.
Their efforts support increased regulatory approval and acceptance of Raman analysis for safetycritical applications worldwide. Standardization enables Raman data to reliably travel between countries and jurisdictions.
Advancing Applications Through Global Collaboration
International collaborations now allow Raman scientists to solve more complex challenges. Multi-institutional projects aim to expand the frontiers of Raman techniques and their applications domain.
For example, the US National Science Foundation funded a multi-university partnership applying deep learning to improve stand-off Raman imaging capabilities. This could enable noninvasive analysis of materials at a distance.
Another project connects researchers in Singapore, Malaysia and Australia to develop compact fiber-optic probes and micro-Raman systems for rapid on-site identification of water impurities, aquatic organisms, and microplastics in waterways across Southeast Asia.
Global firms share instrumentations to enable groups from developing countries to conduct food adulteration screenings using portable Raman systems. This builds analytical capacities worldwide.
Open-access online spectral repositories enable scientists in remote locations to access global reference datasets for material identifications without physical access to instrumentation.
Future Directions in Global Raman Spectroscopy
As instrumentation becomes even smaller, affordable and connected, new applications in fields like environmental monitoring, telemedicine and process automation will emerge globally.
Standards based on high-quality reference datasets and guidelines adopted worldwide will drive greater harmonization. Interoperable instrumentation could facilitate trans-border sensing networks.
Deeper machine learning may enable fully automated sample recognition and open Raman analysis to non-expert users on a wider scale. Collaborations between industry and academia will spawn innovative applications of Raman methods on a global scale.
Overall, global engagement and open-exchange of knowledge have expanded the reach of Raman spectroscopy — an analytical technique that started as a specialized curiosity is now empowering scientists and innovators worldwide to solve critical challenges facing humankind.
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