Raman Spectroscopy

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Contributors: Abed Haddad, Marissa Bartz, Catherine H. Stephens

Overview

• Technique: Raman Spectroscopy
• Formal name: Raman Spectroscopy
• Summary description of this technique:

The Raman effect can be described as a two-photon scattering phenomenon. When matter is impinged upon by light, a small fraction of the incident radiation is scattered, while the remaining radiation is then reflected, absorbed or transmitted by the sample. The light source induces a momentary dipole moment (µ) characterized by: μ = α E, where (α) is a measure of molecular polarizability and (E) represents the electric field strength. In elastic scattering, known as Rayleigh scattering (see diagram), the incoming photon causes an emitted photon of equal energy, unlike inelastic scattering, known as Raman scattering (see diagram), where incoming photons generate photons having more or less of the incident energy.

Diagram of the Rayleigh and Raman scattering processes. Adapted from: Smith E, Dent G. Modern Raman spectroscopy: a practical approach. Second edition. Hoboken, NJ: Wiley; 2019.

Modern Raman instruments utilize various lasers in the visible, near-infrared, or ultraviolet ranges to promote high energy scattering and enough polarization of the molecule to excite a molecule into a momentary virtual state. This state is characterized as “virtual” because it does not correspond to a well-defined electronic state, and its volatility leads to an instantaneous emission of a photon. Most of the emitted photons are scattered elastically; nonetheless, a small amount scatter at either higher or lower energy in relation to the incident beam (hν₀). Stokes Raman scattering occurs at lower energy (hν₀ - hν₁), and Anti-Stokes Raman (hν₀ + hν₁) occurs at higher energy.

Image including raman spectra seen at right.

Raman spectra of pigments identified in samples taken from Interior, Mother and Sister of the Artist by Edouard Vuillard (1893). CO: Chrome Orange. V: Vermilion. RL: Red Lead. ZY: Zinc Yellow. CY: Chrome Yellow. U: Ultramarine blue. The top spectrum was acquired by SERS and shows the presence of eosin Y, an organic lake pigment. Adapted from Haddad A, Hartman D, Martins A. Complex Relationships: A Materials Study of Édouard Vuillard’s Interior, Mother and Sister of the Artist. Heritage 2021;4:2903–17.

Basic Instrumental Components

High energy excitation sources

Monochromatic light sources, the most common being lasers. Raman scattering and excitation wavelength are intimately linked and  the Raman scattering strength (E)  is inversely proportional to the fourth power of the excitation wavelength .Consequently, a stronger Raman signal is achieved at shorter excitation wavelengths. However, shorter wavelengths can produce significant fluorescence and cause photodecomposition of the sample. Conversely, long wavelength sources can be used at higher power without causing photodecomposition of the sample and eliminating or reducing fluorescence in many cases. However, they require more sensitive detectors and other instrumental considerations to obtain well resolved spectra. The most common excitation sources used in cultural heritage analysis include: Argon ion laser (488 and 514.5 nm), Krypton ion laser (530.9 and 647.1 nm), Helium–Neon (He–Ne) (632.8 nm), Near Infrared (IR) diode lasers (785 and 830 nm), and Neodymium–Yttrium Aluminum Garnet (Nd:YAG) lasers (1064 nm). Other sources are available, such as tunable lasers and UV lasers, and often require specialized instrumentation to operate. Lenses and filters Optical components are key for moving the laser beam from the source to the sample and back to the detector. Lenses both focus the light onto the sample and collect the scattered light to direct back to the detector array. Lenses can occur before the sample (beam narrowing and expansion), at the sample (objective lenses), and after the sample (beam collection and focusing). Filters strip the sample-reflected beam from non-elastically scattered light, such as Rayleigh radiation. Two common filters are used in Raman spectroscopy: notch filters which remove radiation at the frequency of the laser; and edge filters, which remove all light above a certain frequency so that only Stokes, inelastic scattering is recorded.

Diffraction gratings

Diffraction gratings (or prisms in some cases) are used to separate the constituent wavelengths of the collected Raman scatter onto different pixels of the detector. The position and orientation of the grating in relation to the laser path and the detector  adjust the range in wave numbers (cm-1). This range can be adjusted in the parameters window when setting up a measurement. Detectors The most common detector used in Raman spectroscopy is a charge coupled device (or CCD detector) is a sectored piece of silicon in which each sector is separately addressed to the computer. In this way, it is possible to discriminate each frequency of the scattered light and therefore construct a spectrum. This type of detector is most effective up to 830 nm or so. Dispersive instruments with excitation in the range from 830 to 1550 nm use InGaAs (Indium Gallium Arsenide) detectors in an analogous way to CCD

Detectors

The most common detector used in Raman spectroscopy is a charge coupled device (or CCD detector) is a sectored piece of silicon in which each sector is separately addressed to the computer. In this way, it is possible to discriminate each frequency of the scattered light and therefore construct a spectrum. This type of detector is most effective up to 830 nm or so. Dispersive instruments with excitation in the range from 830 to 1550 nm use InGaAs (Indium Gallium Arsenide) detectors in an analogous way to CCD

Details

What this techniques measures

Raman spectroscopy today is a well-established technique used in the investigation of cultural heritage materials. Its characteristic molecular specificity, non-destructivity (both of sample and in-situ analysis), and the variety of materials it can analyze (minerals, gems, organic and inorganic pigments, glass, ceramics, and to a lesser extent binding media, varnishes, and plastics) make Raman spectroscopy a versatile and desirable tool for conservators and conservation scientists.

Limitations of this technique

Normal Raman spectroscopy might not characterize well samples in complex organic media that contain oil, waxes, and  natural and synthetic resin among others, as these materials can fluoresce greatly with monochromatic illumination. This leads to a saturated broad spectrum with little to no defining features. Lasers operated at high power can cause localized heating, photobleaching, thermal degradation, or outright bruning. Short wavelength can also cause samples to fluoresce which can affect the quality of the spectrum. Raman is also not an elemental technique and as such not appropriate for the characterization of metal alloys in general. It is worth noting that Fourier-transform Raman spectrometers, dispersive Raman microscopes augmented by confocality, and surface-enhanced Raman spectroscopy (SERS) have seriously augmented the use of Raman spectroscopy for highly excitable, or fluorescent, samples such as dyes and lake pigments.

Can/how can this technique be made quantitative?

This technique can be made quantitative with rigorous calibration, which includes standards. Quantitation can be done by measuring the relative intensities or integrating the area of certain peaks and comparing those results to other peaks in the spectrum or an internal standard. 

Samples

Phases it can be used to examine (gas, liquid, solid)

Most commonly used Raman equipment is able to analyze solids regardless of the sample interface (microscope stage, fiber optic, direct laser, etc…). Liquids and gasses can be examined using specialized equipment, such as microfluidic cells among others.

Is this technique non-destructive?

When operated at low laser power, in-situ analysis of objects is extremely safe from thermal degradation. Sampling for analysis with confocal setups or for SERS is required, but the sample size is negligible and smaller than is necessary for complementary techniques such as u-FTIR due to the use of high magnification objective lenses.

Time to run one experiment?

Raman spectroscopy is extremely sample dependent, as each material will have a different response to the laser stimulus. Understanding how to best calibrate your parameters to the type of sample and the resulting spectra is nuanced and could take several acquisitions before arriving at a satisfactory and fully interpretable spectrum.

Sample preparation methods

Little to no sample preparation is required for analyzing samples with Raman spectroscopy. For SERS, samples could be hydrolyzed with acids (HNO3, HF) prior to exposure to the SERS substrate of choice.

Applications

Examples of how this technique is used in the field?

Common analytes include:

● Mineral and organic pigments

● Organic and inorganic pigments

● Enamels and glazes

● Corrosions producers and efflorescence

● Artistic varnishes waxes and adhesives

● Some polymers and plastics

Risks associated with using this technique?

Lasers are classified for safety purposes based on their potential for causing injury to humans’ eyes and skin. Most laser products are required by law to have a label listing the Class. It will be listed either most commonly using Arabic or Roman numerals. This classification depends on the maximum laser power

• Class I: < 0.39 mW
• Class II: < 1 Mw
• Class IIIR: 1-5 mW
• Class IIIB: 5-500 mW
• Class IV: > 500 mW

Some systems are designed to only operate with the entirety of the laser path blocked off from the user, whereas other schemes (open architecture, portable, etc…) are not and require care when in operation.

Budgetary Considerations

Approximate cost to purchase equipment for this technique?

Pricing for instrumentation varies greatly, as the Raman system can be tailored to satisfy specific requirements or needs by modifying the sample interface, the number of lasers, system automation, among other factors. On the whole, stationary, confocal models are on the higher end whereas more portable models cost far less.

Annual cost to maintain or run?

The main components that require some regular assessment are the lasers, as their emission power degrades with use and might need replacement. Other areas of maintenance are optical alignment, and if using an automated system, health of optical motors.

Case Studies

[provide description and links]

Additional Information

Complementary Techniques

Raman and Infrared spectroscopies. Raman spectroscopy is similar to infrared spectroscopy, where both techniques rely on vibrations of specific molecular groups for identification of materials, and the intramolecular vibrational signatures depend on several factors including, mass of atoms, bond strength, molecular environment, and molecular geometry.  While Raman spectroscopy uses visible or near-Infrared laser light to excite the sample, infrared spectroscopy utilizes a broadband infrared source. Raman spectroscopy measures the light that is inelastically scattered from the sample source whereas infrared spectroscopy measures infrared light that is transmitted or reflected from a sample.

In both, molecular symmetry and structure dictate the spectroscopic selection rules through which vibrational spectra are produced. This makes the techniques more or less complimentary, as vibrational modes inactive in infrared spectroscopy are active in Raman spectroscopy, and vice versa. In order for a sample to be Raman active, a vibration must involve a change in the polarizability (or “shape”) of the electron cloud, which means that the Raman spectra favor symmetric or nonpolar groups. Infrared spectroscopy, on the other hand, favors polar groups, since it requires a shift in the dipole moment (distributions of positive and negative charges) of the molecule.

The spectral windows of Raman and FTIR spectroscopy depend on instrumental considerations, such as optics and excitation sources. Raman spectroscopy can produce well resolved spectra between 4000 and 100 cm^-1, whereas infrared spectrometry can extend in much higher wavenumbers.

Variations of this technique

Portable Raman Spectroscopy

Improvements in instrumental technology have led to the production of portable and equipment, which can be favorable since smaller institutions lack access to expensive and stationary set ups. This has allowed analyses to progress without cleaning or sampling by directing the laser beam directly at the object.  These qualities helped to alleviate many of the apprehensions concerning invasive object sampling and allowed for a robust understanding of cultural heritage materials. Portable instrumentation is available in a wide range of excitation wavelengths, ranging from green to near-IR, and delivering the beam to the material in question can be accomplished through lenses, fiber optic cables, liquid cells, among others.

FT-Raman

Fourier Transform-Raman spectrophotometers (FT-Raman) were introduced to improve the detection system when operating in the near-IR region, such as emplyping \ 1064 nm laser excitation. FT-Raman spectrophotometer uses a Michelson interferometer and continuous wave laser such as Nd–YAG which emits the radiation at 1064 nm. InGaAs and germanium (Ge) detectors are operated at cryogenic temperatures in order to reduce noise and thus raise the signal-to-noise ratio. TheInterferometer is required when using higher wavelengths because the intensity of the scattered signal is dramatically decreased when radiation moves to higher values., and it can collect weaker spectra.  This technique can help in avoiding intense fluorescence resulting from certain materials, such as dyes, lake pigments and, natural resins, among others.

Surface Enhanced Raman Spectroscopy (SERS)

Normal Raman spectroscopy is quite weak, since it is carried out at an excitation wavelength far from any optical resonance of the molecular system. Surface Enhanced Raman Spectroscopy (SERS) produces large signal enhancements when target compounds are near metal substrates as observed by Fleischmann, Hendra and McQuillan in 1974 on chemically roughened silver electrodes. The substrate is of utmost importance in SERS, and they are often made of variations on silver or gold. The most commonly used substrates used in cultural heritage include nanoparticles of various shapes in colloidal suspension, chemical deposited onto solid surfaces, embedded within gel matrices.

References, Resources, Databases, Publications

References

• Correia AM, Clark RJH, Ribeiro MIM, Duarte MLTS. Pigment study by Raman microscopy of 23 paintings by the Portuguese artist Henrique Pousão (1859–1884). J Raman Spectrosc 2007;38:1390–405.
• Košařová V, Hradil D, Hradilová J, Čermáková Z, Němec I, Schreiner M. The efficiency of micro-Raman spectroscopy in the analysis of complicated mixtures in modern paints: Munch’s and Kupka’s paintings under study. Spectrochim Acta A. 2016;156:36–46.
• Li Z, Wang L, Chen H, Ma Q. Degradation of emerald green: scientific studies on multi-polychrome Vairocana Statue in Dazu Rock Carvings, Chongqing, China. Heritage Science. 2020;8:64.
• Madden O, Cobb KC, Spencer AM. Raman spectroscopic characterization of laminated glass and transparent sheet plastics to amplify a history of early aviation ‘glass’: Raman spectroscopic characterization of historic aviation ‘glass.’ J Raman Spectrosc. 2014;45:1215–24.
• Needham A, Croft S, Kröger R, Robson HK, Rowley CCA, Taylor B, et al. The application of micro-Raman for the analysis of ochre artefacts from Mesolithic palaeo-lake Flixton. J Archaeol Sci Rep 2018;17:650–6.
• Nevin A, Osticioli I, Anglos D, Burnstock A, Cather S, Castellucci E. Raman Spectra of Proteinaceous Materials Used in Paintings: A Multivariate Analytical Approach for Classification and Identification. Anal Chem. 2007;79:6143–51.
• Pozzi F, Lombardi JR, Bruni S, Leona M. Sample Treatment Considerations in the Analysis of Organic Colorants by Surface-Enhanced Raman Scattering. Anal Chem. 2012;84:3751–7.
• Robinet L, Eremin K, Cobo del Arco B, Gibson LT. A Raman spectroscopic study of pollution-induced glass deterioration. J Raman Spectrosc. 2004;35:662–70.
• Ropret P, Kosec T. Raman investigation of artificial patinas on recent bronze – Part I: climatic chamber exposure. J Raman Spectrosc. 2012;43:1578–86.
• Vandenabeele P, Wehling B, Moens L, Edwards H, De Reu M, Van Hooydonk G. Analysis with micro-Raman spectroscopy of natural organic binding media and varnishes used in art. Anal Chim Acta  2000;407:261–74.
• Vítek P, Ali EMA, Edwards HGM, Jehlička J, Cox R, Page K. Evaluation of portable Raman spectrometer with 1064nm excitation for geological and forensic applications. Spectrochim Acta A: Molecular and Biomolecular Spectroscopy. 2012;86:320–7.
• Publications
• Clark RJH. Applications of Raman Spectroscopy to the Identification and Conservation of Pigments on Art Objects. In: Chalmers JM, Griffiths PR, editors. Handbook of Vibrational Spectroscopy. Chichester, UK: John Wiley & Sons, Ltd; 2006
• Smith E, Dent G. Modern Raman spectroscopy: a practical approach. Second edition. Hoboken, NJ: Wiley; 2019.
• Vandenabeele P. Practical Raman spectroscopy: an introduction. The Atrium, Southern Gate, Chichester, West Sussex, United Kingdom: Wiley; 2013.

Review Articles

• Casadio F, Daher C, Bellot-Gurlet L. Raman Spectroscopy of Cultural Heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing. Top Curr Chem  2016;374:1–51.
• Committee AM, No 67 A. Raman spectroscopy in cultural heritage: Background paper. Anal Methods. Royal Society of Chemistry; 2015;7:4844–7.

Databases

• The Infrared and Raman Users Group (IRUG)
• Synthetic Organic Pigment Research Aggregation Group (SOPRANO)
• Raman Spectroscopic Library of Natural and Synthetic Pigments (UCL)
• Cultural Heritage Science Open Source (CHSOS)
• Conservation and Art Materials Encyclopedia Online (CAMEO)

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