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Spatially offset Raman spectroscopy

Abstract

Spatially offset Raman spectroscopy (SORS) is a spectroscopic technique that allows for the non-invasive chemical characterization of diffusely scattering materials, ranging from opaque plastics to biological tissues. SORS has been explored for a range of applications, including disease diagnosis, the detection of explosives through unopened containers and the in-depth, non-destructive analysis of pharmaceutical products and objects of art. This Primer introduces the reader to the basic concepts underpinning SORS, details best practices for its implementation, highlights its use across multiple fields and provides insight into its limitations. The Primer concludes by discussing potential applications and envisaging future developments in the field.

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Fig. 1: Raman scattering.
Fig. 2: The SORS concept.
Fig. 3: Schematic of the SORS instrumental set-up.
Fig. 4: Variants of SORS.
Fig. 5: Representative SORS spectra from a two-layer system.
Fig. 6: Analysing the contents of a plastic container.
Fig. 7: Analysing neurotransmitters using SESORS.

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Acknowledgements

This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) grant EP/R020965/1.

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Introduction (N.S.); Experimentation (P.M. and S.M.); Results (P.M. and S.M.); Applications (C.C.); Reproducibility and data deposition (N.S.); Limitations and optimizations (P.M.); Outlook (N.S); Oversight of Primer (P.M.).

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Correspondence to Claudia Conti, Nick Stone or Pavel Matousek.

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Related links

Bruker OPUS software: https://www.bruker.com/products/infrared-near-infrared-and-raman-spectroscopy/opus-spectroscopy-software.html

ChemSpider database: http://www.chemspider.com/Spectra.aspx?st=R

CytoSpec: http://www.cytospec.com/ftir.php

Gaussian software package: https://gaussian.com/

GRAMS/AI: https://www.thermofisher.com/order/catalog/product/INF-15000?SID=srch-srp-INF-15000#/INF-15000?SID=srch-srp-INF-15000

KnowItAll Raman Spectral Database Collection: https://sciencesolutions.wiley.com/solutions/technique/raman/knowitall-raman-collection/

Matlab: https://www.mathworks.com/products/matlab.html

Origin: https://www.originlab.com/

RRUFF: https://rruff.info/

Solo: https://eigenvector.com/software/solo/

WiRE: https://www.renishaw.com/en/raman-software--9450

Glossary

Rayleigh scattering

The elastic scattering of electromagnetic radiation by particles smaller than the wavelength of the radiation.

Polarizability

The degree to which a molecular dipole changes in response to an external electric field.

Raman scattering

The inelastic scattering of photons, where the frequency of the scattered photon is different from that of the incident photon.

Photon shot noise

Fluctuations of the detected number of photons, caused by the inherent particle-like properties of photons.

Lambert’s cosine law

A law describing the cosine dependence of light emission intensity with respect to the angle of incidence from the surface normal.

Solid angle

A measure of the amount of the field of view that an object occupies from a particular point.

Acceptance angle

The maximum incidence angle of an optical ray that is transmitted to the spectrograph, measured from the optical axis of the spectrograph.

Cosmic rays

High-energy protons and atomic nuclei that move through space at nearly the speed of light.

Surface plasmon resonance

A resonant oscillation of nanoparticle conduction electrons induced by incident light; its spectral properties are dependent on nanoparticle size, shape and metal type.

Monte Carlo simulations

Numerical algorithms that rely on the random sampling of events.

Imaging phantoms

Specially prepared samples that mimic the properties of real biological tissue for the purposes of optical imaging.

Polymorphs

Identical chemicals of different crystalline forms.

Etaloning

Wave-like modulation of charge-coupled device (CCD) sensitivity across the sensor caused by light interference and associated with back-illuminated CCDs.

Read-out noise

Noise induced by charge digitization circuitry, imprinted on the signal when it is read.

Thermal noise

Noise induced by thermal fluctuations of charge carriers within a detection element.

Instrument response function

In the context of spatially offset Raman spectroscopy, a combined spectrograph–detector spectral intensity profile in response to illumination by a spectrally uniform light source.

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Mosca, S., Conti, C., Stone, N. et al. Spatially offset Raman spectroscopy. Nat Rev Methods Primers 1, 21 (2021). https://doi.org/10.1038/s43586-021-00019-0

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