Simple Description of Raman Spectroscopy

Raman Spectroscopy

Simple Description of Raman Spectroscopy

• Radiation incident upon a molecule can undergo several types of scattering, including:
• Rayleigh (elastic) scattering where the photon energy remains the same
• Raman (inelastic) scattering, in which the photon energy can decrease (stokes), or decrease (anti-stokes); the change in energy being stored in (released from) vibrational motion
• Raman scattering occurs with a much lower probability (~1,000,000,000 times less) than Rayleigh scattering
• Energy changes are measured as a frequency shift, and reported as wavenumbers (inverse centimeters) for convienience
• In our experimental setup, the Raman scattered light is collected using a spectrograph and CCD detector
• The Raman scattered light is dependent on the vibrational modes of the molecule, and is therefore different for every molecule, providing a fingerprint for the molecules being studied
• In biological Raman spectroscopy it is advantageous to use longer wavelength laser excitation in order to reduce fluorescence from the sample

NIR Raman spectroscopy is well suited for probing the biochemical/morphological composition of human tissue. This methodology has been reviewed extensively in the literature.1-3 Several features of NIR Raman spectroscopy are especially important to the study of human tissue. In this type of vibrational spectroscopy, the spectra are products of molecular vibrations occurring within specific chemical bonds. Raman spectra collected in the so-called “fingerprint region” consist of unique combinations of sharp bands that allow identification, and even quantifation, of the chemical species involved. NIR excitation wavelengths have relatively small extinction coefficients and large penetration depths in human tissue (approximately 1 mm), providing the opportunity to observe subsurface structures. The small absorption coefficient also precludes photolytic sample decomposition. NIR wavelengths are easily transmitted via optical fibers; the use of optical fiber probes for NIR Raman spectroscopy has been clearly demonstrated for non-biological samples4-9, and is beginning to emerge for biological systems.10-17 In contrast to vibrational spectra obtained via mid-IR absorption, water is a relatively weak absorber in the NIR, and water interference is not a problem in a Raman experiment. Finally, the strong fluorescence interference from biological tissue samples encountered with visible excitation wavelengths is significantly reduced in the NIR region.

REFERENCES

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2. Mahadevan-Jansen A and Richards-Kortum R, “Raman Spectroscopy for the Detection of Cancers and Precancers”, J. Biomed. Optics,1(1), 31-70 (1996).

3. Manoharan R, Wang Y and Feld MS, “Review: Histochemical Analysis of Biological Tissues using Raman Spectroscopy”, Spectrochemica Acta Part A,52, 215-249 (1996).

4. Cooney T, Skinner H and Angel S, “Comparitive Study of Some Fiber-Optic Remote Raman Probe Designs. Part I: Model for Liquids and Transparent Solids”, Appl Spectrosc,50(7), 836-848 (1996a).

5. Cooney TF, Skinner HT and Angel SM, “Comparitive Study of Some Fiber-Optic Remote Raman Probe Designs. Part II: Tests of Single-Fiber, Lensed, and Flat- and Bevel-Tip Multi-Fiber Probes”, Appl Spectrosc,50(7), 849-860 (1996b).

6. Huy NQ, Jouan M and Dao NQ, “Use off a Mono-Fiber Optrode in Remote and in Situ Measurements by the Raman/Laser/Fiber Optics (RLFO) Method”, Appl Spectrosc,47(12), 2013-2016 (1993).

7. Li Y-S and Ma J, “Optical-Fiber Raman Probe with Tilted-End Fibers”, Applied Spectroscopy,51(2), 277-279 (1997).

8. Ma J and Li Y-S, “Optical-Fiber Raman Probe with Low Background Interference by Spatial Optimization”, Appl Spectrosc,48(12), 1529-1531 (1994).

9. Schoen CL, Cooney TF, Sharma SK and Carey DM, “Long fiber-optic remote Raman probe for detection and identification of weak scatterers”, Applied Optics,31(36), 7707-7715 (1992).

10. Buschman H, Römer T, Puppels G, van der Laarse A and Bruschke A, “In Vitro Intravascular Histochemistry of Human Atherosclerosis by Raman Spectroscopy”, Circ,98(suppl.I), I-296 (1998b).

11. Buschman H, Römer T, Wach M, Marple E, van der Laarse A, Bruschke A and Puppels G, “Human Coronary Atherosclerosis Studied in Vitro by Catheter-Based Transluminal Raman Spectroscopy”, 2nd European Conference on Cardiac PET Imaging, Groninen, The Netherlands (1998a).

12. Buschman HP, Marple ET, Wach ML, Bennett B, Bakker Schut TC, Bruining HA, Bruschke AV, van der Laarse A and Puppels GJ, “In Vivo Determination of the Molecular Composition of Artery Wall by Intravascular Raman Spectroscopy”, Analytical Chemistry,72(16), 3771-3775 (2000).

13. Mahadevan-Jansen A, Mitchell M, Ramanujam N, Utzinger U and Richards-Kortum R, “Development of a Fiber Optic Probe to Measure NIR Raman Spectra of Cervical Tissue In Vivo”, Photchem Photobiol,68(3), 427-431 (1998).

14. Puppels G, van Aken T, Wolthuis R, Caspers P, Bakker Schutt T, Bruining H, Römer T, Buschman H, Wach M and Robinson Jr. J, “In vivo Tissue Characterization by Raman Spectroscopy”, SPIE Proc,3257, 78-83 (1998).

15. Shim M and Wilson B, “Development of an In Vivo Raman Spectroscopic System for Diagnostic Applications”, J Raman Spectrosc,28, 131-142 (1997).

16. Shim M, Wilson B, Marple E and Wach M, “Study of Fiber-Optic Probes for in Vivo Medical Raman Spectroscopy”, Appl Spectrosc,53(6), 619-627 (1999).

17. Shim MG, Song L-MWK, Marcon NE and Wilson BC, “In Vivo Near-infrared Raman Spectroscopy: Demonstration of Feasibility During Clinical Gastrointestinal Endoscopy”, Photochemistry and Photobiology,72, 146-150 (2000).