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Raman Spectroscopy
When monochromatic light is directed on a molecule, the light can be scattered or absorbed. Most of the scattered light will be at the same frequency as the incident light. This is known as Rayleigh scattering or elastic scattering.
However, a small fraction of the light (~1 in 107 photons) will be inelastically scattered at frequencies different from the incident photons. The energy difference between the incident and scattered light is proportional to the vibrational energy of the scattered molecules.
This process of energy exchange between scattering molecules and the incident light is the Raman effect.
Figure 1
Energy level diagram of various vibrational spectroscopic transitions
Figure 1 schematically illustrates the energy transitions underlying Raman and infrared spectroscopies. A plot of the intensity of the scattered light versus the energy difference (or shift) is a Raman spectrum (Figure 2). Each peak corresponds to a given Raman shift from the incident light energy, hvo.
What Kind of Information Do We Obtain with Raman Spectroscopy?
Raman spectroscopy provides details on the chemical composition, molecular structure, and molecular interactions in cells and tissues. Therefore, biochemical information on proteins, lipids, carbohydrates and nucleic acids, as they relate to tissue health and pathology, can be obtained. In addition, with polarized Raman spectroscopy information is gained regarding the alignment and/or orientation of molecules within tissues.
Figure 2
Raman spectra of enamel and dentin in human tooth.
How Does it Work?
Figure 3
Schematic of a common sampling geometry for ex vivo Raman spectroscopy using a microscope
The basic components for Raman spectroscopy include a light source (normally a laser), collection optics to gather the Raman-scattered light, and a detection system (Figure 3). Furthermore, for biomedical applications, it is generally preferable to work with laser excitation in the near-infrared region (e.g. 785 nm or 830 nm) so the problems of tissue fluorescence and tissue damage are minimized.
It was not until the recent development of highly sensitive charge-coupled device detectors working in the near-IR region that biological and biomedical uses of Raman spectroscopy became practical for clinical applications.
Figure 4
in vivo fibre-optic Raman sampling configuration.
Biomedical Raman studies can be performed using a Raman microscope for mapping of ex vivo tissue samples or, more recently, via the use of specialized fibre optic probes coupled to a portable Raman system for in vivo measurements (Figure 4).
Raman microscopy
Raman fibre optic sampling
How Can Raman Spectroscopy Be Used?
Future Raman developments will focus on developing custom designed in vivo fibre optic probes and spectrographs for point spectroscopy and Raman imaging. These techniques will be applied to various applications such as those in the Oral-Dental Health and Coronary Artery Disease Programs. Oral-Dental Health and Coronary Artery Disease Programs.
Related Information
Read more about Raman Spectroscopy:
- Bioluminescence Imaging
- Diagnostic Technologies
- Diffuse Reflectance Spectroscopy
- Fluorescence Imaging
- Frequency Modulated Spectroscopy
- High Throughput Diagnostics
- Molecular Imaging - Spectroscopy
- MRI Studies
- Non-linear Optical Microscopic Imaging
- Optical Coherence Tomography
- Speckle Imaging
- Thermal Imaging
