Raman Spectroscopy In Raman Spectroscopy, a substance is irradiated with a narrow-band light at a high optical power to detect the weak emission lines which arise from spontaneous or stimulated Raman Scattering. The spectrum of Raman Scattered light contains information about molecular vibrations, and helps identify key properties of the material - such as wavenumber scale for example, which is essential in pharma-applications.
Raman Spectroscopy is an analytical tool used to determine key properties of inorganic and organic materials. The chemical bonds within the material scatter incident light, producing a characteristic Raman Spectrum. The technique is non-destructive, requires minimal sample preparation and provides rapid results. Raman Spectroscopy can be used to quickly identify a sample and to provide information about properties such as structure, phase, temperature, crystallinity, polymorphism, protein folding, hydrogen bonding, intrinsic stress and strain and contamination.
The Raman effect was discovered by the scientist C. V. Raman in 1928 using sunlight as the light source. In 1930 Raman won the Nobel Prize in physics for this discovery.
Samples are illuminated using monochromatic laser light in the visible, near infrared or near ultraviolet range. The photons interact with the bonds and electron clouds of the molecules and are scattered differently depending on the vibrational, rotational, and other low-frequency modes of the system. Optics are used to collect and filter the scattered light, which is then dispersed onto a detector. Computer software analyzes this input and arranges the data to produce a Raman spectrum. Specialized Raman techniques include spatially-offset Raman Spectroscopy, surface-enhanced Raman Spectroscopy, polarized Raman Spectroscopy, resonance Raman Spectroscopy and tip-enhanced Raman Spectroscopy.
Raman Spectroscopy is based on the principle that not all light scattered from a molecule is elastically scattered. Elastic scatter, in which incident photons scattered from the molecule have the same wavelength, is termed Rayleigh scatter. Most of the scatter from a molecule is Rayleigh scatter; however, a small but measurable fraction consists of photons that are inelastically scattered. These photons are scattered at wavelengths different from the incident photons, corresponding to the energy of the vibration of the molecules and their structure. This process is termed the Raman effect and the inelastic scatter is termed Raman scatter. Usually, Stokes Raman scatter is observed, in which the scattered photon has a longer wavelength (lower energy) than the incident photon due to the molecule being in a ground state. Anti-Stokes Raman scatter is observed when the molecule is in a vibrationally excited state and the scattered photon has a shorter wavelength (higher energy) than the incident photon.
A Raman Spectrum features peaks of Raman scattered light, showing their intensity and wavelength position. Each bond or group of bonds within a molecule has a unique molecular vibration and structure. The interaction of light with these vibrating molecular bonds produces unique corresponding peaks on the spectrum. This creates what is essentially a chemical "fingerprint”"which can be used to quickly and reliably identify a sample and to characterize its composition. Spectra from Stokes Raman scatter and Anti-Stokes Raman scatter contain the same frequency information. Most Raman spectra are representations of Stokes scatter as most molecules at room temperature exist in the ground state. Anti-Stokes scatter is used for contact thermometry and when Stokes scatter is not easily observable.
Raman Spectra can often be combined in mapping systems to produce images of a sample. This allows for the visualization of the distribution of components within a sample as well as variation in properties such as crystallinity and polymorphism.
Samples that can be analyzed using Raman Spectroscopy include pure chemicals, solutions and mixtures of inorganic and organic solids, powders, liquids, gels, slurries and gases. Both general characteristics as well as subtle compositional and structural properties can be determined. Raman Spectroscopy is a non-destructive, non-contact technique, making it invaluable when sample integrity must be preserved or sampling is to be done in situ, in vitro or in vivo. Chemical reactions can be analyzed as they occur without interference. Drug-cell and chemical-skin interactions can be studied. Forensics or art and archaeology samples can remain unchanged. A typical spectrum takes only seconds to produce and minimal sample preparation means high throughput.
Raman Spectroscopy is used across several industries, including:
A wide range of wavelengths can be used for Raman Spectroscopy, including but not limited to: near ultraviolet (244 nm, 257 nm, 325 nm, 364 nm), visible (457 nm, 473 nm, 488 nm, 514 nm, 532 nm, 633 nm, 660 nm), and near infrared (785 nm, 830 nm, 980 nm, 1064 nm). The choice of which laser to use is dependent on the type of application and several factors have to be considered. Raman scattering intensity is inversely proportional to wavelength so infrared lasers will produce weaker scattering as compared to lasers emitting in the visible to ultraviolet range. Certain types of materials are better analyzed by certain wavelength ranges. Visible wavelength lasers are useful for inorganic materials, while near infrared and near ultraviolet lasers are utilized when fluorescence suppression is desired as fewer materials absorb at these wavelengths. Choice of wavelength also contributes to achievable spectral resolution. Spectral resolution is the amount of detail in the Raman Spectrum. Low to medium spectral resolution is sufficient for general screening and identification of materials. High spectral resolution is required in order to determine characteristics such as polymorphism, crystallinity, protein folding, hydrogen bonding and intrinsic stress/strain. The higher the spectral resolution needed the longer the total measuring time. Laserglow Technologies offers several laser systems for Raman Spectroscopy. Contact our staff for application specific recommendations.