What is Raman Spectroscopy

Raman Spectroscopy - Similar to an infrared spectrum, a Raman spectrum consists of a wavelength distribution of bands corresponding to molecular vibrations specific to the sample being analyzed.  In practice, a laser is focused on the sample, the inelastically scattered radiation (Raman) is optically collected, and directed into a spectrometer, which provides wavelength dispersion, and a detector converts photon energy to electrical signal intensity. Historically, the very low conversion of incident radiation to inelastic scattered radiation (1 in 10⁹) limited Raman spectroscopy to applications that were difficult to perform by infrared spectroscopy, usually aqueous solutions.

Real-time chemical analysis can be performed in a non-contact manner.  The wavelengths and intensities of the scattered light can be used to identify functional groups of molecules because each compound has its own unique Raman spectrum which can be used as a finger print for identification. It has found wide application in the chemical, polymer, semiconductor, and pharmaceutical industries because of its high information content.

Why Raman?

• Easy sampling of solids, powders, gels, liquids, slurries, and aqueous solutions

• No sample preparation

• Sampling through windows, transparent containers, blister packs, or by immersion

• Remote sampling using fiber optic probes (up to 100 meters)

• Sharp spectral peaks for quantitative and qualitative analysis

Why FT-Raman?

In principle, an interferometer has several basic advantages over a classical dispersive instrument. These advantages are:

• Multiplex advantage (Fellgett advantage) All source wavelengths are measured simultaneously in an interferometer, whereas in a dispersive spectrometer they are measured successively. A complete spectrum can be collected very rapidly and many scans can be averaged in the time taken for a single scan of a dispersive spectrometer.
• Throughput advantage (Jacquinot advantage) For the same resolution, the energy throughput in an interferometer can be higher than in a dispersive spectrometer, where it is restricted by the slits. In combination with the Multiplex Advantage, this leads to one of the most important features of an FT-Raman spectrometer: the ability to achieve the same signal-to-noise ratio as a dispersive instrument in a much shorter time.
• Connes advantage The wavenumber scale of an interferometer is derived from a HeNe (helium neon) laser that acts as an internal reference for each scan. The wavenumber of this laser is known very accurately and is very stable. As a result, the wavenumber calibration of interferometers is much more accurate and has much better long term stability than the calibration of dispersive instruments.
• Negligible stray light Because of the way in which the interferometer modulates each source wavelength. There is no direct equivalent of the stray light found in dispersive spectrometers.
• Constant resolution Resolution is constant at all wavenumbers in the defined spectral range but the signal-to-noise ratio varies across the spectrum. FT-Raman instruments have a much higher optical throughput than dispersive instruments and do not use slits to define the resolution. Instead, the resolution is defined by the J-stop (Jacquinot stop) aperture size, which does not change during data collection. In dispersive instruments, throughput is typically optimized by adjusting the slit width during the scan. Thus, signal-to-noise is constant but resolution varies.
• No discontinuities Because there are no grating or filter changes, there are no discontinuities in the spectrum.