3.2: Fourier Transform Near-Infrared Raman Spectroscopy

3.2.1 Theory

Raman spectroscopy is an emission technique which involves irradiating a sample with a laser and collecting the scattered radiation. Most of the scattered radiation has the same wavelength as the laser. A very small fraction (approximately one millionth) of the scattered radiation is displaced from the laser wavelength by values corresponding to the vibrational frequencies of the sample. This is the Raman signal.

The major features of Raman spectroscopy are:

1. The information on molecular structure is complementary to that obtained from infrared spectroscopy.

2. Functional groups that give weak absorptions in the infrared, such as S S, C=C, and N=N, give strong Raman signals.

3. It is a non-contact, non-destructive technique.

4. No sample preparation is required.

5. Glass is an ideal optical window material.

6. Dangerous or delicate samples may be examined in sealed containers.

Conventional Raman spectrometers use visible lasers to irradiate the sample with wavelengths between 400 and 800 nm. At these wavelengths 90% of 'real-world' samples fluoresce and no useful Raman data can be collected. When a near-infrared laser (i.e., near-infrared Raman spectroscopy) is used to irradiate a sample, fluorescence is unlikely to occur and at least 80% of samples will give useful Raman spectra. Thermal degradation of coloured and delicate samples is also reduced. The intensity of Raman scattering is dependent on the wavelength of the excitation source, therefore a near-infrared laser produces a much weaker Raman signal than a visible laser. However, Fourier transform technology provides the signal-to-noise advantage necessary to overcome...