Technology
NIR FT Raman
NIR FT Raman spectroscopy
While stimulating the Raman effect this can be covered by simultaneously stimulated fluorescence. Both effects show large differences in their intensities by several orders of magnitude. In this case it is almost impossible to collect a Raman spectrum. The collection of Raman spectra becomes like searching a needle in a hay stack.
Choosing an excitation wavelength far away of any electronic transitions avoids stimulating fluorescence basically. If the Raman effect is stimulated in the near infrared (NIR) region at a wavelength of 1064 nm many materials do not show fluorescence anymore. However, since the intensity of the Raman scattering is depending on the fourth power of the excitation wavelength stimulating Raman spectra at longer wavelengths results in a strong decrease of the Raman effect.
Technological advantages of NIR FT Raman spectroscopy
This intensity decrease can be compensated by using spectrometers equipped with an interferometer which enables a higher throughput of the instrument. It provides an interferogram that can be transformed to a spectrum by calculating a Fourier transformation (FT). The higher throughput of interferometer based spectrometers is known as the Jacquinot advantage and is based mainly on transporting the scattered light through large circular apertures in the instrument.
For NIR FT Raman spectroscopy typically Germanium (Ge) and Indium Gallium Arsenide (InGaAs) detectors are used which are most sensitive in this spectral range. Additionally, the throughput of fiber probes is very high. This enables to transmit as well the laser light as the Raman scattering over distances of several meters with almost no intensity decrease.
Limitations of NIR FT Raman spectroscopy by self absorption of organic solvents and water
Organic solvents, the sample itself, and especially water are partly absorbing the Raman scattering in the spectral range of NIR FT Raman spectroscopy. However, at the excitation wavelength of 1064 nm the absorption spectrum of water has a local minimum and the Raman scattering can be excited in water at a depth of several millimeters almost without decreasing the laser intensity. Additionally, in water the strongest absorbing spectral range only coincides with the Raman scattering of disulphide bonds. So NIR FT Raman spectra can be measured with an excellent signal to noise ratio at low penetration depths.
