The Comparison between a Near IR spectrometer (diffuse reflectance) and a Raman/FT-Raman spectrometer
In a previous article, a near IR spectrometer operating in diffuse reflectance geometry was compared with a FTIR/FT-NIR spectrometer. In this article the attention is turned to comparing a near IR spectrometer to a Raman/FT-Raman spectrometer. Each one will have its advantages and disadvantages and one has to recognize in which application, what is the technique of choice.
To begin with, one has to describe the technique of Raman spectroscopy. Contrary to near infrared spectroscopy which utilizes overtones and combination bands of vibrational spectra of molecules and is basically an absorption process, Raman spectroscopy is a scattering process. Normally, photons from a laser beam are scattered from a molecule with a center of symmetry and the majority of the scattered photons are at the same frequency as the source laser photons (Rayleigh scattering). However a small percentage of photons are scattered at frequencies which is lower than the photon laser frequency (Stoke lines) and even a smaller percentage which will be scattered at higher frequencies with respect to the incident photon (Anti-Stoke lines). These frequency shifted spectra are called the Raman spectra of the molecule.
Figure 1, compares the molecular transitions of the Raman spectra with that of the IR spectra
Figure 1: Comparison of Raman and IR absorption spectra
As is visible from the diagram, the Stoke and Anti-Stoke photons are offset from the laser frequency by a frequency difference which corresponds to a vibrational transition within the molecule. Hence measuring the Raman scattering spectra would provide information regarding the vibrational transitions within the molecule. It is also important to recognize that Raman scattering is an inelastic scattering process in which the incident radiation loses energy or gains energy through the scattering process.
For the near IR absorption spectra, the data is normalized with respect to the incident radiation and hence is an absolute measure, however for the Raman spectra, the data is not normalized and is relative. One may perform certain smoothing operations to remove the DC offset or the noise but the measurement is relative.
One has to recognize that the molecules which exhibit Raman scattering or in other words are Raman active, may not be near IR active and vice versa. The question is what conditions need to be met for a molecule to be near IR active (or IR active, since near IR is just the overtones and the combination bands of IR) or Raman active. For a molecule to be Raman active, the incident radiation needs to change the polarizability of the molecule and it needs to have a center of symmetry. For Raman activity to be predominant, the incident radiation needs to change the dipole moment of the molecule and hence the molecule should not have a center of symmetry. This way, the Raman activity or the IR activity are complementary to each other. Most of the molecules which are Raman active, are not IR active and vice versa. Figure 2 shows the concept.
Figure 2: Comparison of Raman and IR active molecules
Raman spectroscopy can distinguish between bonds consisting of two similar atoms but different bonds such as C-C, C=C and C☰C bonds. Example of bonds which show NIR activity but no Raman activity are C-H, N-H and O-H bonds which all have different atoms forming the bond.
One can distinguish between Raman scattering and IR spectroscopy by measuring the relative frequency at which the sample scatters versus the absolute frequency at which a sample absorbs the radiation.
Now that the basics of the Raman scattering and the NIR absorption process have been discussed, one needs to explain the instrumentation for each category and the different applications in which each technique is preferable to use.
Originally, Raman spectroscopy was always done using a visible laser as the excitation source. This was necessary because Raman effect is a very weak effect and in order to observe it, one needs a strong source. However since Rayleigh scattering is so predominant, one needed to build instrumentation to block the very strong Rayleigh scattered line to be able to observe the Raman scattered line. Figure 3, shows an example of such an instrument.
Figure 3: Setup for a Raman spectrometer to block the Rayleigh line
A notch filter centered at the same frequency as the laser line is utilized to block the strong Rayleigh line and the Raman scattered radiation goes through a two stage monochromator to increase the signal to noise ratio and falls on the detector. In some other spectrometer designs, instead of a notch filter, two stage Czerny-Turner spectrometer are used to block off the Rayleigh line and a higher focal length spectrometer is used to resolve the Raman lines. This is in contrast with diffuse reflectance spectrometer in which a broad-band near IR source such as a halogen-Tungsten lamp embedded within the body of the spectrometer shines on the sample and the diffuse reflectance is detected. Figure 4 shows the setup for the NIR (diffuse reflectance spectrometer).
Figure 4: Set-up for a specrto-reflectometer in diffuse reflection mode
The NirvaScan spectrometer introduced by Allied Scientific Pro has the above configuration.
Applications for each spectrometer: A huge drawback for Raman spectrometer as compared to diffuse reflectance spectrometers such as NirvaScan spectrometer (introduced by Allied Scientific Pro) is that the Raman signal can be easily swamped by the fluorescence signal whereas the NIR technique is insensitive to fluorescence. For this reason Raman spectrometer designers have moved to an excitation source in the near IR region of the spectrum (1064 nm YAG laser) and although the strength of the Raman signal decreases as the fourth power of wavelength, the fluorescence signal is highly diminished. FT-Raman systems have also been introduced which use the multiplexing and throughput advantage of Fourier Transform spectroscopy and have higher signal to noise ratios. Another technique to reduce the fluorescence signal for Raman is photo-bleaching but for some samples neither a FT-Raman technique nor a photo bleaching processes reduces the fluorescence signal and the choice between Raman and diffuse reflectance NIR is definitely in favour of diffuse reflectance NIR spectrometer. Figure 5 shows the schematic for an FT-Raman spectrometer.
Figure 5: Simple schematic of a FT-Raman system
There are some similarities between the two techniques which are as follows:
● Both techniques could be used to probe samples with large water content. Raman signal from water is pretty weak and NIR water absorption is not strong either.
● As far as the size of the instruments are concerned, both techniques have small hand-held portable sizes.
● Both can be used to probe the bulk of a sample contrary to FTIR which is mostly used for surface investigation due to strong mid-IR signals.
There are some further disadvantages for Raman spectrometers as follows:
● The acquisition time for Raman and FT/Raman spectrometers could be much longer than diffuse reflectance and matter of 10s of seconds as opposed to order of 1 second for NIR spectroscopy.
● FT-Raman system do not work for heated samples above 250° C because the blackbody radiation could swamp the Raman signal. NIR in diffuse reflectance could still work in elevated temperatures.
● The cost of a Raman/FT-Raman spectrometer is higher than a NIR diffuse reflectance spectrometer because they have to use a laser source.
● It is difficult to work with black coloured samples with Raman spectrometers because they heat up and the background emission would suppress the Raman signal.
In summary, for samples which have high fluorescence signal, the diffuse reflectance NIR spectrometer such as NirvaScan spectrometer (introduced by Allied Scientific Pro) is a better choice Raman/FT-Raman spectrometers. Cost needs to be taken into account as well for the choice as well since portable NIR spectrometers are more economical.
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2- Analytical vibrational spectroscopy, NIR, IR and Raman, Fran Adar, Spectroscopy online journal, volume 26, issue 10, Oct 2011.
3- Fourier Transform Raman Spectroscopy, D.Bruce Chase, J. Am. Chem Soc. 108, 1986.