10. Raman Spectroscopy Email to: ijvs@soton.ac.uk
Molecular vibrations give rise to both mid infrared absorptions (i.e. their frequencies
lie between 200 & 3700cm-1 (50 - 2.6µm wavelength)) but also Raman
scattering. Raman scattering is an inelastic scattering process defined in Figure 1.
If a specimen is illuminated by a monochromatic source - these days invariably a laser - the scattered light will consist of light at the frequency of the source Vo ± V vibrational. If the source has a wavelength of 1 micron i.e. 10,000cm-1 and the sample vibrates at 500cm-1 then two incredibly weak scattering 'sidebands' will occur at 9500 and 10,500cm-1. These bands are the Raman effect predicted in 1923 by Smekal and discovered five years later by Sir C.V. Raman.[1] Thus, Raman Spectroscopy is an alternative method for us to record the vibrational characteristics of a sample in the full knowledge that this very vibrational behaviour constitutes an excellent fingerprint for the specimen under analysis.[2] In Figure 2, I show the combined mid infrared and the Raman spectra of a specimen. You will notice that to compare the two spectra, infrared at the top Raman at the bottom, the axes are different in each case: in the infrared we plot transmission against the spectroscopic frequency. In the Raman, the plot is scattering intensity vs wave-number shift from the wave-number value of the laser. In this way, the two measurements of the vibrational properties of our molecule are directly comparable.
What do we see? The overall spectrum is similar in the Raman and in the infrared, but the shapes of the two spectra are totally different. If both techniques are measuring the same set of molecular properties this seems odd. In infrared spectroscopy, we find that resonance occurs between the electromagnetic radiation bathing the sample when a molecular vibration contorts in such a way that the molecular dipole periodically varies. If it doesn't - no resonance - no absorption of the mid infrared radiation. Raman scatter occurs when the polarizability of the molecule varies as it vibrates. The polarizability is best described to students as the ease of which electrons can be induced to slosh around a molecule as a field is applied. This polarizability (the sloshability!) can vary as a molecule contorts and in some vibrations this does indeed occur. Hence, some vibrational motions give rise to infrared absorption, some to Raman bands and some to both but in this latter case one would not expect the intensities of the interactions to be the same in each spectrum. One of those dreadful rules of thumb which work but are always letting us down can now be enunciated. Symmetrical vibrations tend to give strong Raman bands, asymmetric ones give prominent absorptions in the mid infrared. Also the "group frequencies" are dissimilar. Group frequencies are an incredibly old concept going back to Coblentz at the turn of
the century (1900 not 2000!). Specific chemical functional groups often have a vibrational
frequency associated with the group, which enables infrared users to carry out molecular
diagnosis. Excellent examples are
To summarise: we can examine the vibrational characteristics of a molecule by two
complimentary techniques - mid infrared absorption and Raman scattering. Obviously, to do
the analytical job properly one ought to record both but hardly anyone does - or rather did!
A simple description of the Raman effect will be found in Ref 3. Raman spectroscopy has, since it's discovery been dogged by fluorescence. If a sample is only slightly fluorescent or if it is contaminated with a trace of a fluorescer, the fluorescence will be stronger than the very weak Raman lines and inundate them. As a result, Raman spectroscopy has failed to attract analysts. What is the point of applying a method where impure or degraded specimens are inaccessible? As a result, there has been a tendency to use infrared along with nmr and mass spectroscopy as front-line analytical methods. Late in the 1980s, commercial Raman systems began to appear that used near infrared
laser sources (hence avoiding the fluorescence problem)[4,5]. An even more exciting
development was that the new Raman facilities were offered as accessories on FTIR
instruments. These F-T Raman accessories are now offered by all of the leading
manufacturers and thousands are now in service. The combined FTIR/FT Raman system provides
the analyst with a complete vibrational spectroscopic facility all in one instrument. The Raman experiment is a scattering process so the experiment involves hitting a
sample with intense monochromatic light, collecting and analysing the scatter. To compare
the two processes, infrared absorption and Raman scatter study Figure 3. On the left, I
draw the optical system favoured by all the various producers except Nicolet of America.
This manufacturer prefers to collect the scattered light with a gold surfaced mirror
rather than a lens but the principle is exactly the same - laser light onto the sample -
collect the scattered light in the reverse direction… On the right of Figure 3, I
draw a simple transmission experiment and quote the Beer Lambert Law. Clearly the amount
of radiation absorbed can be controlled both by varying the thickness of the sample and
the concentration of the analyte. Under the left-hand diagram you will see the Raman
equivalent of the Beer Lambert Law. Note that the intensity can only be varied by changing
the concentration of the analyte and the laser power. Sample thickness is not a useful
variable but the mean volume irradiated is of importance.
Let us then compare and contrast FTIR and FT Raman
So the obvious question arises - why should I consider using Raman as an
analytical procedure when I already have available a compete suite of more conventional
techniques? The answer is that for specific analyses where Raman works well, the ability
to record spectra from unprepared samples in glass vessels or through glass windows and
the possibility of operating remotely are very attractive. The experiment is entirely
open-ended - the laser impinges on the sample and the scattered light is collected in the
reverse direction so sample size and shape are irrelevant. Now, the Raman scattering of
some components is selective i.e. scattering from one compound can be much more intense
than from others and this can be exploited - many actives give much stronger spectra than
excipients and this can be used to make quantitative measurements even when the active is
present at low levels. Frequently, different polymorphs or hydrates give really different
Raman spectra hence production measurements and control or formulation monitoring and
shelf life studies are all in prospect. In addition, Raman provides a laboratory
analytical tool for the synthetic chemist complimenting FTIR, NMR and Mass Spectrometry. The account above extols the virtues of FT-Raman - near infrared sources and an accessory on an FTIR. There is an alternative method of recording spectra, which is much closer to the experiment carried out by Raman himself in 1928. Rather than using the Fourier Transform/Interferometer as the basis one buys a dedicated Raman instrument which incorporates a Charge Coupled Device # detector. Because these detectors operate in the visible and very deep red, these instruments incorporate a VISIBLE laser. # An up-market version of the detector in your Camcorder. The latest CCD detectors are incredibly sensitive and are used in the most advantageous
way in the instruments on offer. As a result, they have much more sensitivity than their
FT Raman competitors.[6] "Much more" is 100 to 1000x so it is really worthwhile.
In Figure 4, I show the block diagram of a typical CCD based instrument. Several points
are noteworthy - these instruments very frequently use a modified conventional visible
microscope as the sample system. They will also work through optical fibres so these
visible laser CCD systems are the best Raman machines on offer WHERE THEY ARE APPLICABLE.
Thus, it is worth considering briefly the types of sample where Raman analysis can be
useful, comparing CCD and FT machines.
Sample types in Pharmaceutical Analysis
However the real point is that the individual analysis will, in effect, define the approach to be adopted. Thus, well-endowed laboratories have both systems. To give you a feel for the quality of good Raman spectra here is an example.
References
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