Analytical Vibrational Spectroscopy - NIR, IR, and Raman

Sep 30, 2011

Molecular vibrations can be exquisitely sensitive to molecular structure, and as such, measurements detecting them have gained favor for analytical purposes. Easy-to-use instrumentation to detect molecular vibrations is well developed in the mid-infrared (IR), near-IR (NIR), and visible (Raman) regions of the spectrum. The instrumentation has been optimized for each spectral range, but the physical processes producing the spectra are all different. Deciding which technique will be the measurement of choice will depend on the sample format, the required sample environment, the available database, and which process provides the most relevant information to answer the required question. In this installment, I will attempt to guide you through the maze of choices and describe what is being measured, how it is being measured, and what the advantages and disadvantages of these techniques are.

What is being measured? Vibrations can be detected by direct absorption of light into fundamental levels, by absorption into overtones and combinations of those levels, and by scattering off those levels with lasers. The first process is called mid-infrared (IR) absorption, named as such because the vibrational energies are in the mid-IR part of the electromagnetic spectrum. While at Cornell University (Ithaca, New York), William W. Coblentz measured IR absorption spectra of a large number of organic compounds from which he was able to catalog group characteristic bands. Even though he was not the first to measure mid-IR spectra, he is credited with developing the application of IR spectroscopy for structural diagnostics (1). The second process is near-infrared (NIR) absorption, named as such because the overtones and combinations of CH, NH, and OH vibrations and their angle-deformations occur in the near-IR part of the electromagnetic spectrum. According to Jerry Workman (2), the development of NIR spectroscopy for analytical purposes goes back to about 1960 when spectra–structure correlations for a number of functional groups were documented (3). The scattering process is called the Raman effect because C.V. Raman was the first to report the phenomenon (4,5) that had actually been predicted about 5 years earlier by Smekal (6). In fact, Raman received the Nobel Prize for this work in 1928.

By its nature, a scattering experiment is quite different from any absorption experiment. In an absorption experiment, one is always ratioing the spectrum of transmitted or reflected light to the spectrum of the light source; thus the signal intensity is absolute and the spectrum should be invariable from instrument to instrument, unless there are effects of spectral resolution. On the other hand, it is very difficult to normalize the scattering intensity to absolute units; one can correct for the laser intensity, but that is only the first of the phenomena that affect the spectral intensity. One important effect is that of the laser wavelength; everything else being equal, a shorter-wavelength laser will produce a stronger signal than a longer-wavelength laser because of the physics of the scattering phenomenon itself. In addition, if the sample is colored (has an electromagnetic absorption in the vicinity of the laser wavelength), the signal will be enhanced and the relative intensities will change drastically. Another factor affecting the signal is the volume of excitation (laser-irradiated volume). Then the optics used to image the laser-irradiated volume on the entrance slit will determine the angular aperture from which the light is collected, as well as the size of the image on the slit, which will, in turn, determine how much light is detected. And if the sample is turbid or opaque, the size of the illuminated volume will be affected. To conclude this topic, one can see that it is not easy to produce a Raman spectrum representing absolute scattering intensity, thereby making the application of the technique less straightforward than the two absorption phenomena.

There also is the issue of the polarity of the photons. Who among us remembers group theory? The point is that a photon is a shorthand name for a bundle of energy described by coupled oscillating electric and magnetic fields that move through space. The smallest unit of energy (the energy of a photon or light quantum) is proportional to the reciprocal of the wavelength (λ), called the wavenumber.

The interaction between the photons and the molecules is by the coupling with the photon's electric field, which of course is a polar quantity. That means that for a molecular vibration to absorb radiation, not only must the energy of the vibration match the photon energy (v), but the vibration has to produce an electric dipole. That is why polar groups like carbonyls are strong IR absorbers. In the case of Raman scattering, there are two photons, so the molecular vibration must have even character. (Two odd photons make "even" character and will couple with a symmetric-type vibration.) That is why a vibration like a C=C double bond is strongly Raman active. The implication of this is that IR and Raman spectroscopies are complementary — vibrations that appear in one do not appear in the other if there is a center of symmetry; or, when there is no center of symmetry, a vibration tends to be strong in one effect and weak in the other.

So what about NIR absorption? Again we have an absorption process, but each photon makes a transition to more than one vibration so that the energy range is high enough to reach the NIR range of the spectrum. The observed spectra are attributed to overtones of CH, NH, and OH stretches and combinations of the stretches and angle deformations; the region between 700 and 1600 nm is typically assigned to overtones (14,300–6250 cm-1), and the region between 1600 and 2500 nm to combinations (6250–4000 cm-1). Because there is only one photon, the symmetry of the sum of the vibrations has to be compatible with the odd character of the electric field. That would imply, for instance, that a combination of a symmetric and asymmetric XH vibrations would occur in the "overtone" region. It should be noted that the longer-wavelength detectors that are sensitive to the combination region are noisier and more expensive (in the case of the multichannel arrays this is quite significant) than the shorter-wavelength detectors that can measure overtones. An important point that should be made is that because of the overall weakness of the absorptions in the NIR, relatively thick materials can be measured, even in transmission.