Analytical Vibrational Spectroscopy - NIR, IR, and Raman - - Spectroscopy
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Analytical Vibrational Spectroscopy - NIR, IR, and Raman

Volume 26, Issue 10, pp. 14-23


The biggest factor in the definition of the instrumentation for each measurement is what is compatible with the wavelength range. For instance, the NIR region is still compatible with glass optics, which means that fiber optics can be used. However, the detectors used in this region of the spectrum are different from those used in the visible and tend to be based on materials like InGaAs (to 1.7 μm) and extended InGaAs (to 2.5 μm), but other materials such as InSb, Ge, and Pb salts have also been used. The light can be analyzed into a spectrum by using either a grating-based dispersive spectrometer or a Fourier-transform (FT) interferometer. When the dispersive instrument is equipped with a multichannel detector, such as an InGaAs array, all wavelengths are detected simultaneously. Such a dispersive instrument with a multichannel detector will benefit from the multiplex or Fellget's advantage, which was originally defined on FT systems.

For mid-IR measurements, FT instruments are now used almost exclusively. Before the 1960s, IR measurements were made with prism-based dispersive systems equipped with single-channel detectors. The development of digital control and then computers of manageable size provided much of the computing power necessary to engineer FT-IR instruments compatible with industrial and university laboratories. The conversion from dispersive to interferometer-based systems provided powerful new capabilities. Compared to a spectrometer with a light-limiting entrance slit, the entrance aperture of an interferometer accepts essentially all of the available energy (Jacquinot advantage). During the acquisition of the interferogram, the detector is "seeing" all of the wavelengths simultaneously so every point in the transformed spectrum reflects the same state (or combination of states) of the sample (Fellget's advantage). To accurately perform the conversion from the interferogram to a spectrum, the instruments use a laser to search for the zero-crossing of the interferogram; this provides superb wavelength tracking, so the instruments are much less susceptible to environmentally caused drift (Connes' advantage). Because of the rapid evolution of FT-IR instruments, their usage has become universal — thousands and even tens of thousands of instruments have been delivered with ever more sampling accessories. A corollary of this is the assembly of rather large IR data bases that have enormous benefits for identification of unknowns.

As will be discussed later, NIR was developed largely for commercially interesting products because of several technical advantages, not the least of which is the use of glass optics. NIR instruments are easier to construct than mid-IR instruments. They can be small (grating-based) spectrometers or small interferometers, either one with a single-channel detector, and they can also be constructed with a multichannel detector (MCD) on a grating-based spectrograph. The downside of the use of NIR spectra is the lower information content in the spectra that is exhibited in broad overlapping bands superimposed on a large scattering background which has to be dealt with in data treatment (this aspect will be discussed in the next section). However, the limitation of low information content has been overcome by the extremely high signal-to-noise in the data that enables sophisticated multivariate analysis to extract composition information from the data sets.

Raman instruments come in essentially two designs — dispersive and Fourier transform. In fact, the earliest instruments built during the 1930s and 1940s were prism-based dispersive systems. In the 1960s grating-based instruments became the rule, with the Toronto mercury lamp source getting replaced by gas lasers that became available. But the universal implementation of Raman equipment was hindered by fluorescence, which was experienced in almost all industrial materials. In 1986, Bruce Chase at Dupont, in collaboration with Tomas Hirschfeld, experimented with Raman detection on the emission port of an FT-IR instrument, using a Nd:YAG laser emitting at 1064 nm (7). The motivation for this effort was that the laser photons have energy low enough that they are unable to excite fluorescence, at least in organic systems. FT-Raman systems were commercialized first as accessories to FT-IR instruments, and then as stand-alone products.

FT-Raman systems became quite popular and reinvigorated interest in Raman spectroscopy by the analytical community. The instruments were continuously improved until they came close to hitting the photon shot-noise limit. During this period, dispersive Raman instrumentation also reinvented itself. Large double monochromators with water-cooled lasers and single-channel detectors were replaced by compact spectrographs with air-cooled lasers and multichannel detectors. Spectral acquisition times for equivalent spectral resolution and signal-to-noise dropped by two orders of magnitude. Red lasers eliminated much of the fluorescence interference seen with green argon laser excitations. In addition, the microscope as a sampling accessory, which had been introduced much earlier in 1972, provided ease of sampling and added fluorescence rejection. Today, these instruments are being used with lasers between 1064 nm and 244 nm for a wide range of applications.

An important advantage of Raman spectroscopy is that the spectrum can include frequencies between 5 cm-1 and 400 cm-1 — a region that is inaccessible to routine IR systems. This provides information on metal oxide vibrations and phonon modes in molecular crystals. Metal oxides are of importance to the paint industry, to catalysis, to conservation of cultural heritage, to additives in textiles and paper, and so forth. Molecular phonons are of high importance in the pharmaceutical industry where the crystalline form of the active pharmaceutical ingredient (API) can determine bioavailability.

Vibrational spectroscopy is valuable not only because of the information that it can potentially provide. Sampling is often done in a noncontact mode, sometimes in controlled, isolated environments (such as reactors), and the measurement is nondestructive. Furthermore, in the visible and NIR region of the spectrum, light can penetrate many containers made of glass or plastic so a measurement can be made without breaching the container integrity. On top of this, visible and NIR light can be transmitted via fiber optics to and from a container making remote sampling a reality.

Figure 1: Jablonski diagram illustrating mid-IR absorption, NIR absorption, Raman processes, Rayleigh scattering, and fluorescence.
The serious reader can obtain more information on these types of instrumentation in Volume I of Handbook of Vibrational Spectroscopy, edited by John Chalmers and Peter Griffiths (8).

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