History
Sir William Herschel was the first to recognize the existence of infrared in 1800. Interest in IR was not explored further for 80 years. During 1882-1900 several investigations were made into the IR region. Abney and Festing photographed absorption spectra for 52 compounds and correlated absorption bands with the presence of certain organic groups in the molecule (Smith).
W. W. Coblentz laid the real groundwork for IR spectroscopy. Starting in 1903 he investigated the spectra of hundreds of substances, both organic and inorganic. His work in the rock salt region, from 0.7 to 18 (m, was so thorough and accurate that many of his spectra are still usable. The experimental difficulties of the early researchers were many. They not only had to design and build their own instruments but all the components too. Obtaining a spectrum was a tedious job requiring 3-4 hours or more since each point in the spectrum had to be measured separately and at least 10 points per micrometer were measured. After World War II advances in electronics made it possible to obtain a spectrum in 1-2 hours (Smith).
The end result of this early work was the recognition that each compound had a unique IR spectra and that certain groups, even when they were in different molecules, gave absorption bands that were found at approximately the same wavelength.
Applicability
The IR absorption spectrum of a compound is its most unique physical property. The samples can be liquids, solids, or gases. They can be organic or inorganic. The only molecules transparent to IR radiation under ordinary conditions are monatomic and homonuclear molecules such as Ne, He, O2, N2, and H2. One limitation of IR spectroscopy is that the solvent water is a very strong absorber and attacks NaCl sample cells.
In terms of a comparison of physical properties, a melting point, refractive index, or specific gravity gives only a single point of comparison with other substances. An IR spectrum, in contrast, gives a multitude of such points. Not only can the position of bands be compared but their intensity as well since the intensity is indicative of the number of a particular group contributing to an absorption. IR is usually preferred when a combination of qualitative and quantitative analysis is required. It is often used to follow the course of organic reactions allowing the researcher to characterize the products as the reaction proceeds.
Definitions
A term often encountered in IR spectroscopy is wavenumber ((), whose relationship to wavelength ( is ( (cm-1) = (104/() where ( is measured in micrometers. The wavenumber may be visualized as the number of wavelengths per centimeter.
Instrumentation
All IR spectrometers have the following elements in common, the source, optical system, detector, and amplifier. In the region 100-4000 cm-1 the most popular sources are the Globar and the Nernst glower, which are heated electrically to about 1500 C. The purpose of the optical system is to channel the radiation along the proper path. Mirrors are used rather than lenses because lenses are subject to chromatic aberration.
Because infrared radiations are essentially radiant heat, thermal detectors are used to detect changes in the radiations. Thermal detectors are made as small as possible to reduce their heat capacity so that for a given amount of energy there will be a large temperature rise. In order to make the detector rapid in response it must be able to dissipate the heat very rapidly. There are three main types of detector.
The thermocouple uses the principle that the change in temperature of a junction of two dissimilar metals creates an electromotive force (emf) which may be measured. The bolometer uses the principle that the electrical resistance of a pure metal or semi-conductor is temperature-sensitive. If a constant potential is applied to such a detector the variation of the resistance with temperature may be measured by the variations in the current flowing in the circuit. In the Golay cell the detector is a small metal cylinder enclosed by a blackened metal plate at one end and a flexible metallised diaphragm at the other. The cylinder is filled with a gas and sealed. As IR radiation falls on the blackened plate the gas in the cylinder expands deforming the diaphragm. Light from a lamp inside the detector is focused on the diaphragm. The light is reflected from the metallised diaphragm and falls onto a photocell. Movement of the diaphragm moves the light beam across the photocell. The output of the photocell is proportional to the expansion of the gas.
Early IR spectrometers used optical amplification of the detector signal obtained from a galvanometer or radiometer. Current instrumentation uses chopped radiation with electronic amplification.
An infrared spectrometer may use either a single beam or double beam design. In the single beam design light from the radiation is focused and passed through a sample contained in a special cell. After passing through the sample the emergent light beam is dispersed by a monochromator, either a prism or a diffraction grating, into its component wavelengths. The spectrum is scanned by slowly rotating the prism or grating. The main difference in a double beam spectrometer is that the original light is split into two beams, one of which passes through the sample and the other through a reference cell. The instrument records the difference in intensity of these two beams. The double beam spectrometer is especially useful if the spectrum is to be measured in solution. In this case the reference cell would contain pure solvent and any absorption due to the solvent would be canceled out.
The prism and cells used in IR spectrometers cannot be made of glass because glass absorbs strongly in the infrared region of interest. The prism and sample cell walls are usually made from large NaCl or KBr crystals.
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If we build an instrument in such a way as to allow a sample of unknown material to be held in position while various wavelengths of the IR region shine on it in turn, we can find which wavelengths are absorbed and which are not. This scan can be plotted on a graph and is called the infrared spectrum of the material. A spectrum is a plot of absorbance (or transmittance) versus wavelength, frequency, or wavenumber. For IR spectra, we usually plot wavenumber (in units of cm-1) which is the reciprocal of wavelength calculated as follows: cm-1= 1/(m x 104 .
