Sample beam infrared spectroscopy

Wadsworth and coworkers (13, 14) have found considerable evidence for surface polarization in double-beam infrared spectroscopy. Not only do new differential peaks due to adsorption appear in the spectrograms but also the bands due entirely to the adsorbent are frequently appreciably shifted by adsorption. This occurred, for example, in calcium fluorite treated with oleic acid, in samples of bentonites taken from aqueous solutions of different pH, and in various minerals treated by flotation collectors. In fact, it is more the rule than the exception that the spectrograms of finely divided solids dispersed in the KI or KBr window exhibit distortion due to adsorption, whether adsorption occurs at the solid-aqueous solution or at the solid-vapor interface. For example, Eyring and Wadsworth (13) found that two (differential) peaks were produced by adsorption on willemite either from the vapor or aqueous solution of hexanethiol. These peaks were due to the influence of adsorption of the hexanethiol on the Si-O bands of the willemite and occurred at about 9.2 and 12.3 microns. 

Miller and Willis" obtained infrared spectra of antioxidants from polymer films. They compensated with additive free polymer in the reference beam. Infrared spectroscopy is more specific than ultraviolet spectroscopy, but some workers find that the antioxidant level in polymers is too low to give suitable spectra. In-situ spectroscopic techniques are not likely to be of value then, in the analysis of samples of unknown composition. 

Infrared spectroscopy is routinely used for the analysis of samples in the gas, liquid, and solid states. Sample cells are made from materials, such as NaCl and KBr, that are transparent to infrared radiation. Gases are analyzed using a cell with a pathlength of approximately 10 cm. Longer pathlengths are obtained by using mirrors to pass the beam of radiation through the sample several times. 

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

It is important to appreciate that Raman shifts are, in theory, independent of the wavelength of the incident beam, and only depend on the nature of the sample, although other factors (such as the absorbance of the sample) might make some frequencies more useful than others in certain circumstances. For many materials, the Raman and infrared spectra can often contain the same information, but there are a significant number of cases, in which infrared inactive vibrational modes are important, where the Raman spectrum contains complementary information. One big advantage of Raman spectroscopy is that water is not Raman active, and is, therefore, transparent in Raman spectra (unlike in infrared spectroscopy, where water absorption often dominates the spectrum). This means that aqueous samples can be investigated by Raman spectroscopy. 

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

Infrared spectroscopy has been used in combination with different thermal experiments as a convenient tool of analysis. For example, IR-EGA (infrared evolved gas analysis) was used for obtaining information on different thermal and combustion processes [89]. A simple IR attachment where the sample can be pyrolysed close to the IR beam is available (Pyroscan/IR available from CDS Analytical). A gas stream can be controlled to flush the pyrolysate. 

Infrared and Raman spectroscopy, coupled with optical microscopy, provide vibrational data that allow us to chemically characterise geochemical sediments and weathered samples with lateral resolutions of 10-20 pm and 1-2 pm respectively. Fourier transform infrared spectroscopy involves the absorption of IR radiation, where the intensity of the beam is measured before and after it enters the sample as a function of the light frequency. Fourier transform infrared is very sensitive, fast and provides good resolution, very small samples can be analysed and information on molecular structure can be obtained. Weak signals can be measured with high precision from, for example, samples that are poor reflectors or transmitters or have low concentrations of active species, which is often the case for geochemical sediments and weathered materials. Samples of unknown... 

Infrared Spectroscopy The infrared spectroscopy bands observed following adsorption of NO at beam temperature (ca. 310 K) are given in Table III. Detailed results of the IR spectroscopy studies are presented elsewhere (39). Typical results are shown in Figures 2 and 3. These are the beam temperature spectra of 1% Fe/Al203 and 1% Fe/Si02 in NO. The samples were initially treated for 4 h in... 

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