FT-IR instrumentation

Like NMR spectrometers some IR spectrometers oper ate in a continuous sweep mode whereas others em ploy pulse Fourier transform (FT IR) technology All the IR spectra in this text were obtained on an FT IR instrument... 

In principle, emission spectroscopy can be applied to both atoms and molecules. Molecular infrared emission, or blackbody radiation played an important role in the early development of quantum mechanics and has been used for the analysis of hot gases generated by flames and rocket exhausts. Although the availability of FT-IR instrumentation extended the application of IR emission spectroscopy to a wider array of samples, its applications remain limited. For this reason IR emission is not considered further in this text. Molecular UV/Vis emission spectroscopy is of little importance since the thermal energies needed for excitation generally result in the sample s decomposition. 

Whilst nothing can improve upon the disadvantage of low molar absorption coefficients, instrumental designs and improvements with ratio recording and FT-IR instruments have virtually overcome the accuracy and instrumental limitations referred to in (b) and (c) above. As a result, quantitative infrared procedures are now much more widely used and are frequently applied in quality control and materials investigations. Applications fall into several distinct groups ... 

The essential instrumentation is divided into three parts (a) the pyrolyser, (b) the gas chromatograph and (c) the MS or FT-IR instruments. In this chapter interest focuses on pyrolysers as the other instruments are discussed elsewhere. 

Fourier transform spectroscopy technology is widely used in infrared spectroscopy. A spectrum that formerly required 15 min to obtain on a continuous wave instrument can be obtained in a few seconds on an FT-IR. This greatly increases research and analytical productivity. In addition to increased productivity, the FT-IR instrument can use a concept called Fleggetts Advantage where the entire spectrum is determined in the same time it takes a continuous wave (CW) device to measure a small fraction of the spectrum. Therefore many spectra can be obtained in the same time as one CW spectrum. If these spectra are summed, the signal-to-noise ratio, S/N can be greatly increased. Finally, because of the inherent computer-based nature of the FT-IR system, databases of infrared spectra are easily searched for matching or similar compounds. 

Increasingly a readily available option on middle-lR FT-IR instruments. 

Essentially just an FT-IR instrument coupled to a GC, thus allowing IR spectra of compounds eluting from the GC column to be obtained. More useful for structure elucidation rather than quantitative studies. The detector is sensitive to the 10 ng level. Used as a tool for qualitative identification. There are some examples of quantitative applications, e.g. determination of propandiol in acyclovir cream. ... 

FT IR instruments can have very high resolution (<0.001 cm-1). Moreover since the data undergo ana-log-to-digital conversion, IR results are easily manipulated Results of several scans are combined to average out random absorption artifacts, and excellent spectra from very small samples can be obtained. An FT IR unit can therefore be used in conjunction with HPLC or GC. As with any computer-aided spectrometer, spectra of pure samples or solvents (stored in the computer) can be subtracted from mixtures. Flexibility in spectral printout is also available for example, spectra linear in either wavenumber or wavelength can be obtained from the same data set. 

Several manufacturers offer GC-FT IR instruments with which a vapor-phase spectrum can be obtained on nanogram amounts of a compound eluting from a capillary GC column. Vapor-phase spectra resemble those obtained at high dilution in a nonpolar solvent Concentration-dependent peaks are shifted to higher frequency compared with those obtained from concentrated solutions, thin films, or the solid state (see Aldrich, 1985). 

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. 

The overall simplicity of an FT-IR compared to a dispersive instrument is also an advantage. For example, a single instrument can be easily converted to study the near, mid or far-infrared frequency region whereas with the dispersive method, three totally different instruments are required. To improve resolution with an FT-IR instrument, the basic design is only slightly modified while for a dispersive instrument different optical components are required. 

However, the improved sensitivity of FT-IR allows one to obtain better sensitivity using the conventional sampling accessories and expand the range of sampling techniques. Emission, diffuse reflectance and photoacoustic spectroscopy represent new areas where FT-IR reduces the difficulty of the techniques considerably. Greatly improved results are also achievable from reflection spectroscopy. Special effects such as vibrational circular dichroism can be observed using FT-IR instrumentation. 

When light is directed onto a sample it may either be transmitted or reflected. Hence, one can obtain the spectra by either transmission or reflection. Since some of the light is absorbed and the remainder is reflected, study of the diffuse reflected light can be used to measure the amount absorbed. However, the low efficiency of this diffuse reflectance process makes it extremely difficult to measure 120) and it was speculated that infrared diffuse reflection measurements would be futile 120). Initially, an integrating sphere was used to capture all of the reflected light121) but more recently improved diffuse reflectance cells have been designed which allow the measurement of diffuse reflectance spectra using FT-IR instrumentation 122). 

The application of FT-IR instrumentation arises from the need for a high signal-to-noise and the multiplex advantage is helpful in this regard. The light output... 

FT-IR INSTRUMENTATION AND DATA ACQUISITION. Most spectra were recorded with a Mattson Instruments Sirius 100 FT-IR spectrometer... 

This work was supported by the Public Health Service through grant GM-29864 to RM. Further support for the update of FT-IR instrumentation came via the Busch bequest to Rutgers University. Ve thank Prof. A. Blume for a sample of 4-d DPPE. 

These developments in Raman instrumentation brought commercial Raman instruments to the present state of the art of Raman measurements. Now, Raman spectra can also be obtained by Fourier transform (FT) spectroscopy. FT-Raman instruments are being sold by all Fourier transform infrared (FT-IR) instrument makers, either as interfaced units to the FT-IR spectrometer or as dedicated FT-Raman instruments. 

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