Dispersive IR Instrumentation

With the dispersive IR instrument, it is important to use computer control and data acquisition. For conventional experiments also, the attractiveness of such a system has been pointed out by Peri (24). 

Let us begin with the instrumentation. Dispersive IR instruments, similar to the double-beam instruments described for UV-VIS spectrophotometry, have been used in the past but have become all but obsolete. While some laboratories may still use these instruments, we will not discuss them here. 

Due to limitations in signal-to-noise ratio available for the then common dispersive IR instruments, peptide and protein vibrational spectroscopic studies shifted to emphasize Raman measurements in the 1970s 29-32 Qualitatively the same sorts of empirical correlations as discussed above have been found between frequencies of amide bands in the Raman and secondary structure. However, due to the complementary selection rules for Raman as compared to IR and to the multi-component nature of these polymeric spectral bands, the... 

What are the major advantages of Fourier transform IR instruments over dispersive IR instruments ... 

Dispersive Infrared Spectroscopy The dispersive IR spectrometer generally incorporates an IR broadband source, sample cell, a diffraction grating and one or more IR detectors. Dispersive IR instruments may provide simultaneous or sequential measurements. Respectively, the instrument may have a fixed grating and many detectors, or a movable grating and a single detector. In some cases, the grating may be replaced by one or more optical filters to resolve the desired wavelengths. A reference cell and associated optics to perform simultaneous differential analysis are also incorporated to improve sensitivity or reliability of measurement. 

FTIR spectrometers [7] in particular have been popular for the past few years. The number of suppliers of FTIRs worldwide is large, and with close competition the cost per instrument has recently dropped significantly. As a result, dispersive IR instruments have become of less importance because of the competition with the faster, more sensitive, and computerized FT methods. 

Figure 16-11 shows schematically the arrangement of components in a typical IR spectrophotometer, l.ike many inexpensive dispersive IR instruments, it is an optical null type, in which the radiant pow cr of the reference beam is reduced, or attenuated, to match... 

Ouantitative IR absorption methods differ somewhat from ultraviolet-visible molecular spectroscopic methods because of the greater complexity of the spectra, the narrowness of the absorption bands, and ilie instrumental limitations of IR instruments. (Quantitative data obtained with older dispersive IR instruments were generally signilicantly inferior in quality to data obtained with UV visible speclrophotomelers. 1 he precision and acciir icyof measurements with modern F TIR iustrumenis, however, is distinctiv betler than those... 

ATR technique is used commonly in the near -infrared for obtaining absorption spectra of thin films and opaque materials. However, ATR spectra can be obtained using dispersive IR instruments, but the higher-quality spectra are obtained using FTIR spectrometers. 

Figure 5.28 (a) Box diagram of a dispersive IR instrument, (b) Box diagram of an FTIR instrument and the interferometer. The beam splitter creates two components that recombine at the detector. 

The scope for IR spectroscopic techniques for direct in-polymer additive analysis is much broader than for extracts. In many real-life cases the form of the sample as presented for analysis is not at all suitable for routine transmission spectroscopy, which would, of course, have been the only method feasible with dispersive IR instruments. Most real-life samples are much too intensely absorbing or scattering for this to be possible. Yet, this does not preclude their routine measurement with Fourier transform spectrometers with the variety of sampling modes. In situ infrared analysis has been used for a host of analytical problems, as indicated in Table 1.10. 

The preceding examples with fentanyl have shown that GC-MS coupled with the ion cluster technique can be extremely useful for the recognition of metabolites in complex matrices. Nevertheless, the ion cluster technique requires the availability of the labeled parent drug. Infrared (IR) detection also provides three-dimensional information and can be considered as an alternative to MS detection. The use of Fourier-transform infrared (FTIR) detection allows short-time analysis and great sensitivity compared with dispersive IR instrumentation. Consequently, FTIR has become an important detection mode for GC. It can provide the identification of isomeric compounds and information on the functional groups present. Although GC-FTIR is less sensitive than GC-MS, the information obtained from both techniques is complementary. 

The IR beam, suitably modulated (by a chopper for dispersive IR instruments or by the Michelson interferometer in the FT-IR instruments) is directed on to the sample. If the sample absorbs the Incident radiation at any particular wavelength, the absorbed radiation is converted to heat by nort-radiat1ve processes. This heat is transferred to the surrounding gas by thermal diffusion. The gas expands and contracts at the modulation frequency resulting in a pressure wave within the sealed cell. This pressure wave is detected by the microphone and the signal from the microphone then becomes the output of the photoacoustic detector that is processed by the IR instrument detector electronics. 

Multichannel dispersive IR instruments utilize a multichannel detector (Fig. 3.2). A multichannel detector is an array of many detector elements. Using a multichannel detector improves the signal-to-noise ratio proportional to the square root of the number of detector elements. The use of a multichannel detector has improved the performance of near IR instrumentation. 

There are a number of advantages to Fourier Transform IR instrumentation relative to dispersive IR instrumentation including ... 

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