OTHER COMPONENTS OF FT-IR SPECTROMETERS

Polymeric materials then, whether natural (such as cellulose, resins, and proteins) or synthetic (such as polyolefins, nylons, and acrylics), behave in reproducible ways when exposed to pyrolysis temperatures. This permits the use of pyrolysis as a sample preparation technique to allow the analysis of complex materials using routine laboratory instruments. Pyrolytic devices may now be interfaced easily to gas chromatographs, mass spectrometers, and FT-IR spectrometers, extending their use to solid, opaque, and multicomponent materials. Laboratories have long made use of pyrolysis for the analysis of paint flakes, textile fibers, and natural and synthetic rubber and adhesives. The list of applications has been expanded to include documents, artwork, biological materials, antiquities, and other complex systems that may be analyzed with or without the separation of various layers and components involved. 

Combining the values of Fellgett s and Jacquinot s advantages, FT-IR spectrometers should be about 2000 times more sensitive than grating spectrometers that operate in the mid-infrared. In practice, however, smaller values are found. To understand why this is the case, we must consider other components in these spectrometers. The same types of source are used in both types of instruments, so we will neglect any discussion of this component. This cannot be said of the detector, however. 

The computer system and its relationship with other components of an FT-IR spectrometer are illustrated in Figure 5.11. In this figure, a single computer is depicted as confrolling the entire spectrometer. If a multiprocessor system is employed instead, a host computer controls the entire system, and slave processors exclusively carry out their individual functions such as data acquisition and numerical computation. 

In the example shown in Figure 13.18, the ATR spectra of layers of a laminate film cut obliquely by using a knife with an edge of synthetic diamond are shown. As an FT-IR spectrometer with a microscopic ATR apparatus was used, each of the measured spectra corresponds to an individual component without overlapping of the spectra by other layers. The individual layers were readily identified as a nylon, a polyester, probably PET, and a polypropylene. 

Thus, the two beams reflected by Rf and R are mixed at B, although their planes of polarizations are orthogonal to each other, and advance to Pout- The plane of Pout is rotated anticlockwise by 45° about the x-axis, and its wires are parallel to the x-axis. Therefore, only the z-polarized component is transmitted from Pout and advances to the detector. Interference between the two beams refleeted by Rf and R occurs during this step. The spectroscopic measurement process after this is exactly the same as that of a conventional FT-IR spectrometer. The x-polarized component reflected by Pout along the z-axis is not utilized for any purpose. 

The basic component of most Fourier Transform Infrared spectrometers is the Michel son interferometer. This is not the only interferometer used in FT-IR, but it is employed more often than other designs. A treatment of many other interferometer designs is available. The Michel son interferometer in a Fourier Transform Infrared spectrometer replaces the monochromator in a dispersive instrument, although the functions cannot be correlated. A monochomator divides a continuous bandwidth into its component frequencies, whereas an interferometer produces interference patterns of the bandwidth in a precise and regulated manner. It should be noted that this type of interferometer is not restricted to the infrared region and its use can be extended to the visible and millimeter regions of the electromagnetic spectrum. 

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