Infrared grating spectrometer

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

Figure 1. Comparison of the IR spectra of amine salts taken in KBr pellets on a Perkin-Elmer 337 Grating Infrared Spectrometer. AB, AS, and DAD are 1,4-diaminobutane, 3-aminopropyltriethoxysilane, and 1,12-diaminododecane, respectively [2].

Infrared Spectrometers. Infrared spectroscopy is one of the most powerful tools for quantitative and qualitative identification of molecules, and this led to the early development of prism and grating spectrophotometers. Typically, these instruments cover the region from 400 to 4000 cm, give a resolution of 1 to 4 cm, and require calibration with polystyrene films or with standard gases such as H2O, CO2, CH4, or This al-... 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

Conventional infrared spectrometers are known as dispersive instruments. With the advent of computer- and microprocessor-based instruments, these have been largely replaced by Fourier transform infrared (Fllk) spectrometers, which possess a number of advantages. Rather than a grating monochromator, an FTIR instrument employs an interferometer to obtain a spectrum. 

Infrared spectrometers have been commercially available since the 1940s. At that time the instruments relied on prisms to act as dispersive elements, but by. the. mid 1950s, = diffraction gratings had been introduced into dispersive machines. The most significant advances in infrared spectroscopy, however, have come about with the introduction of Fourier-transform Spectrometers. This type of instrument employs an interferometer and explbits the well established mathematical process of Fourier transformation. FT-IR spectroscopy has dramatically improved the quahty of infrared spectra and has minimised the time required to obtain data. Thus j with the improvements to computers achieved in recent years, infrared spectroscopy has made great strides. 

The dispersive element is contained within a monochromator. Figure 2.3a shows the optical path of an infrared spectrometer which uses a grating monochromator. Dispersion occurs when energy falling on the... 

Figure 2.3a The optical path of a double-beam infrared spectrometer with a grating monochromator...

Figure 8.2. A schematic diagram of a typical double-beam infrared spectrometer. The symbols Ml, Ml,... indicate mirrors SI and SI indicate slits and Gl and G2 indicate gratings. Courtesy of the Perkin-Elmer Corporation.

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