Dispersive infrared spectrometers double-beam

Figure 10.7 Schematic diagram of spectrometers and analysers in the infrared, (a) Single beam analyser containing a fixed monochromator or a filter used when a measurement at a single wavelength will suffice (b) dispersive spectrometer, double beam system. In contrast to spectrophotometers in the UV/Vis, the sample, located prior to the monochromator is permanently exposed to the full radiation of the source, knowing that the energy of the photons in this region is insufficient to break the chemical bonds and to degrade the sample (c) Fourier transform single beam model.

The chemist often obtains the spectrum of a compound by dissolving it in a solvent (Section 2.6). The solution is then placed in the sample beam while pure solvent is placed in the reference beam in an identical cell. The instrument automatically subtracts the spectrum of the solvent from that of the sample. The instrument also cancels out the effects of the infrared-active atmospheric gases, carbon dioxide and water vapor, from the spectrum of the sample (they are present in both beams). This convenience feature is the reason most dispersive infrared spectrometers are double-beam (sample -I- reference) instruments that measure intensity ratios since the solvent absorbs in both beams, it is in both terms of the ratio h / 4 and cancels out. If a pure liquid is analyzed (no solvent), the compound is placed in the sample beam and nothing is inserted into the reference beam. When the spectrum of the liquid is obtained, the effects of the atmospheric gases are automatically canceled since they are present in both beams. 

Figures 4a euid 4b show the infrared transmission spectra of a high-purity float-zoned silicon wafer and of a Czochralski wafer, respectively (J 3). The spectral features due to the presence of oxygen and carbon in the Czochralski wafer are clearly shown in Figure 4c, rt ich is the difference spectrum of the Czochralski wafer relative to that of the float-zoned wafer. This difference spectrum was obtained using a double beam dispersive spectrometer, with the float-zoned wafer in the reference beam and the Czochralski wafer in the sample beam. The broad band at 1107 cm and the smaller band at 515 cm" are due to interstitial oxygen, and the band at 605 cm is due to substitutional carbon.

The dispersive infrared analyzer, which is similar to the double-beam spectrometer, is capable of analyzing components of liquid and gaseous process streams. In contrast, the nondispersive infrared analyzer, which is a filter photometer, is more suited to the selective analysis of components of gaseous process streams. In general, the design and operation of nondispersive infrared analyzers are simpler than those of dispersive infrared analyzers. Also, the use of filtering techniques with the nondispersive infrared analyzers increases the selectivity of the analysis, but this can also reduce sensitivity. These infrared analyzers are commonly used for the analysis of... 

IR spectroscopy became widely used after the development of commercial spectrometers in the 1940s. Double-beam monochromator instruments were developed, better detectors were designed, and better dispersion elements, including gratings, were incorporated. These conventional spectrometer systems have been replaced by Fourier transform IR (FTIR) instrumentation. This chapter will focus on FTIR instrumentation and applications of IR spectroscopy. In addition, the related techniques of near-infrared (NIR) spectroscopy and Raman spectroscopy will be covered, as well as the use of IR and Raman microscopy. 

A review covering instrumentation—sources of infrared radiation, slit system, dispersing elements, detectors, amplification and recording, single and double-beam and fast-scanning spectrometers, preparation and examination of samples, applications of infrared spectroscopy, and infrared spectroscopy and coal structure. 232 references. 

In order to make rapid, accurate comparisons of a sample and a reference, double-beam instruments are frequently used. Since it is essential that the two beams are as similar as possible, a single source is used and the optics arranged to pass equal intensities of the beam through the sample area and through the reference area, and then to disperse and detect them alternately. This is shown schematically in Figure 1 for an infrared spectrometer. 

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