Nondispersive Spectrometers

Infrared (IR) spectroscopy offers many unique advantages for measurements within an industrial environment, whether they are for environmental or for production-based applications. Historically, the technique has been used for a broad range of applications ranging from the composition of gas and/or liquid mixtures to the analysis of trace components for gas purity or environmental analysis. The instrumentation used ranges in complexity from simple filter-based photometers to optomechanically complicated devices, such as Fourier transform infrared (FTIR) spectrometers. Simple nondispersive infrared (NDIR) insttuments are in common use for measurements that feature well-defined methods of analysis, such as the analysis of combustion gases for carbon oxides and hydrocarbons. For more complex measurements it is normally necessary to obtain a greater amount of spectral information, and so either Ml-spectrum or multiple wavelength analyzers are required. 

Braden B, Haisch M, Duan LP et al (1994) Clinically feasible stable-isotope technique at a reasonable price - analysis of 13C02/12C02-abundance in breath samples with a new isotope selective nondispersive infrared spectrometer. Zeitschrift fur Gastroenterologie 32(12) 675-678... 

Azad et al. [ 186] used a similar technique for the determination of selenium in soil extracts using a nondispersive spectrometer, with which it was possible to observe fluorescence from the 196.1, 214.3 and 204.0 lines simultaneously, thus enabling a detection limit of 10 ng/ml to be observed using discrete sample introduction via the hydride generation technique. In this method, soil... 

Fourier transform (FT) IR spectroscopy is one of several nondispersive optical spectroscopies based on interferometry. A two-beam interferometer first proposed by Michelson is the basis of most modern FT-IR spectrometers, as exemplified by the schematic of the Bruker Equinox 55 spectrometer (Bruker Optik, Ettlingen, Germany) in Fig. 2. Simply described, the interferometer comprises a beam splitter and two mirrors. A collimated beam of IR energy is split at the beam splitter into equal halves. Half of the energy travels through the beam splitter to one of the mirrors, which is positioned at a fixed distance away from the beam splitter. The reflected beam travels perpendicular to the incident beam to a moving mirror. IR radiation reflects off the fixed and moving mirrors and recombines at the beam splitter. The recombined IR beam projects from the interferometer towards the detector on an optical path perpendicular to the source beam. 

Fig. 17 Carbon monoxide concentration traces for IM16 reference cigarette smoke runs a with 3-m multipass and b HWG gas cells (dashed line nondispersive IR (NDIR) analyzer, solid line FT-IR spectrometer using partial least-squares calibrations) [43]...

Figure 5.2. Schematic of a nondispersive, FT-Raman spectrometer. A single detector monitors photons with all Raman shifts, after each has been modulated by a multiplexer such as an interferometer. Raman spectrum is obtained by Fourier transformation of the detector output (interferogram).

At the time of this writing, the Raman spectrometer market is approximately split between dispersive (spectrograph/CCD) and nondispersive (FT-Raman) instruments. Both types have their pros and cons, which enter into a selection for a given application. Several generalizations are listed in Table 5.3. These... 

Both dispersive and nondispersive spectrometers can exhibit preferential transmission of light depending on its polarization. The efficiency of diffraction gratings and the polarization sensitivity of the beamsplitter can cause errors in the observed depolarization ratio, depending on several variables such as experimental geometry and Raman shift region. For this reason, it is often important to place a polarization scrambler between the sample and any polarization-sensitive components of the spectrometer other than the polarization analyzer itself. In addition, it is good practice to measure p for a few known systems to verify accuracy of the apparatus. 

Most implementations of filter-based nondispersive spectrometers occur in Raman microscopy and imaging, where their large clear aperture is particularly valuable. These applications are discussed further in Chapter 11, but the nature and performance of tunable filters as wavelength analyzers are outlined here. 

A different approach to liquid crystal filters led to the development of a nondispersive Raman spectrometer used commercially in a Raman microscope... 

Infrared spectroscopic techniques have long been used to analyze gas streams in industrial chemical processes. Recently, with the advent of fastscan infrared spectrometers, they have been used as gas chromatograph detectors. One requirement of their use, needless to say, is that the compound must possess one or more infrared absorption band. By means of a carrier gas. the evolved gas sample from a pyrolysis chamber can be readily passed through an infrared cell for analysis. Infrared systems that can be employed include (1) nondispersive analyzers, (2) dispersion spectrometers. 3) band-pass filter-type instruments, and (4) interference spectrometers all these techniques have been adequately reviewed by Low (87). 

Fig. 18. A Schematic representation of a diffuse reflectance spectrometer. For (i) nondiffuse dispersed illumination the source at the left and the detector at the integrating sphere are used. For (ii) diffuse nondisperse illumination (in the case of fluorescent samples) a source directly attached to the sphere and the detectors on the left upper side are applied. B Temperature-regulated and evacuable diffuse reflectance sample cell according to [32]. Reprinted from [32] with permission of Academic Press, Inc...

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