Non-dispersive spectrometers

Some detectors employ the optothermal effect the absorbed modulated infrared radiation heats the sample and its environment, thus producing sound waves which are recorded with a microphone. They can be combined with scanning spectrometers and interferometers. A Golay cell (Golay, 1949) measures the optothermal pressure change by a light beam which is deflected by a reflecting membrane. The first infrared process spectrometer, the URAS, alieady employed the absorption bands of a detector gas to specifically analyze the concentration of this particular gas in a sample. This is a non-dispersive spectrometer already mentioned in Sec. 1. 

One type of non-dispersive spectrometer employs filters to isolate the wavelength desired the other type... 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion so as to isolate the signals of the elements to be determined and to make the full selectivity of the methodology available. In optical atomic spectrometry, this involves the use of dispersive as well as of non-dispersive spectrometers. The radiation from the spectrochemical radiation sources or the radiation which has passed through the atom reservoir is then imaged into an optical spectrometer. In the case of atomic spectrometry, when using a plasma as an ion source, mass spectrometric equipment is required so as to separate the ions of the different analytes according to their mass to charge ratio. In both cases suitable data acquisition and data treatment systems need to be provided with the instruments as well. 

Since the triumphant advance of interferometers in the infrared range, spectrometers are nowadays distinguished into interferometers or non-dispersive spectrometers and dispersive instruments. There are two main arguments for proving the superiority of interferometry over dispersive spectroscopy ... 

Hadamard transform [17], For example the IR spectrum (512 data points) shown in Fig. 40.31a is reconstructed by the first 2, 4, 8,. .. 256 Hadamard coefficients (Fig. 40.38). In analogy to spectrometers which directly measure in the Fourier domain, there are also spectrometers which directly measure in the Hadamard domain. Fourier and Hadamard spectrometers are called non-dispersive. The advantage of these spectrometers is that all radiation reaches the detector whereas in dispersive instruments (using a monochromator) radiation of a certain wavelength (and thus with a lower intensity) sequentially reaches the detector. 

Infrared analyses are conducted on dispersive (scanning) and Fourier transform spectrometers. Non-dispersive industrial infrared analysers are also available. These are used to conduct specialised analyses on predetermined compounds (e.g. gases) and also for process control allowing continuous analysis on production lines. The use of Fourier transform has significantly enhanced the possibilities of conventional infrared by allowing spectral treatment and analysis of microsamples (infrared microanalysis). Although the near infrared does not contain any specific absorption that yields structural information on the compound studied, it is an important method for quantitative applications. One of the key factors in its present use is the sensitivity of the detectors. Use of the far infrared is still confined to the research laboratory. 

In the process of developing PRMs, it is necessary to study and establish measurement methods which are used to analyzed the purity of raw gases and verify the stability of the gas mixture kept in the cylinder. Up to now, NRCCRM has been equipped with several series of analytical techniques including atmospheric pressure ionization mass spectrometer, gas chromatograph, infra-red spectrophotometer with long-path gas cell, chemiluminescent, non-dispersive infra-red, minor 02 and H20 analyzer and so on. 

If the flame background emission intensity is reduced considerably by use of an inert gas-sheathed (separated) flame, then an interference filter may be used rather than a monochromator, to give a non-dispersive atomic fluorescence spectrometer as illustrated in Figure 14.36-38 Noise levels are often further reduced by employing a solar blind photomultiplier as a detector of fluorescence emission at UV wavelengths. Such detectors do not respond to visible light. The excitation source is generally placed at 90° to the monochromator or detector. Surface-silvered or quartz mirrors and lenses are often used to increase the amount of fluorescence emission seen by the detector. 

Non-dispersive systems are very compact, and potentially less expensive than dispersive spectrometers. For this reason they may be used for multi-channel instruments, measuring upto six elements almost simultaneously.39,40 Although... 

Figure 14 Essential components on a non-dispersive atomic fluorescence spectrometer...

A typical IR spectrometer consists of the following components radiation source, sampling area, monochromator (in a dispersive instrument), an interference filter or interferometer (in a non-dispersive instrument), a detector, and a recorder or data-handling system. The instrumentation requirements for the mid-infrared, the far-infrared, and the near-infrared regions are different. Most commercial dispersive infrared spectrometers are designed to operate in the mid-infrared region (4000-400 cm ). An FTIR spectrometer with proper radiation sources and detectors can cover the entire IR region. In this section, the types of radiation sources, optical systems, and detectors used in the IR spectrometer are discussed. 

Non-dispersive multiplex spectrometers include Hadamard transformation spectrometers and Fourier transform spectrometers and are particularly useful for the case of very stable sources. In both cases the information, such as intensities at various wavelengths, is coded by a multiplex system, so that it can be recorded with a conventional detector. A suitable transformation is then used to reconstruct the wavelength dependence of the information. In Hadamard spectrometry use is made of a codation of the spectrum produced by recombining the information with the aid of a slit mask which is moved along the spectrum [66],... 

Fleet ME, Muthupari S, Kasrai M, Prabakar S (1997) Sixfold coordinated Si in alkali and alkali-CaO silicophosphate glasses by Si -edge XANES spectroscopy. J Non-Crystal Solids 220 85-92 Fontaine A, Baudelet F, Dartyge E, Guay D, Itie JP, Polian A, Tolentino H, Tourillon G (1992) Two time-dependent, focus-dependent experiments using the energy-dispersive spectrometer at LURE. Rev Sci Instrum 63 (1, Pt. 2B) 960-965... 

In the recirculation system concentrations were monitored by means of non-dispersive IR spectrometers (CO Ultramat, Siemens CO2 Uras 2T, Hartmann Braun) and a magnetic device (O2 Magnos, Hartmami Braun). 

Hutton, R.C. and Preston, B. (1980) A simple non-dispersive atomic-fluorescence spectrometer for mercury determination, using cold-vapour generation. Analyst, 105, 981-984. 

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