Non-dispersive instruments

5 Cold Vapour Method. The capability of the cold vapour technique for the determination of mercury has been examined by using non-dispersive instruments. Mercury is first reduced with tin(ii) chloride to metallic mercury, and the mercury vapour is fed in an argon stream to the excitation cell. A mercury lamp is used for the excitation measurement, and the fluorescence signal is detected by a photomultiplier. A monochromator or filter is required in this technique. 

6 Hydride Generation Method. Hydrogen flames have been employed in AFS for the atomization of gaseous hydrides. These flames are very suitable for hydride generation methods because of the low background emission. 

1 Non-dispersive Instruments. In non-dispersive equipment, the atoms act as their own monochromators. In these instruments element-specific, line-like 

The optimization of the signal/noise ratio is very important in AFS. In non-dispersive equipment, the intensity of the signal is directly proportional to the intensity of the incident beam, space angle, quantum yield, and transmission efficiency of the optics. The last factor may be improved by using mirrors because no colour deflections occur, unlike the case of lenses. 

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Quantitative IR spectioscopic measurements for environmental analysis are often carried out by using simple non-dispersive instruments. 

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. 

The interferometer used in a non-dispersive instrument is a device that divides the beam of radiation into two paths and recombines the two beams after a path difference has or has not been introduced. The basic concept of the interferometer was introduced by Michelson almost a century ago (Fig. 2). It consists of a stationary mirror, a moving mirror, and a beam splitter. The radiation from the infrared source is divided at the beam splitter half the beam is passed to a fixed mirror and the other half is reflected to the moving mirror. The two beams are later recombined at the beam splitter and passed through the sample to the detector. For any particular wavelength, the... 

Analyses of gases and vapours tend to utilize the teehniques deseribed on page 308. Many of these methods were traditionally limited to laboratory analyses but some portable instruments are now available for, e.g., gas ehromatography (Table 10.16) and non-dispersive infra-red speetrometry (Table 10.17). 

Fig. 19.4 Non-dispersive continuous stream infrared analyser. Reproduced by permission of Beckman Instrument Co.

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. 

The unique appearance of an infrared spectrum has resulted in the extensive use of infrared spectrometry to characterize such materials as natural products, polymers, detergents, lubricants, fats and resins. It is of particular value to the petroleum and polymer industries, to drug manufacturers and to producers of organic chemicals. Quantitative applications include the quality control of additives in fuel and lubricant blends and to assess the extent of chemical changes in various products due to ageing and use. Non-dispersive infrared analysers are used to monitor gas streams in industrial processes and atmospheric pollution. The instruments are generally portable and robust, consisting only of a radiation source, reference and sample cells and a detector filled with the gas which is to be monitored. 

OIC Analytical instruments produce the fully computerized model 700 total organic carbon analyser. This is applicable to soils and sediments. Persulphate oxidation at 90-100°C non-dispersive infrared spectroscopy is... 

With the exception of instrumental dry combustion methods [32], the techniques referred to above for the analysis of organic (and total) carbon in sediments are time consuming (e.g. 2-3h). An instrumental technique described by Van Hall and Stenger [33] makes use of a non-dispersive infrared detector and measures the carbon dioxide resulting from the combustion of the carbonaceous compounds. Total and inorganic carbon can be differentiated by the use of different combustion columns and temperatures. 

Abstract In routine chemical measurements traceability can be achieved by using analytical instruments calibrated against primary reference materials. In the present work the calibration of a C02 non-dispersive infrared (NDIR) analyzer with measuring range 0 2000 mol/mol of C02 and a resolution of 5 mol/mol is reported. A procedure with working reference gas mixtures (WRMs) has been adopted, which requires seven calibration points. Primary reference gas mix-... 

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... 

In the laboratories of BASF (Badische Anilin- and Soda-Fabrik) at Ludwigshafen, the importance of infrared spectroscopy for industrial purposes was realized as early as the 1930 s. The first IR instrument with a modulated beam was built by Lehrer in 1937 and modified to a double beam instrument with optical compensation in 1942. Luft described the first non-dispersive infrared analyzer in 1943. He used the gas to be analyzed as absorber in a photo-acoustic detector cell. Thus, the instrument was sensitive only to this gas. He also provided a survey of early industrial applications of infrared spectroscopy (Luft, 1947). 

Emissions-measuring instruments used in this program consisted of non-dispersive infrared (NDIR) analyzers for CO, CO2, and NO. A modified flame ionization detector (FID) was used to measure HC. Modifications insured that the instrument would detect the unbumed... 

The NO analysis involved considerable problems. Eventually we used a Beckman NDIR (non-dispersive infrared) for NO measurements because of good signal stability and because the instrument read NO directly, not total NOj,. However all of the water had to be removed from the gas because moisture was recorded as NO. The NO-NO2-H2O interaction was eliminated in the water trap by removing the NO2 before the trap. After trying several approaches, we finally used a saturated solution of sodium sulfite at room temperature as the NO2 absorber... 

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. 

Although these are the most important considerations in designing a parallel reactor module, another important point is the analysis. While in principle each analytical instrument is suitable and can be connected to the exit of such a multiple-pass reactor, one must ensure that the instrument works with relatively small amounts of sample, as well as low flow rates. For the model reaction under investigation in our reactor, the CO-oxidation, non dispersive IR is used. As C02 concentrations are relatively high under our conditions, the analysis chamber can be kept short, and purge times are therefore also short. Analysis times are around 4 minutes, this being determined mainly by the purge times of the tubes and the sample chamber. However, any analytical techniques, such as mass spectrometry, GC, etc., can in principle be used in connection with this set-up. 

Figure 1. Experimental set-up. FID flame ionization detector, CLD chemiluminescence detector, GC gas chromatograph, TCD thermal conductivity detector, NDIR non-dispersive infra-red instrument, PC personal computer.

Continuous analysis instruments equipped with flame ionization, chemiluminescence, and infrared detectors were used to measure the concentrations of total hydrocarbons, nitrogen oxides and carbon monoxide, respectively. The concentration of total hydrocarbons was measured by a JUM FID 3-300 hydrocarbon analyzer with a flame ionization detector. NO, NO2 and NOx was measured by an ECO Physics CLD 700 EL-ht chemiluminescence detector. CO was measured with either a Beckman Industrial Model 880 non-dispersive infrared instrument or an NDIR instrument from Maihak (UNOR 6N). 

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