Nondispersive infrared detector

CO and C02 are removed from the air, which is then directed through a suitable catalyst to a nondispersive infrared detector (NDIR)... 

Infrared spectroscopy is widely used in industry. The applications range from quality control and quality assurance of raw materials to customer complaints (troubleshooting) to quantitative analysis and online process monitoring and control. Most of the instrumentation is based on a Fourier transform infrared (FTIR) spectrometer but specialist applications use instruments as diverse as a nondispersive infrared detector through to mid-infrared diode lasers. 

Combined analyses of DOC and DON are performed in catalytic HTO systems, where the combustion of the water samples takes place generally at —680°C in the presence of Pt-Al203 catalyst. After the gas mixture containing the combustion products (CO2, NO, H2O, etc.) is dried, it passes in series through a nondispersive infrared detector (NDIR), for determination of DOC as CO2, and through a chemiluminescent detector, for determination of DON as NO2 [57,84,148,149,155]. Merriam et al. [146] proposed a catalytic oxidation at 680°C with Pt-A Oa and CuO as catalysts for the analysis of only TDN in soil solutions and throughfall, as alternative choice to noncatalytic HTO and PO methods. 

Reference methods for criteria (19) and hazardous (20) poUutants estabHshed by the US EPA include sulfur dioxide [7446-09-5] by the West-Gaeke method carbon monoxide [630-08-0] by nondispersive infrared analysis ozone [10028-15-6] and nitrogen dioxide [10102-44-0] by chemiluminescence (qv) and hydrocarbons by gas chromatography coupled with flame-ionization detection. Gas chromatography coupled with a suitable detector can also be used to measure ambient concentrations of vinyl chloride monomer [75-01-4], halogenated hydrocarbons and aromatics, and polyacrylonitrile [25014-41-9] (21-22) (see Chromatography Trace and residue analysis). 

A flame-ionization, total hydrocarbon analyzer determines the THC, and the total carbon content is calculated as methane. Other methods include catalytic combustion to carbon dioxide, which may be deterrnined by a sensitive infrared detector of the nondispersive type. Hydrocarbons other than methane and acetylene are present only in minute quantities and generally are inert in most appHcations. 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

FIGURE 13.46 Nondispersive Infrared analyzer based on (d) interference filters and (b) gas correlation cechnictues. M = mirror, D = detector, S source, F = filter disk. WO = motor, FB = baud pass filter. SD = synchronous detection. C = correlation cell. N nitrogen filter. 

Fig. 4. Nondispersive infrared analyzer with a Veingerov.type detector...

Fig. 8. Dual-channel nondispersive infrared analyzer with a solid- slate detector. Infrared Industries, Inc)...

Air drawn through a vacuum pump into the gas cuvette of a nondispersive infrared spectrophotometer IR absorption by CO is measured using two parallel IR beams through sample and reference cell and a selective detector, detector signal amplified concentration of analyte determined from a calibration curve prepared from standard calibration gases (ASTM Method D 3162-91, 1993). 

There are two main approaches to the oxidation of OC in water samples to C02 combustion in an oxidizing gas and UV-promoted or heat-catalyzed chemical oxidation. Other approaches are sometimes used, but are much less widespread.11 Carbon dioxide, which is released from the oxidized sample, can be detected in several ways, including conductivity detection, nondispersive infrared (NDIR) detection, or conversion to methane and measurement with a flame ionization detector (FID).1213 The limits of detection in TOC determination can be as low as 1 pg L 1, and the dynamic range can span many orders of magnitude. The precision of the method is usually very good, and the analysis can be completed in a few minutes. Another advantage is the very small amount of sample required—from 10 to 2000 pL. 

Several methods have been developed which seek to escape the inherent nonlinearity of the nondispersive infrared analyzer by the use of other detectors. The flame ionization detector commonly used in gas chromatography has many useful characteristics, including sensitivity and linearity of response, but it does not respond equally to all carbon compounds it does not respond at all to carbon dioxide. Therefore, the organic compounds must be converted to a single organic compound... 

Nondispersive infrared analyzer, on-line Flame ionization detector, on-line Chemiluminescence detector, on-line... 

Note Key CL = chemiluminescent EC = electro-chemical FID = flame ionization detector GC = gas chromatograph HC = hydrocarbons IR = nondispersive infrared LA = line absorption Para. = paramagnetic RS = Raman spectroscopy. 

There are a few common methods of measuring oxygen concentration in the gas phase. Electrochemical sensors and paramagnetic sensors are typically used to measure oxygen concentration on a wet and dry basis, respectively. Carbon monoxide (CO) is most commonly measured using a nondispersive infrared technique. A gas sample flows between an infrared radiation source and an infrared detector. Carbon monoxide absorbs infrared radiation, hence the difference in intensity proportional to the concentration of CO in the gas sample. 

CO and NO conversions were measured using conventional nondispersive infrared and chemiluminescence analyzers respectively. Hydrocarbon conversions were measured with a heated flame ionization detector analyzer. 

Instrumentation. Almost every imaginable type of gas detector or analyzer has been utilized in EGA, including hygrometers, nondispersive infrared analyzers, and gas chromatographs. Absorption of the products into solution permits analysis by coulometry, colorimetry, ion selective electrode measurements, or titrimetry. The most important analyzers are Fourier transform infrared (FTIR) spectrometers and, preeminently, mass spectrometers [19J. The two latter methods can be used to record spectra repetitively, thereby producing a lime-dependent record of the composition of the gas phase, from which EGA curves can be constructed for selected species. 

In addition to being a green house gas, CO2 is an important component for metabolism process of plant and many living creatures [34]. Thus, reliable and selective CO2 detectors are needed for a variety of applications including environmental and health monitoring [35, 36], fire detection [37], and controlling of fermentation [38]. There are several types of commercially available CO2 gas sensors and most of them are based on nondispersed infrared (NDIR) and electrochemical methods. 

Table 14.15 Various infrared detectors used in optical nondispersive infrared gas sensors...

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