NDIR-detection

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. 

Using TLC-NDIR detection, it is possible to differentiate between important industrial products of biogenic and nonbiogenic origin. Mineral oil diesel and bio-diesel can be clearly distinguished from each other. The same applies to mineral oil-based lubricants and bio-lubricants based on rape oil, whose chromatograms are shown in Fig. 107a,b. 

Figure 107. Comparison of (a) a mineral oil-based lubricant with (b) a rape oil-based lubricant with the aid of NDIR detection technology...

Other examples of instruments that can be used for real-time monitoring of VOCs are gas analyzers based on non-dispersive infra red (NDIR) detection and Fourier transform infrared (FTIR) detection. The use of FTIR technique may under certain circumstances enable identification and quantification of individual VOCs at low concentrations (Hicks et al., 1992 and Bunding Lee et al., 1993). 

In the above example, the new impurities were relatively harmless, but in pharmaceutical analysis, the potential dangers are much higher and so there is a need to ensure that all identified and unidentified impurities are accounted for. Other multi-modal approaches in TLC include GC/TLC [133] which has largely been superceded by GC/MS today [131]. Others include SFC/TLC, combining the chromatographic technique to a flame iortisation or flame-thermionic ioitisation detector (TLC/FID/FTID) or non-dispersive infrared (TLC/NDIR) detection [131]. 

A non-dispersive IR (NDIR) detector permits the continuous detection of a certain selected constituent in mixed gases. This apparatus was constructed for practical purposes at the Institute of Geological Sciences (London) [68] a detector for five evolved gases, such as H2O, CO 2, CO, SOj and O2, is coupled to a DTA furnace, realizing a coupled simultaneous DTA-NDIR determination. NDIR detection limits are an order of magnitude lower than those for an X-ray diffractometer. Details can be found elsewhere [68,69,93). 

The deprotection of carbobenzyloxy protected phenylalanine was carried out in a low-pressure test unit (V= 200 ml) equipped with a stirrer, hydrogen inlet and gas outlet. The gas outlet was attached to a Non Dispersive InfraRed (NDIR) detector to measure the carbon dioxide. During the reaction the temperature was kept at 25 °C at a constant agitation speed of 2000 rpm. In a typical reaction run, 10 mmol of Cbz protected phenylalanine and 200 mg of 5%Pd/C catalyst were stirred in a mixture of 70 ml ethanol/water (1 1). The Cbz protected phenylalanine is not water-soluble but is quite soluble in alcoholic solvents conversely, the water-soluble deprotected phenylalanine is not very soluble in alcoholic solvents. Thus, the two solvent mixture was used in order to keep the entire reaction in the solution phase. Twenty p.1 of the corresponding modifier was added to the reaction mixture, and hydrogen feed was started. The hydrogen flow into the reactor was kept constant at 500 ml/minute and the progress of the reaction was monitored by the infrared detection of C02 in the off-gas. 

Optical NDIR-methods are not suitable to detect oxygen due to the lack of a dipole moment. Tab. 5.3 gives an overview of commercial oxygen sensors. 

Detection techniques. Detection techniques commonly used for CO include two infrared techniques, TDLS and NDIR (also known as gas filter correlation, GFC), and gas chromatography with various detectors. The principles behind TDLS and NDIR have been... 

The application of a commercial NDIR instrument to ambient CO measurements is described by Parrish et al. (1994) precision (lcr) of 2 ppb with 1-h averaging times could be obtained. A similar detection principle has been used to measure middle-tropospheric CO from the space shuttle (Reichle et al., 1990). 

The most recent generation of NDIR analyzers have evolved to satisfy the frequently harsh industrial environments encountered. These analyzers utilize solid-state sensors for the detection of infrared radialion. Most frequently used sensors are lead selenide (PhSc). thermopiles, or pyroelectric detectors. The gas analyzers generally are configured as single-path instruments, dual-beam with a reference palh. or dual-channel with a reference filter. 

The more expensive, permanently installed NDIR units often include data loggers, which can store thousands of periodic readings along with their times and dates. Some of these units can also detect other gases, such as CO, H2S, or 02. 

According to IPCS (1999), the analyzer system consists of an analyzer as well as sample preconditioning components fitted with a moisture control system such as the Non-Dispersive-Infrared (NDIR) analyzer. The infrared absorption near 4.6 pm, characteristic of CO, is used to measure its concentration. The most sensitive analyzers can detect CO concentrations as low as 0.05 mg/m (0.044 ppm). The NDIR analyzer designed by Luft (1962) is considered appropriate because it is little affected by flow rate, requires no wet chemicals, has a short response time, and is sensitive over wide concentration ranges. 

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... 

By coupling the Chromarod separation technique described in Section 11.4 with the reliable and robust NDIR (nondispersive infrared) technique for detecting the CO2 formed in the combustion, completely new ways of analyzing hydrocarbons are opened up. This principle of detection has the advantage that it is specific for CO2, i.e. cross-sensitivities are largely avoided. 

Carbon monoxide and nitric oxide were measured by nondispersive infrared analysis (NDIR), propylene by flame ionization detection (FID),... 

The results obtained with the Skalar 12 instrument were checked with a TOC Analyser 1010 (O I Analytical), which is based on a wet oxidation method with 100 g L" persulphate at 100 C (see Table A4.4). Carbon is detected by NDIR, this method does not involve a UV source, which is believed to cause the decreased oxidation efficiencies described above. The standard used for this instrument... 

The limitation of NDIR technology for gas detection depends on the uniqueness of the absorption spectrum of a particular gas. IR sensors can detect gases in inert atmospheres (little or no oxygen present), are not susceptible to poisons, and can be made very specific to a particular target gas. 

To evaluate RMM performance regarding methane conversion and hydrogen recovery, it is necessary to measure the content of the output stream from the reformer and membrane modules. The composition of the reformed gas and retentate streams was detected by an ABB analyzer. CHi, CO and CO2 concentrations were measured by the online non-dispersive infrared (NDIR) multiple analyzer, ABB URAS14. H2 was analyzed using the thermal conductivity detector ABB Caldos 17. A Perkin Ehner Gas Chromatographer unit (CLARUS 500) was used to analyze the composition of the permeate streams. 

G460 Multi-gas detector from GfG Instmmentation is a mgged, compact instrument for simultaneous detection of up to 6 gases. 02, LEL, CO and H2S sensors are warranted for three full years. Additional sensor choices include a wide selection of toxic as well as infrared (NDIR) sensors for combustible gas and PID for toxic VOCs. GfG Instrumentation, www.gfg-inc.com. Circle 332... 

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. 

NDIR type CO2 sensors allow highly specific detection via the absorption of CO2 in the infrared region [39-41]. However, because of bulk size, limited operation temperature range (<328 K) and high cost, their applications are not widespread. The use of light emitting diode (LED) has allowed dramatic reduction in the size of the optics system allowing NDIR sensor dimension... 

Another major research theme related to silicon is the development of MEMS-based analytical instruments for gas sensing, such as ntiniaturized silicon photoacoustic and thermal conductivity gas sensors, MEMS Fourier transform and other spectrometers, and integrated nondispersive IR absorption (NDIR) sensors. Unfortunately, there are several problans to be overcome here. According to Bogue (2007), part of the difficulty lies with the fact that nuniaturization is not always particularly beneficial miniaturized NDIR optical sensors suffer from low sensitivity due to the necessarily short optical path lengths silicon photoacoustic sensors cannot offer the low limits of detection that characterize their conventional counterparts, and as yet, MEMS spectrometers only offer low resolution. 

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