TDLAS spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) has been used to measure oxides of nitrogen during flight (71). By tuning the laser to specific infrared absorption bands, the technique can selectively measure each compound. Detection limits are higher (25-100 pptrv for a 3-min response time) than the best chemiluminescent methods, and the instrumentation is less amenable to aircraft operations than the chemiluminescence techniques because of weight and size. 

In-situ measurement technique of water vapor concentration in gas flow channels in PEMFCs using tunable diode laser absorption spectroscopy (TDLAS)31-36 is also shown with fundamental descriptions on its measuring principle and validity of a practical system. Localized current density and through-plane water-back transport index are obtained with variation of vapor concentration along the gas channel taken into account. Demonstrative results showing that effect of the micro porous layer (MPL) on variation of through-plane water-back transport index is shown in an operating PEMFC. 

TUNABLE DIODE LASER ABSORPTION SPECTROSCOPY (TDLAS) AS A DIAGNOSTIC TOOL FOR IN-SITU DETECTION OF WATER VAPOR CONCENTRATION IN PEMFCs... 

Localized remote sensing for chemical compositions in operating PEMFC can be performed by tunable diode laser absorption spectroscopy (TDLAS). Measurement of water vapor and carbon dioxide has been reported by using TDLAS.31-36 In TDLAS measurement, the absorption of the laser beam passing through the gas sample is expressed with the partial pressure of the absorbing species, Pa (atm), by Beer s law,... 

Chapters 4-6 address specific diagnostic methods in PEFCs. Martin et al. provide a detailed review of methods for distributed diagnostics of species, temperature, and current in PEFCs in Chapter 4. In Chapter 5, Hussey and Jacobson describe the operational principles of neutron radiography for in-situ visualization of liquid water distribution, and also outline issues related to temporal and spatial resolution. Tsushima and Hirai describe both magnetic resonance imaging (MRI) technique for visualization of water in PEFCs and tunable diode laser absorption spectroscopy (TDLAS) for measurement of water vapor concentration in Chapter 6. 

Recently, very sensitive and selective measurements became possible by tunable diode laser absorption spectroscopy (TDLAS). Diode lasers that lase in the mid-infrared region give extremely high resolution (3 x 10 " cm ) and can tune the emission line to one of many vibration-rotation bands by changing the laser temperature and current (10-100K, 0.1-2.0 A). The TDLAS measurement is usually carried out at reduced pressure to avoid band broadening due to molecular collision. Practically interference-free measurements are possible with a typical detection limit of sub-ppbv levels (100 m path, at 25 Torr), although the TDLAS system is still expensive and under development. 

Various methods have been developed for remote gas sensing. These include differential optical-absorption spectroscopy (DOAS), differential absorption lidar (DIAL), and a number of methods that use spectroscopic methods with an atmospheric path in place of a laboratory long-path cell, for example tunable diode laser absorption spectroscopy (TDLAS) and Fourier transform infrared (FTIR) spectroscopy. 

The method just described is becoming increasingly popular in the monitoring of environmental trace gases, including chemical combustion products, utilizing semiconductor diode lasers. Diode lasers are small and relatively inexpensive, and thus mobile systems for in-situ measurements can and have been devised. The method is now commonly known as tuneable diode laser absorption spectroscopy (TDLAS). For a few examples of its application see Chapter 28. 

A typical modem set-up for modulated absorption spectroscopy, with a multipass cell, is shown schematically in Figure 6.11 the equipment components indicated in this figure can easily be recognized in the photograph of a TDLAS system for the analysis of flowing gases in Figure 6.12. 

The methods most widely in use now for understanding and monitoring chemical processes that affect our environment and the atmosphere are those of TDLAS, and remote absorption/Raman spectroscopy based on lidar (absoiption-Hdar/ Raman-lidar). Application examples of these two techniques are outlined in Sections 28.1—28.3 and Sections 28.4-28.6 respectively. The chapter will conclude with the description of some less-developed techniques, which, however, provide information not easily obtained, or not accessible at all. All of them are based on ionization in one form or other, and include laser-induced breakdown spectroscopy (LIBS), matrix-assisted laser desorption ionization (MALDl) and aerosol TOFMS (ATOFMS). Examples of these are provided in Section 28.7. 

Figure 28.3 Typical TDLAS response from a multi-gas mixture, using a QCL source in intra-pulse spectroscopy mode. Within the tuning interval, several absorption lines of CH4, H2S, NO2 and SO2 are observed...

In the early days of tuneable diode laser spectroscopy, as is true for many modem-day applications, the experimental set-up of a TDLAS system was comprised of a gas cell as its centrepiece. This sampled the analyte across a closed volume allowing for the control of the internal and external parameters (e.g. ambient temperature, gas pressure in the cell, etc.). Then, flow-type systems were introduced later, in which the overall configuration was modified so that the gas could be continuously exchanged in a flow through the cell, normally assisted by a pump at the exit of the cell, as shown in Section 28.2. 

TDLAS (and other optical spectroscopy) measurements and is of comparable dimensions to fully commercial units, allowing forrealistic feed with different mixtures of fossil fuels or domestic waste. In the measurements carried out by Ebert et al. (2003), the concentration of various species was monitored... 

LPAS Laserphotoacoustic spectroscopy TDLAS Tuneable diode laser spectroscopy... 

The tunable diode laser absorption spectroscopy (TDLAS) sensor is a real-time, non-invasive device that is used to measure water vapor concentration and gas flow velocity in the duct connecting the freeze-drying chamber and the condenser using... 

Kuu, W. Y, Nad, S. L Sacha, G 2009. Rapid determination of vial heat transfer parameters using tunable diode laser absorption spectroscopy (TDLAS) in response to step-changes in pressure set-point during freeze-drying./. Pharm. Sci. 98 1136-1154. 

Schneid, S., Gieseler, H., Kessler, W., Pikal, M. J., 2007. Tunable Diode Laser Absorption Spectroscopy (TDLAS) as a residual moisture monitor for the secondary drying stage of freeze-drying. Proceedings of AAPS Annual Meeting and Exposition, San Diego (CA), USA. 

Experimental methods - applications of tunable diode laser absorption spectroscopy (TDLAS)... 

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