Detection gaseous chemicals

Although employing inorganic radicals in solid electrolytes opens the possibility to detect gaseous oxides by a potentiometric method, it is a chemical choice rather than a material choice for solid electrolytes. Since salts involving inorgaific radicals are not usually good solid electrolytes due to limited chemical stability. 

CATSI The compact atmospheric sounding interferometer (CATSI) is a commercial product utilizing Fourier Transform infrared (FTIR) spectrometry in the 3-to 18-pm IR spectral range for the detection and discrimination of gaseous chemical air pollutants. This sensor has been used by the Army for the measurement of agent phenomenology. 

Figure 16.7 Ion mobility spectrometer. The ions are admitted into the analyser tube hy controlling the polarity of the entry grid. Below, an example of a recording obtained from gaseous compounds. Armies in many countries use this technique for the detection of chemical warfare agents.

In order to gain more fiexibihty in the detection of gaseous chemical species other than the mobile species of the electrolyte, electrodes of the second kind are used. The second kind of electrodes occurs when the analyte reacts reversibly with the mobile ions in the electrolyte, forming an intermediate phase, separate or dissolved phase in the electrolyte. An equilibrium is thus established between the gas and the intermediate phase. For the reference electrode, an electrode of the first kind is utilized. Typical examples of the so-called type II sensors (Fig. 3b) are CO2 and SO3 sensors with K2CO3 [11] and Ag2S04 [7] solid electrolytes, respectively. 

Lee C-Y, Wu G-W, Hsieh W-J (2008) Fabrication of micro sensors on a flexible substrate. Sens Actuators A 147 173-176 Li P, Li X (2006) A single-sided micromachined piezoresistive SiOj cantilever sensor for ultra-sensitive detection of gaseous chemicals. J Micromech Microeng 16 2539-2546... 

Other applications of this technique, which can detect gaseous compounds down to the sub-ppm range can be found in the environmental monitoring of laboratories, workshops, storage areas of chemicals, refineries, service stations etc. (survey of the maximum tolerable working place... 

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity only while present in the synthesis gas. This distinction is fully discussed in the book by Nielsen. Permanent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons do not interact nearly as strongly with the catalyst. It is very difficult to detect temporary poisons by means of post-analytical methods. The principal temporary poisons are oxygen, carbon oxides, and water. Since the catalyst also contains percent amounts of oxygen... 

W. Ruch, ed.. Chemical Detection of Gaseous Pollutants, Ann Arbor Science PubHshers, Inc., Ann Arbor, Mich., 1966. 

Anions of another group were derivatized with formation of gaseous chemiluminescing species. Chemical reaction - gas extraction has been used with chemiluminescence detection in the stream of canier gas in on-line mode. Rate of a number of reactions has been studied as well as kinetic curves of extraction of gaseous products. Highly sensitive and rapid hybrid procedures have been developed for the determination of lO, BrO, CIO, CIO, NO,, N03, CrO, CIO, Br, T, S, 803 with detection limits at the level of pg/L, duration of analysis 3 min. 

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

Up to date, several experimental techniques have been developed which are capable of detecting some of these particles under ordinary thermodynamic conditions. One can use these methods to keep track of transformations of the particles. For instance, it is relevant to mention here the method of electron paramagnetic resonance (EPR) with sensitivity of about 10 particles per cm [IJ. However, the above sensitivity is not sufficient to study physical and chemical processes developing in gaseous and liquid media (especially at the interface with solids). Moreover, this approach is not suitable if one is faced with detection of particles possessing the highest chemical activity, namely, free radicals and atoms. As for the detection of excited molecular or atom particles... 

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