Abstract
This article presents design issues of high-sensitive laser absorption spectroscopy systems for nitrogen oxides (NOx) detection. Examples of our systems and their investigation results are also described. The constructed systems use one of the most sensitive methods, cavity enhanced absorption spectroscopy (CEAS). They operate at different wavelength ranges using a blue—violet laser diode (410 nm) as well as quantum cascade lasers (5.27 μm and 4.53 μm). Each of them is configured as a one or two channel measurement device using, e.g., time division multiplexing and averaging. During the testing procedure, the main performance features such as detection limits and measurements uncertainties have been determined. The obtained results are 1 ppb NO2, 75 ppb NO and 45 ppb N2O. For all systems, the uncertainty of concentration measurements does not exceed a value of 13%. Some experiments with explosives are also discussed. A setup equipped with a concentrator of explosives vapours was used. The detection method is based either on the reaction of the sensors to the nitrogen oxides directly emitted by the explosives or on the reaction to the nitrogen oxides produced during thermal decomposition of explosive vapours. For TNT, PETN, RDX and HMX a detection limit better than 1 ng has been achieved.
Highlights
Nitrogen oxides are compounds of nitrogen and oxygen
The sensor consists of a pulsed laser diode, diffraction grating and mirror, optical cavity, detection module with photomultiplier tube (PMT), digital signal processing unit with special software (Figure 16)
We presented some developed optoelectronic sensors designed to detect some nitrogen oxides
Summary
Nitrogen oxides are compounds of nitrogen and oxygen They play a significant role in many different fields of technology and science. They are important greenhouse gases and one of the compounds responsible for the generation of acid rains. Many explosive materials are sources of these gases as the result of their decomposition products This phenomenon can be applied in explosives detection using spectroscopy [1]. Their detection limit varies from ppt-level (e.g., integrated cavity output spectroscopy—ICOS [6]) to hundreds of ppm (e.g., differential optical absorption spectroscopy—DOAS [7])
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