Stability evaluation of calibration gas mixtures containing both NO and NO2

Christophe Lachaud, Air Liquide (FR)

NO and NO2 are air polluting compounds which are well-known to produce ozone and acid rain. NO spontaneously reacts with oxygen to produce NO2 which is then transformed into NO and ozone by reactions with oxygen in the presence of ultraviolet light.  This reaction forms oxygen radicals.  Further reactions in the atmosphere between these oxygen free radicals and VOCs under ultraviolet light, will then recycle NO2 into NO. This is why NO and NO2 are considered by environmental agencies as major pollutants.
Vehicle emissions account for 50% of the global NOx emissions. The rest is generated by incinerators, cement manufactures, petroleum refineries… [1]. Despite governmental efforts to reduce emissions, environmental agencies will continue to have large analytical needs for the coming decades.
Accurate measurements of these pollutants require strong calibration protocols for the laboratory analyzers which in turn require accurate calibration gas standards. The literature describes two ways to produce standards [2]: permeation tubes and gas cylinders. Drawbacks of permeation tubes are the strong influence of the gas used for dilution, external pressure and temperature on the permeation rate and quality of gas exiting the tube. Additionally, mixtures containing multiple components necessitate the use of multiple tubes.  In contrast, cylinders can be filled with several species in the same mixture to simplify the calibration.  Nevertheless, several laboratories report instability of cylinders containing both NO and NO2 [2], [3] and prefer the use of permeation tubes. This instability is overcome by recent work that shows the influence of cylinder treatment to reduce the gas surface reactivity [4]
This presentation will outline a shelf-life study of a gas mixture containing both NO and NO2 with other atmospheric pollutants employing a specialized cylinder treatment. Multiple cylinders filled at the same time were analyzed at fixed time intervals to monitor the concentration of each species for several months. This cylinder passivation technology along with our filling protocols allows guaranteeing homogeneity better than 1% for each batch. Stable mixtures with the appropriate homogeneity between cylinders will aid laboratories in simplifying their calibration protocols on standard process or specific applications such as automated remote calibration systems.
[1] « Nitrogen Oxides (NOx), why and how they are controlled »; Technical bulletin; Environmental Protection Agency
[2] E. Chilton et al.; « Nitrogen Dioxide Calibration Standards for Portable Monitors »; National Institute for Occupational Safety and Health; IC 9482
[3] D.J. Robertson et al.; « Stability and Analyses of Nitric Oxide in nitrogen »; Journal of Air Pollution Control Association; 1977; Vol 27, 8
[4] M. Carre et al.; « Low concentration nitrogen dioxide calibration mixtures: Cylinder preparation and stability effect»; NEN GAS 2011


Pen Profile

Christophe Lachaud graduated from the University of Paris XI in 2006 with a degree in Analytical Chemistry. He has been with Air Liquide since 2000. Christophe spent nine years in R&D focusing on the analysis of organometallic precursors for electronic and photovoltaïc industry. In 2013 he became the manager of the analytical laboratory of the specialty gases filling plant in Mitry-Mory and was nominated as an Air Liquide Local Senior Expert in analysis in 2015. He is involved in the AFNOR E29G commission concerning gas analysis.

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