The evaluation of the nitrogen dioxide (NO2) content in ppb nitric oxide (NO) mixtures is a difficult task due to the low concentration level of NO2 and its equilibrium with NO.
The paper presents the results of the analysis performed on more than 50 cylinders to evaluate the NO2 in ppb mixtures of NO in nitrogen. The evaluation of the NO and NO2 concentration is usually performed using a chemiluminescence analyzer: the converter has a very high performance and is able to convert almost all nitrogen compounds present in the mixture introduced. Therefore, the analyser recognizes the sum of all nitrogen compounds present in the mixture, including NO2, as NOx. This leads to an overestimation of the concentration of NO2.
Thus is important to find the difference between all the nitrogen compounds (Reactive Nitrogen) and NO2. This evaluation can be done using an UV analyzer that is able to detect directly and selectively NO2, without any conversion and virtually free of interference at 450 nm.
The study reports the analysis performed on the 400 and 800 ppbmol NO mixtures using both a chemiluminescence analyzer and an UV analyzer.
The results show that, while the chemiluminescence signal gives an NO2 concentration from 2 to 15 ppbmol, the concentration at UV analyzer was different and lower. Thus, the presence of ppb of reactive nitrogen (most of it is supposed to be HNO3 but this should be confirmed) introduces an overestimating of the NO2 content when the mixtures are controlled with a chemiluminescence analyzer. One of the problem found in the studies was the calibration of the UV instrument; this situation has been overcome through the production of stable calibration mixtures in cylinder at ppb concentration of NO2.
Giorgio Bissolotti received his PhD in Environmental Engineering in 1987. Currently he holds the position of Research Manager of SIAD. Since the ‘90s he is working in the specialty gases field and of calibration mixtures. In this area, the calibration mixtures prepared and used for environmental purposes are of great importance. Since 2001 he is Manager of the LAT Center n. 143 of SIAD in which metrological calibration mixtures are produced. He was for 15 years a member of the EIGA (European Industrial Gases Association) working group WG4 “Special Gases” and is currently a member of the EIGA-IGC (Industrial Gases Council).