Using the HPR-20 to Detect NO and Nitrogen Dioxide

The Hiden HPR-20 gas analysis system has been extensively applied in the fields of thermal and catalysis analysis.

The reduction of NO_x emissions from power plants and car exhausts is one of the most important aspects of catalysis research. There are several challenges involved in this research to satisfy the requirements of current emission legislation without increasing the catalyst cost. Some of the commercially available NO_x reduction technologies include NO storage and reduction (NSR), and selective catalytic reduction (SCR) with NH₃, urea and hydrocarbons.

NSR technology includes two cyclic steps, which occur on a lean NO_x trap (LNT) catalyst. The LNT catalyst readily stores NO₂ as compared to NO. For this reason, NO should be oxidized to NO₂ to achieve an acceptable level of NO_x storage.

This application note describes the measurement of NO₂ using the m/z 46 peak and the NO/NO₂ ratio in a research application using the Hiden HPR-20 QIC. The experiment emphasizes the capability of the QIC series gas analysis systems to measure low levels of NO in high concentrations of NO₂.

Experimental

The Hiden HPR-20 QIC gas analysis system is configured for repeated gas and vapor analysis. The Hiden QIC quartz-lined sampling interface can operate at temperatures up to 200ºC, providing faster response times of less than 300 milliseconds for most of the common vapors and gases. The HPR-20 system used in this experiment consists of a corrosion resistant pumping system, triple-stage mass filter, quadrupole probe with 100amu mass range capability, and QIC capillary inlet with 2m heated capillary operating up to 160ºC.

Test Results

The results of NO₂ measured in a carrier gas of He are shown in Figure 1, which clearly indicates that the HPR-20 system can be used to measure a unique peak at m/z 46. This is a critical aspect as NO₂ quickly and easily fragments to NO⁺, causing difficulty in deconvolution of analysis of any NO_x mixtures with NO and NO₂. As a result, there is an increase in the uncertainty of the NO and NO₂ concentration results. Assuming the major peak at m/z 30 represents 100%, the peak at m/z 46 is equivalent to 15% giving a 46/30 ratio of 1:6.6.

Figure 1. Profile scan for 10000ppm NO₂

The facile fragmentation of NO₂ to NO⁺ also makes it difficult to measure low concentrations of NO in a high background of NO₂. A high background at m/z 30 produced by the NO₂ further blocks any signal due to the NO in the gas mixture. An example MID scan for a wide range of NO concentrations measured at m/z 30 with a background level of 1000ppm NO₂ is shown in Figure 2. It is obvious from the results that the Hiden HPR-20 system is capable of achieving detection limits of over 12ppm.

Figure 2. Detection Limit of NO in 1000ppm NO₂ (Inset: Expansion of detection of 12ppm NO in 1000ppm NO₂). Primary axis represents partial pressure at m/z 30, Secondary axis represents partial pressure at m/z 46.

Figure 3 shows the quantitative information on the NO detection levels obtained through a correlation of the NO peak signal intensity with the NO concentration level. The high degree of linearity denotes high confidence in the detection of ppm levels of NO in a high background of NO₂.

Figure 3. Relationship between partial pressure for m/z 30 and NO concentration (Note: The background at m/z 30 due to NO₂ has been subtracted)

Conclusion

This article has demonstrated the high sensitivities of the Hiden HPR-20 QIC for NO₂ detection using the m/z 46 peak irrespective of a large degree of fragmentation. It has also showed that there is no need for other additional or tandem techniques to achieve low levels of NO detection in a high background of NO₂. Furthermore, the ability to achieve detection levels of over 12ppm of NO has highlighted another significant advantage of the Hiden QMS system.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

For more information on this source, please visit Hiden Analytical.