Measurement and Testing
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2. EXPERIMENTAL RESULTS
In order to produce a calibration curve avoiding any matrix effect in the quantitation, it is recommended to use a gas or a mixture of gases with a similar composition of the actual sample matrix. This is important to enhance the efficiency of the detection free from interferences. The scope of analysis for this application is extended to Liquefied Petroleum Gases (LPG) since the sample inlet is connected to a temperature controlled vaporiser unit in accordance to the customer needs.
Accordingly with the calibration range, the analyser is set for continuous injection (for concentrations up to 1 ppm vol) or loop injection type (for concentrations above 1 ppm vol of total NOx species). For process analysers, it is possible to measure continuous and loop injections simultaneously with accuracy values less than 5% full scale [F.S.]. Figure 1 presents results for a calibration curve produced using a NO standard in nitrogen balance, diluting with ethene gas. According to those results, the calibration curve presents good linearity correlation between zero and 25 ppb NOx. Additionally, resolution for 25 ppb is good enough to measure concentrations as low as 10 ppb total NOx.
Similar internal validation results have been found using propane and a mixture of propane and ethene as diluent gases, probing the robustness of the method using different hydrocarbons.
Repeatability results for 15 ppb vol total NOx are presented in figure 2. In agreement with those findings, the total nitrogen oxides analysed is very stable at low concentrations with a repeatability reported at less than 2% F.S.
The C.I. analytics NOx application in hydrocarbon streams has been extensively tested presenting high sensitivity, a broad linearity range (including low ppb concentrations), fast response, high accuracy and a good degree of repeatability and reproducibility. More importantly, it is a very versatile option since it has been designed for laboratory as well as on line monitoring of NOx, following international recognised safety standards depending on the final analyser location.
3. CONCLUSIONS
Dry colorimetric detection offers a good option for NOx monitoring at very low concentrations for laboratory and online analysis. The technique presents a low detection limit of 10ppb vol in hydrocarbon matrices. To conclude, the analytical parameters such as repeatability, reproducibility and accuracy fit the needs of clients where strict monitoring of these gases is crucial for their product process and for quality control.
4. REFERENCES
[1] EPA 456/F-99-006R, Technical bulletin. Nitrogen oxides (NOx), why and how they are controlled, United States Environmental Protection Agency, November 1999.
[2] Zhang R., Tie W., Bond D.W., Impacts of anthro-pogenic and natural NOx sources over the U.S. on tropospheric chemistry, PNAS 2003 100(4), 1505-1509.
[3]
www.atsdr.cdc.gov/toxfaqs/tfacts175.pdf, Agency for Toxic Substances and Disease Registry ATSDR.
[4] Environmental health criteria 188, Nitrogen Oxides, United Nations Environment Programme, International Labour Organisation, World Health Organization 1997, International Programme on Chemical Safety.
[5] Coughlin J. G., Rose L. A., Bain D.J., Elliott E. M., The influence of Marcellus Shale Extraction emissions on regionally monitored dry reactive nitrogen deposition, Environ. Sci. Technol., 2017, 51, 3542-3549.
[6] Villena G., Bejan I., Kurtenbach R., Wiesen P., and Kleffmann J., Interferences of commercial NO2
instruments in the urban
atmosphere and in smog chamber, Atmos. Meas. Tech., 2012, 5, 149–159.
Figure 2: Repeatability tests for 15 ppb total NOx diluted with ethene. The measurements presented are consecutive, collected by the same analyst with a C.I. Analytics laboratory unit analyser.
Author Details L. Lorena Torres, C.I. Analytics • 2085 Boulevard Industriel, J3L 4C5, Chambly, Quebec, Canada • Web:
www.cianalytics.com
For More Info, email: email:
8248ad@reply-direct.com
AUGUST / SEPTEMBER •
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