Effects of Isotopic Calibration Gases on IR Quantification Analyzer Techniques to Measure CO and CO ₂ in Engine Emissions Testing

Infrared spectroscopic methods are the most common methods in the automotive industry for measuring carbon monoxide (CO) and carbon dioxide (CO₂) gases. Concentrations of both gases, which are emitted from the combustion of fuels, are required to be determined accurately in order to follow strict environmental regulations. Appropriate analytical techniques and accurate calibration gas mixtures are therefore critical for successful measurements. Regulatory documents such as the EPA’s Code of Federal Regulations 40 (CFR 40) part 1065.250, UN ECE-R83, and (EU) 2017/1151 recommend a nondispersive infrared (NDIR) analyzer to measure CO and CO₂ concentrations in raw or diluted exhaust gas samples. Over the last decade, Fourier Transform Infrared (FTIR) spectrometry has been validated and recommended in engine exhaust certification testing as well as in engine and vehicle development activities.

The variation in the isotopic ratio of ¹³C/¹²C in natural atmospheric CO₂ is in the range of ± 2‰ however, artificial or non-natural sources of CO or CO₂ can potentially have much larger variances. To fully understand the impacts of isotopic composition on the analyzers, the δ¹³C values used in this study were selected to cover a broad range of non-natural isotope ratios (very depleted and enriched). In the present work on both FTIRs and NDIRs, up to 4% deviation in analytical results were observed relative to the base case composition (-12‰ ¹³CO) when the CO/N₂ gas mixture was enriched to 2630‰ with ¹³C content. Analytical deviations measured on NDIR analyzers were more pronounced (4-14%) relative to the base case composition with the change of ¹³C in the CO₂/N₂ mixture from -982‰ to 6783‰. Moreover, the error with FTIR measurements could rise up to a factor of 2 or more depending on the ¹³C and ¹²C band selection and their evaluation methods. Known isotopic gas mixtures and careful evaluation band selection in the FTIR method were observed to reduce the analytical errors. Even though calibration gases were prepared accurately for molecular concentrations, carbon isotopic concentrations far removed from natural abundance showed significant errors in the measurements. It is therefore essential to have either known or natural ratios of carbon isotope calibration gas mixtures for accurate emission measurements.