CO2 isotopes? FTIR help

[polartownjunkie]

im having some unexpected difficulties measuring CO2 with a FTIR. Though the FTIR measures bottled CO2 with great accuracy, I get a strong disaggreement when comparing an NDIR and FTIR's CO2 measurement when measuring diesel emissions.

I have good control of the samples temperature and pressure, so I dont believe that could be the issue
I have tried blending CO2 with other known constituents, looking for a quench effect, though that doesnt seem likely with CO2

is there a chance CO2 from combustion emissions might have several isotopes which may not absorb in the way I expect?

any suggestions?

[Don_Hilton]

My first suspiccion is that this is a matter of other combustion products and the slectivity of the NDIR. Apparently water is known to be a problem. Look at the band pass for the filters used in the NDIR and then the portion of the spectrum you are using in the FTIR measurement to see if that would make a difference.

If you look at the ratio of 13C in carbon from various souces in the terestrial enviornment - such as green plants or fossle fuels, there are some differences but they are not great. USGS reports that the mole fraction of 13C ranges between 0.0096 and 0.0115 - or a variation of 0.11% acorss the range of terestrial sources. ( http://pubs.usgs.gov/wri/wri014222/ )

[polartownjunkie]

thanks don, i will check on my NDIR's spectrum range, though do you think its possible that C13 and C12 ratios from combustion emissions may deviate from ambient C13 and C12 ratios?

thanks for your reply

[Don_Hilton]

I do not expect variation in carbon isotope ratio of more that 0.11% of the total carbon content - or USGS is holding out on us.

Some interestign data in http://nature.berkeley.edu/classes/espm-c220/Brand.pdf