Chromatography Forum

We recently had a GC/IRMS setup at our university here, and I had a question regarding it's theory that I haven't heard back from anyone at the university on yet. When I went in to help set it up, the professor mentioned basically how it works, and that, for carbon, CO2 is analyzed via a mass spectrometer at masses 44, 45, and 46, to determine the isotopic abundance of carbon in a given sample. My question is this: How is there 100% certainty that the mass of the CO2 molecules isn't being affected at all by possible isotopic variations in the oxygen atoms comprising the CO2. Pardon me if it actually does ionize the CO2 and analyze the carbon on an atomic basis, but the way it was explained to me, the entire mass of the CO2 particle is analyzed and is then calculated into an isotopic abundance of C, which seems fraught with uncertainty introduced by the presence of other stable oxygen isotopes (as well as unstable carbon and oxygen isotopes). Maybe I'm being a bit too paranoid? I just wanted to see some of you guys' takes on this, thank you!

You have the right idea: the mass spec is analyzing the molecular mass of carbon dioxide (hence the 44, 45, and 46 traces) and both 13C16O16O and 12C17O16O contribute to the mass 45 signal. How the integration program figures out how much of the mass 45 signal is from carbon-13 versus oxygen-17 is by using the mass 46 signal, which is primarily 12C18O16O, and the fact that oxygen-17 and oxygen-18 fractionate proportionally ("mass-dependently") so they have a constant ratio.

The actual math is a little more complicated than that (but not too much!). If you're interested, there's an annotated Python implementation in Aston. The "Santrock" method is the default one used in Isodat and is more accurate than the "Craig" method (which is only there for historical reasons).

Ah, thank you so much, I actually just got an email from someone here containing the articles in which Craig and Santrock published these findings, very interesting stuff, I had to wonder how they managed that one, glad to see that so much thought was put into this, thank you!