Analysis of 12CO2 and 13CO2 ratio

[sabikh]

Colleagues,

I am starting the analysis of 13CO2 and 12CO2 gas in human breath samples. I have tried the analysis with SPME-GC/MS and a DB-5 capillary column and I am having problems. Is there anyone who has used this system (or a similar system) who I can communicate with to ask further questions.
Thanks

[Ron]

Not enough information here. Are you trying to use a standard GCMS system, or are you using an isotope ratio system? I have seen this type of analysis performed successfully on carbon dioxide in the atmosphere in plant growth chambers using cryogenic concentration followed by IRMS, but this is not an appropriate analysis for a standard benchtop unit.

[HW Mueller]

Ron,
as stated in another chain, we gave up on doing 12C/13C ratio via conversion of oxalic to CO2, because we could not get rid of C coming from other entities (solvent, etc.). Thus, I would be very much interested in finding out how contamination by other components in the atmosphere were ruled out/compensated in the example you mention.

Also, I am still not sure about how isotope ratio MS does a better job than any high resolution MS on isotope ratios in unburned molecules. (We apparently solved the problem by doing isotope ratio with GCMS on the dimethyl ester of oxalic acid)

[CE Instruments]

There seems to be a lot of questions regarding C12/C13 studies and there are a different techniques depending on the sample.

Breathgas analysis can be done directly with a Breatgas system. Thermo used to make the Breathmat especially for this application.

GC/MS quads can be used for C12/C13 studies but struggle with looking at natrually occuring levels. Most work on these sorts of systems is on metabolism studies looking at enriched C13.

High resolution MS can give easier speciation of the isotopes and better sensitivity thanks to better signal to noise.

IRMS with combustion gives accurate isotopic ratios but combusts the sample and then monitors C12/C13 ratios as CO2.

IRMS Can be used without combustion and normally uses dedicated detector for each target mass and can be set up to look at C12/C13 as CO2, and O16/O18

[Ron]

HW,
IRMS was used without combustion, so that eliminated contributions from other sources to the CO2. As stated in the posting by CE Instruments IRMS uses dedicated detectors for the masses detected, and sensitivity is enhanced by doing this.