Dear Robert,

if I understood well then you have measured the CO2 gas liberated from NBS19 against your reference CO2 gas and now you want to calculate the delta value(s) of your ref CO2 gas versus PDB.
It is not difficult if you know how to calculate the delta(b vs. a) if you know the delta(a vs. b), where a and b are two different CO2 gases.
The formula is

delta(b vs. a) = (-delta(a vs. b)*1000)/(delta(a vs. b)+1000)

In this way you can calculate the delta value of ref CO2 against the liberated CO2 from NBS19, but you want to know the delta value(s) of ref CO2 against PDB.
The delta values of the NBS19 vs. PDB is known. You can calculate the delta value(s) of the liberated CO2 vs. PDB using the temperature dependant fractionation factor for the process you liberated CO2 from carbonate by phosphoric acid. You can find it easy in the literature.

Then using the following equation you can calculate what you wanted:

delta(refCO2 vs. PDB) = delta(refCO2 vs. liberatedCO2)*(delta(liberatedCO2 vs. PDB)/1000 + delta(liberatedCO2 vs. PDB).

I hope this helps you.

Best regards
István

Robert van Geldern írta:
I assume this is an easy question but it seems my last math class was a long time ago.
Usually I don't care about the exact value of my CO2 ref gas on our gasbench system since everything is normalizied to reference materials with known values (and working gas cancels out). I just type in a value for the working gas in the isodat standard editor which will give a delta value close to the correct value.

I need to calculate the CO2 value of the working gas now and my idea was just to caluclate the things backwards. Assuming that my reference material (e.g. NBS19) will produce a CO2 with a known value it should be possible the calculate the delta value of the working gas etiher from the raw delta values or the ratios given in Isodat by treating the ref gas as the sample and the NBS 19 as the standard gas. However, sitting in front of Isodat and the equations it is not so obvious anymore...can anyone point me in the right direction?

regards,
Robert

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Dr. FÓRIZS István  ..........  Istvan FORIZS Ph.D.