Quantitative analysis of mass independent fractionation in oxygen isotopes
of stratospheric carbon dioxide.
||Carbon has two stable isotopes 12C and 13C, and oxygen has three stable
isotopes 16O, 17O and 18O. This gives carbon dioxide major three kinds of the molecular weight
; 44, 45, and 46. In case we want to know how much 13C or 17O are included in a sample CO2, we
use mass spectrometer which can determine how much they are in by comparing the molecular weights.
But if the samples are CO2, we canƒőt do it easily. Because 13C16O16O is identical with 12C16O17O
in regard to mass. For this problem, an equation, ˘17O=0.516*Â18O, is substituted.
A sample which hasn't undergone photochemical reaction and is in the equilibrium state, the
equation always comes into existence. (Â=(Rsamples/Rstandard-1)*1000, standard act as a scale
of isotopes, R=(rA)/(pA), rA means rarer isotope and pA means principal isotope.) But what should we do about CO2 the equation doesn't work ? Two methods are suggested before. One is to convert CO2 to H2O with H2 and reduce it with F2 [Brenninkmeijer
et al., 1998]. The other is to exchange oxygen in CO2 for oxygen in CeO2 [Assonov et al., 2001].
First is danger and troublesome in the respect that we should use F2. Second is very similar to our study.
The most remarkable point of this method is we can adopt ˘17O=0.516*Â18O after exchanging oxygen.
So Â13C, Â17O, Â18O is clear eventually.|
Our purpose is to establish ˘17O measurement by GC/C/IR mass spectrometer on this theory.
The ˘17O repeatability was }1.05ń. With this method, we measured stratospheric CO2 and Â17O/Â18O of
samples was 0.86.