Subject:                Re: 99.9atm% carbon

> Thank you Dwight for clarifying that.  This has been our position all
> along. Ecologists here are attempting to calculate mass-balances after
> addition of 99atom% tracer to a system and there is some disagreement
> about how to proceed, or even if the resulting figures will actually
> mean anything......
>
>                                                 Thanks again,
>

Hi Andy, Hi Darren,

Here is my tuppence worth of thought on this matter.

In principle, there would be nothing to add to Dwight's, Tom's and
Andrew's comments, wouldn't it be for the ubiquitous "it all
depends".

The devil, as always, is hidden in the details. Personally, I am not
very happy with the use of 99.9 atom% when used in conjunction
with organic compounds and IRMS. In IRMS, we do not measure
the atom% of carbon-1 or whichever carbon happens to be
enriched; this in contrast to manufacturers/suppliers of labelled
compounds since they use MS to determine the enrichment of the
labelled position. We measure the atom% of the entire molecule,
which, other than for formic acid and its salts, will always be less
than 99.9% unless the entire carbon skeleton is uniformly labelled.
For [13C-1]acetate, the 13C enrichment of the entire molecule is
roughly 50 atom% if C-1 is 99.9% and C-2 is natural abundant, i.e.
1.1%.

So, as I said, it all depends on what your colleagues want to do. Is
the idea to introduce 13CO2 or 13C-carbonate into a system? If
yes, it would certainly be more sensible to base any mass-balance
calculations on APE (or even better MPE, as Dwight pointed out)
or, as suggested, as mole fractions or mole ratios. Tom Brenna's
review article in Mass Spectrom. Rev., vol 16, 227-258 (1997)
includes a section on mass balance, pool mixing and dilution
spaces.

Speaking of which, the latter two will determine not only which
notation to use but also which instrument. If the enrichment in the
target compounds is likely to exceed 0.5 APE you would be better
off using MS anyway as this would exceed the dynamic range of
an IRMS.

Have fun.


Cheers,
                  Wolfram



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Dr. W. Meier-Augenstein, CChem MRSC
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