Nick Ostle wrote:

> Greetings ISOGEOCHEMISTS,
>
> I am using 13C enriched CO2 to pulse label grassland vegetation and
> was wondering if anyone had some tips/references as to the best way to
> separate the O from the 13CO2 do that I can make 18/17/16O
> determinations at natural abundance and enrichment levels?
>
> thanks in advance,
>
> nick
>
>                      Nick Ostle
>       Institute of Terrestrial Ecology
>                 ITE- Merlewood
>              Grange-Over-Sands
>               Cumbria, LA11 6JU
>                      England
>
>            e-mail: [log in to unmask]
>         tel: ++44 (0)15395 32264
>         fax: ++44(0)015395 34705

If I understand your question correctly Nick.  The answer is that when you
analyze carbon13/carbon12 ratios in carbon dioxide in modern mass
spectrometers you get the bonus of measuring the oxygen18/oxygen17 as
well.  If you collect your gas samples carefully (dry) then you can get
the oxygen isotope ratios of ambient carbon dioxide.  For organic matter
its a different story; you will have to have two aliquots one to determine
the carbon isotope ratios and one for oxygen isotope ratios.  Good luck.
Leo