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Pitter,
Beyond the partial reaction of calcite below 1600C, our experience has 
also been that divalent cations such as calcium and barium can create an 
apparent oxygen scavenging problem in the reactor that will reduce 
yields (creating CaO , BaO etc.?) and shift isotope values in those and 
the subsequent several samples or standards, so we never run calcium, 
magnesium or barium compounds in the TCEA. Did you notice any alteration 
of known samples or standards run immediately after the calcite? The 
extent of the effect could be related to many operational factors, 
including the extent of self-scavenging/pacification, and goes away with 
each reactor cleaning. See Straight et al. 2004, Rapid Comm. Mass Spec. 
18: 2897-2903.
Jon

Pitter Ferrio (pvcf) wrote:
> Dear Isotopers,
>
> I have a relatively stupid question. Can anyone tell me what's the 
> oxygen elemental composition of the international standards NBS19 and 
> IAEACO1? In all the official pages (IAEA, USGS, NIST) and reports I 
> have been looking for I could only found that they are both "calcite", 
> but I'm not sure whether they are PURE calcite (CaCO3, 48% O).
> To calibrate our lab standards for d18O in solid organic matter, we 
> ran them together with IAEACH6 and two internal organic standards 
> (benzoic acid and glucose) in a pyrolysis TC/EA (1450ºC). We found 
> that the amount of oxygen for both carbonates was the same, but much 
> lower than the one expected for pure calcite (33-34% O). Both the d18O 
> and the relationship between weight of sample and peak area looked 
> quite stable, so at first glance it doesn't seem to be due to an 
> incomplete pyrolysis. But we would like to know (if possible from an 
> "official source") the expected %O values for these material before 
> ruling out this possibility.
>
> thank you in advance,
>
> Pitter Ferrio
>
>
>
> -- 
>
> Juan Pedro (Pitter) Ferrio
>
> Marie Curie IEF-research fellow
>
> Chair of Tree Physiology, University of Freiburg
> Georges-Köhler-Allee, Gebäude 53/54
> D-79110 Freiburg im Breisgau, Germany
> TEL +49(0)761/203-8310
> FAX +49(0)761/203-8302
>
> e-mail: [log in to unmask] 
> <mailto:[log in to unmask]>
>
> web: http://web.udl.es/usuaris/x3845331/
>
> --
>