Hi John,

Thanks for sharing your insights. :-)

We had a go at analysing IAEA-CH-3 as well as IAEA-C4 and IAEA-C5 as part of a testing exercise organised by Manfred Gröning a few years back.

For samples dried down under vacuum for 7 days we got d18O v. VSMOW values of 31.5 ± 0.1, 25.2 ± 0.1, and 16.4 ± 0.4 respectively.  The larger uncertainty for IAEA-C5 were due to the inhomogeneity of the sample which even affected its d13C value.

Compared to d18O values of subsamples that were left to "equilibrate" with ambient atmosphere and not vacuum dried, d18O values of dried down samples were 0.5 to 1.0 ‰ lower.  For example, d18O of not-dried-down IAEA0CH-3 was 32.0 ± 0.1 while dried-down material showed a d18O value of 31.5 ± 0.1.

Measured d18O values were scale normalized using IAEA-601 and IAEA-602 as scale anchors.  While these reference materials were not vacuum dried (as you emphasized :-) ), we stored these materials in a sealed container purged with dry Argon and in the presence of P2O5 to keep "our powder dry" as it were.


Best,

Wolfram


-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of John Howa
Sent: 27 October 2016 22:31
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] tips for TCEA 18O analysis of cellulose

Lisa,

For bulk organic matter, issues of exchangeability and water absorption will have to be examined on a case-by-case basis; however, for alpha cellulose, it is much more straightforward.

We have used a powdered Sigma cellulose (C1875-100G) in Utah (where it's pretty dry) and have seen little effect between samples that were vacuumed for multiple days and samples that were loaded into silver capsules straightaway and run. But it is a good practice to vacuum the samples to be sure (don't vacuum your benzoic acid!). I would not use sugar if cellulose is available, as it is even more hygroscopic (especially if ground) and is extremely difficult to dry.

As I'm sure you've read, the CO tank should only be used to monitor the instrument performance, not a reference for isotope ratios - this is why it is important to use at least two reference materials covering the expected isotopic range of your samples. Though the IAEA benzoic acids are not ideal, they have a very good pedigree to the VSMOW scale and are quite widely used, and have the advantage of flexibility in sample mass over the reference waters in silver tubes.

Though there are varying reported oxygen isotope ratio values for IAEA-CH-3, it may be useful for you to help verify your methods and calibrate your working standards. The original IAEA TECDOC-825 lists +31.85‰ using nickel pyrolysis and mercuric chloride methods. Posts in the archive give delta values of +31.5‰, +32.0‰, and +32.7‰. The reference below (Evans, et. al.) gives +32.52‰; we get values a bit more positive using TC/EA at +33.2‰. Though there may not be an officially recognized value, hopefully these can assure you that you're on the right track.


Best of luck!

John Howa


Reference: Evans, M. N., et al. "Correction algorithm for online continuous flow δ13C and δ18O carbonate and cellulose stable isotope analyses." Geochemistry, Geophysics, Geosystems (2015).

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