Dear Lisa and others,
Regarding measurement of "dry" or "humid" samples: In the chapter by
Saurer and Siegwolf in Pier de Groot`s book (chapter 23, pp. 497ff) you
can find a comparison between the measurement of "dry" and "humid"
aliquots of cellulose samples. The outcome is that the difference
between these d18O measurements seems not to be constant (Fig. 23.2).
For me that has as consequence that even if one uses a "humid" cellulose
as correction standard for the measurement of "humid" samples, the
results of the samples might be biased in a not proper, correctable way.
We use benzoic acid, dimethoxy benzoic acid and trimethoxy benzoic acid
as correction and quality control standard (is is expected that these
compounds show a huge difference in d18O because their production
process is quite different and most probably using different oxygen
sources with different d18O). But our lab is not a tree ring lab. It
might be advisable for a tree ring lab to use a cellulose samples as
quality control standard. I would use such a material at least to check
routineously if the "extraction" procedure of the cellulose from the
tree ring is reproducible. In case you are interested, please find some
info on our standards below.
In our lab the performance of the measurement of sugar samples rapidly
has increased when we put the "moving carousel" of the autosampler
filled with the sample Ag-capsules in a drying oven at 60° C for at
least 1 hr and immediately put it back in the zero blank autosampler
starting to flush with dry He (before starting the measurement). The
hygrocopicity of cellulose should be smaller in relation to that of sugars.
Benzoic acid Riedel-de Haen 100g 33047
Dimethoxybenzoic acid Acros Organics 25g 115420250
3,4,5-Trimethoxybenzoic acid Acros Organics 99% 500g 13993500
Riedel-de Haen ---> Sigma Aldrich?
On 28.10.2016 11:09, Wolfram Meier-Augenstein (aps) wrote:
> 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.
> -----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
> 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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