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Hello Andrew, 

in my own PhD work, equilibrating at 120 °C overnight in an evacuated stainless steel equilibration vessel with two waters (−268 ± 1 and +113 ± 1‰, respectively), I measured exchangeable H fractions varying between 0.18 and 0.24 for leaf litter samples from a climate transect across the Argentinean Pampas and Patagonia (page 62 in the below GCA paper). Temperature conditions, equilibration time and thorough drying, as pointed out already by Len, play a big role. E.g. time to reach equilibrium is longer for these kinds of samples compared to "more defined" matrices like cellulose.

Source:
- Geochimica et Cosmochimica Acta 152 (2015) 54–71
- Environ. Sci. Technol. 2013, 47, 949−957 + supporting information (for more method details about the used equipment)

Best regards,
Marc Ruppenthal


-----Ursprüngliche Nachricht-----
Von: Stable Isotope Geochemistry <[log in to unmask]> Im Auftrag von Wolfram Meier-Augenstein (pals)
Gesendet: Donnerstag, 20. Dezember 2018 10:28
An: [log in to unmask]
Betreff: Re: [ISOGEOCHEM] help interpreting 2H exchange exp't

The waters Andres used are 210.6 apart which should be enough for most organic compounds / materials.  Even if their native d2H is close to one end-point, the other end-point will provide sufficient shift.

Of course, the wider the gap between d2H values of waters A and B, the better. No argument there.

However, in this case an unfortunate incidence of waters used for equilibration is highly unlikely to be the cause for Andrew's observation.

Even at an F(ex) of 0.05, with a sample d2H value of -177.8 it would require equilibration with SLAP (!) to shift the d2H(total) to -190.


-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of WASSENAAR, Leonard Irwin
Sent: 20 December 2018 09:22
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] help interpreting 2H exchange exp't

Which is really not much once you have factored in all of the uncertainties....  could be +/-7 permil or so on this estimate.

I have seen this a few times in reviews, where researchers unwittingly choose equilibration waters that are way too close together (like just a few 10's of permil in d2H apart), or some unfortunate combination of 2 waters in the natural abundance range that land them in a spot with very little discernment.  Seems like this might be one of those cases...

I propose a new years resolution for org-H equilibrators - with your waters go really  wide (i.e. >300 permil difference) and preferably positive outside the natural delta-range.  And always do a quick pre-estimate model of your expectations as WMA has done below - it can save a lot of head scratching later on.


-----Original Message-----
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of Wolfram Meier-Augenstein (pals)
Sent: Thursday, 20 December 2018 10:05
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] help interpreting 2H exchange exp't

Let's say the exchange fraction of 0.000475 would have been incorrectly determined (for whatever reasons).  Let's also say the actual exchange fraction is 0.05 (as it is more or less for raw cotton; see RCM 2014, 28, 545-552).

With a d2H(non-ex) of -177.8 the choice of equilibration waters would have yielded d2H(total) of -178.7 and -168.2 for water A (-196.1) and water B (+14.5) respectively. The difference between d2H(A) and d2(B) for equilibrated samples would thus be 10.5.




-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of WASSENAAR, Leonard Irwin
Sent: 20 December 2018 08:06
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] help interpreting 2H exchange exp't

Hi Andrew - the devil is always in the prep and analysis details.  Something seems wrong to me with such a low exch-H fraction.  What is the residual moisture content (gravimetrically)?  How did you treat / dry the sample, etc. It's also possible - by a fluke  - that the H isotope fractionations of the 2 waters you used landed you right on the identical value of the non-exch H. I would go far outside the natural range - i.e. very positive on one of the waters (like +500). Len

-----Original Message-----
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of A.J. Tanentzap
Sent: Wednesday, 19 December 2018 20:35
To: [log in to unmask]
Subject: [ISOGEOCHEM] help interpreting 2H exchange exp't

Dear all

I've run an exchange experiment on a solid organic as described in Meier-Augenstein et al. 2011. Rapid Commun Mass Spectrom.

I estimated the non-exchangeable 2H ratio to be -178.  The estimated fraction of exchangeable H in the sample was 0.0005.

When I re-run the sample in a bulk analysis, the total (i.e.
exchangeable + non-exchangeable) ratio was -190.

Would someone be so kind as to explain how this is possible with a near-zero fraction of exchangeable H?

Many thanks!
Andrew
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