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Dear Andrew,

I just saw your emails and this issue is really striking to me.  In my modest opinion, I believe there was something strange in the exchange experiment (e.g. residual moisture).  Did you exchange your materials offline and then transfer to the carousel? Did you dry them in between?  I am also curious to know if you include non-exchangeable materials in your experiments (e.g. polyethylene) and how you corrected the measured results (which (non-)exchangeable stds did you use for calibration?).  And more importantly, you said that the bulk and exchange experiments were run at the same time.  Were they different subsamples?  Because if I consider that your measured values for bulk and exchange exp are the same subsamples, this is more striking, since I would expect something close to -178 for bulk samples as well. 

David


-----Original Message-----
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of WASSENAAR, Leonard Irwin
Sent: 20 December 2018 10:22
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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