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Dear Jennifer,

I tried this as a side project a few years ago, but I was after dD 
ratios as well as d18O.  The problems with direct equilibration are 1) 
standardization and 2) interference of plant material/soil with the 
equilibration, especially that which happens on the surface of the 
platinum coated catalyst beads to facilitate measurements of dD.  In 
both cases, the question boils down to what is going on in the exetainer 
vials besides straightforward equilibration and how do you standardize 
for that.  The extraction line approach, so long as you end up with all 
of the water, is the best way to eliminate this question because you 
will equilibrate with pure water.

Brad

Brad E. Rosenheim, Ph.D.
Postdoctoral Investigator
National Ocean Sciences Accelerator Mass Spectrometer
Department of Geology and Geophysics
Woods Hole Oceanographic Institution

tel.	508.289.3547 (off.)
tel.	508.289.4816 (lab.)
fax.	508.457.2183 (dept.)
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mail.	Brad E. Rosenheim
	MS #8
	Woods Hole Oceanographic Institution
	Woods Hole Road
	Woods Hole, Massachusetts 02543



Jennifer Eikenberry wrote:

> I have recently had requests for analysis of 18O in soil, stem and 
> leaf water.  We have a Delta plus and GasBench II, but we do not have 
> an extraction line.
> Do I have to purchase and set up an extraction line? (West et al. 2006 
> Water extraction times for plant and soil materials used in stable 
> isotope analysis.  Rapid Comm in Mass Spec.)
> Is there any reason why we can't just do a direct equilibration in the 
> exetainer vials? (Scrimgeour 1995. Measurement of plant and soil water 
> isotope composition by direct equilibration methods. Journal of 
> Hydrology).
>
> If direct equilibration is not the best option, what are your thoughts 
> on this new method?  (Vendramini and Sternberg 2007. A faster plant 
> stem-water extraction method. Rapid communications in Mass Spectrometry)
>
> Thanks!
>