Stable Isotope Geochemistry


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Dr W Meier-Augenstein <[log in to unmask]>
Reply To:
Thu, 19 Feb 2004 09:47:27 -0000
text/plain (80 lines)
Hi Len,

I'm currently writing a paper along those lines you mentioned but I'm
not sure if it will be a slam dunk; especially since the waters we have
run thus far on our pyrolysis system (TC/EA-IRMS) are rain, ground and
creek waters (plus IAEA standards), i.e. fresh waters only, nothing with
a high salt content.

Here's a sneak preview from the abstract.

Accuracy and precision of 2H/1H and 18O/16O isotope ratio analysis of
water samples were evaluated with a set of 4 international isotopic
standards obtained from the International Atomic Energy Agency (IAEA) in
Vienna, namely IAEA OH-1, OH-2, OH-3 and OH-4. Typical precision of d2H
and d18O measurement from H2 and CO by the IRMS alone, i.e. independent
of sample conversion was  0.23  and  0.14 , respectively. Typical
precision of d2H measurement by the entire TC/EA-IRMS array was  1.93 
and  0.51  before and after optimisation, respectively. For d18O
measurement the corresponding values were  0.28  and  0.18 ,

The above doesn't preclude me getting two "poor" d18O precisions of 0.38
or 0.78  within a group of 20 water samples referenced against CO and
enveloped by 2 QC standards either side. On the other hand, within the
same group I observe precisions ranging from 0.017 to 0.157 with 8 out
of 20 samples (all run in triplicate) hovering around the 0.08 mark.

To put this into perspective, the true d18O value of my QC standard
(IAEA-OH4) is -15.3 . Over a run spanning 8 groups of 20 samples plus 9
groups of QC standards, the measured d18O value typically ranges from
-14.98 to -15.77, i.e. -15.3 +/- 2 standard deviations.

In my experience thus far, TC/EA-IRMS yields good results for (fresh)
water if one's prepared to take the bother.



> -----Original Message-----
> From: Stable Isotope Geochemistry
> [mailto:[log in to unmask]] On Behalf Of Wassenaar,Len [NHRC]
> Sent: 18 February 2004 16:00
> To: [log in to unmask]
> Subject: Re: CO gas for CF-IRMS
> Harro,
> However, I still do not really understand why anyone would
> want to use CO for measuring 18O in water over so easily
> automated CO2-water equilibration, and especially so for
> salty to more saline samples (ie 18O activity verus
> concentration).  Never got a good answer to this question on
> this list before, and haven't seen a slam dunk paper in the
> literature yet. Anecdotally, I hear nothing but complaining
> from labs who tried to use CO for 18O in waters (very poor
> precisions, huge memory effects, on and on). So why even bother?
> Len
> Leonard I. Wassenaar, Ph.D.
> Environment Canada
> 11 Innovation Blvd.
> Saskatoon, SK
> Canada, S7N 3H5
> tel: 306-975-5747
> fax: 306-975-5143