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Dear Penny,
in your set up CO2 is absorbed with high efficience.
Since only the carbonate ions dominate at this pH you can waite for the
time necessary for DIC-water isotope exchange equilibration at known
temperature and precipitate the DIC quantitatively and fast as BaCO3 by
the addition of a saturated barium chrloide solution (or better dehydrated
barium chloride). This fixes the DIC isotopic ncomposition of DIC. The
BaCO3 is analyzed using McCrea´s classical phosphoric acid method. Then
use the (dissolved) carbonate-water fractionation factor determined
experimentally in a recent GCA paper (John Morse is one of the co-authors)
at the known temperature to calculate the 18O activity of water.
Be carefull: non-quantitative absorption leads to a strong kinetic isotope
fractionation for both C and O isotopes upon hydroxylation of aqueous CO2!
As a rough first estimate you could also add anhydous BaCl2 to an aliquot
of your alkaline water and let gaseous CO2 (just a small relative
fraction) of known O isotopic composition be absorbed in a flow-through
system. Then analyze the washed precipitated barium carbonate. Then use
the known kinetic isotope fractionation factor. This method however is
less precise because the letter partition coefficients is (besides other
factors) influenced by micro-scale zones of decreased pH around the
precipitated barium carbonate.
A third method requires a shift of the pH by the absorption of hugh
amounts of CO2 and then gas-(Na carbonate) solution equilibration using
the standard method.
One could also think about changing the pH prior to CO2 equilibration by
small amounts of concentrated acids (HCl steam; sulfuric acid, phosphoric
acid)... Adding different amounts should give an estimate if you change
the 18O content by the acid addition. Check by mass balance calculations.
Finally, total combustion of a small aliquot of the (DIC poor) solution in
a TC-EA type system will give you the 18O content of the total solution in
a simple measurement.
Good luck,
Michael
*****************************************
Prof.Dr. Michael E. Böttcher
Marine Geochemistry-Geology
Leibniz Institute for Baltic Sea Research
D-18119 Rostock, FRG
>
>>Hi all. I have a curious problem. I've been doing headspace analyses
>>for d18O of water samples using the Gasbench in continuous flow mode
>>on our Delta plus XP.
>>
>>A few samples yielded negligible peaks. I re-ran them, and still no
>>measurable peaks. I decided to check the pH and have found that the
>>samples that won't run are all at about pH 12. Presumably, the CO2
>>gas that I'm flushing with is going into solution in the water
>>rather than remaining a gas in the headspace and exchanging oxygen
>>like good gasses should.
>>
>>I think I need to neutralize the waters, but I'm not sure how to do
>>it without affecting the oxygen values of the water.
>>
>>Has anyone else dealt with this problem? What did you do about it?
>>If need be, I could run the oxygen analyses using our TC/EA, but
>>since I've run all the other samples in this set by headspace
>>equilibration, I'd like to run them all this way.
>>
>>Thanks,
>>
>>~Penny
>>
>>*******************************************************************
>> Dr. Pennilyn Higgins
>> Research Associate
>>
>> "SIREAL"
>> Stable Isotope Ratios in the Environment Analytical Laboratory
>>
>> Department of Earth and Environmental Sciences
>> University of Rochester
>> 227 Hutchison Hall
>> Rochester, NY 14627
>>
>> [log in to unmask]
>>
>>Office: 209b Hutchison Hall Lab: 209 Hutchison Hall
>>Voice : (585) 275-0601 Outer lab: (585) 273-1405
>>FAX : (585) 244-5689 Inner lab: (585) 273-1397
>>
>> http://www.earth.rochester.edu/SIREAL/index.html
>>*******************************************************************
>
>
> --
>
> Prof. dr. Harro A.J. Meijer
> Centrum voor IsotopenOnderzoek (CIO), Rijksuniversiteit Groningen
> Nijenborgh 4, 9747 AG Groningen, Netherlands
> tel +31-50-3634760 fax +31-50-3634738
> http://www.rug.nl/cio
>
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