Dear Penny,

The most simple solution is simply to add more CO2 and let the CO2 do 
the neutralizing. I had this problem of very basic water samples 
years ago (on an off-line system), and modelled the whole system 
using the known 18O (and 13C) fractionations. Whereas the 13C-values 
are obviously very much affected, the 18O-values are hardly affected 
at all. Therefore, adding (much) more CO2 is the safest way of doing 
such samples, although I am not sure how you would accomplish that in 
your automated system.

lots of success,

Harro Meijer

>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
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>Office: 209b Hutchison Hall               Lab: 209 Hutchison Hall
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-- 

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