Ajaz,
I think, you have to use a GC  to separate O2 from N2 before the
conversion.  See our paper :

Kinga Révész, John-Karl Böhlke, Richard L. Smith, and Tadashi Yoshinari,
1999, Stable Isotope Composition of Dissolved O2 Undergoing Respiration in
a Ground-Water Contamination Gradient:  Morganwalp, D.W., and  Buxton, H.T.
eds., U.S. Geological Survey Toxic Substances Hydrology Program ?
Proceeding of the Technical Meeting, Charleston, South Carolina, March
8-12, 1999--Volume 3-- Subsurface Contamination From Point Sources: U.S.
Geological Survey, Water-Resources Investigation Report 99-4018C, p.
323-328.
Revesz,   K.,  Bohlke,  J.K.,  Smith,  R.L.,  Yoshinari,  T.,  1999,"  d18O
     Composition  of  Dissolved  O2  Undergoing Respiration in Contaminated
     Ground-Water.   IAEA  International Symposium on Isotope Techniques in
     Water Resources Development and Management, Vienna, Austria, 10-14 May
     1999.
And where the GC method is described:
Revesz,  K.,  and  Coplen,  T.B.,  1990,  Gas Chromatographic Separation of
     Carbon  Dioxide  and  Nitrous  Oxide  for  Stable Isotopic Analysis of
     Carbon Dioxide. Analytical Chemistry, v. 62, p. 972-973.
Revesz,  K.,  Coplen,  T.B.,  Baedecker, M.J., Glynn, P.D., Hult, M., 1995,
     Methane  Production  and  Consumption Monitored by Stable Hydrogen and
     Carbon  Isotope  Ratios  at  a  Toxic  Waste Site, Bemidji, Minnesota.
     Applied Geochemistry, Vol. 10,  pp.505-516

Kinga Revesz
Chemist, Stable Isotope Laboratory
U.S.Geological Survey
MS 431
Reston, VA 20192
T:703-648-5865
FAX: 703-648-5274












                    Ajaz Karim
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                    Sent by: Stable              Subject:     18-O analysis of dissolved oxygen
                    Isotope Geochemistry
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                    EDU>


                    06/28/00 12:56 PM
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                    Stable Isotope
                    Geochemistry





Dear List Members,

This is concerning 18-O analysis of dissolved oxygen in water by gaseous
equilibration and catalytic oxidation of graphite. I have been extracting
air and air-equilibrated water samples to test our new line. There seems to
be some unreacted O2 at the end (even when reaction time is 30 minutes),
fractionating the samples on the negative side by 1.7 plus minus 0.2
permil.

The difficulty with monitoring gas transfer using vacuum guages is the
large
amount of nitrogen gas in the air or water samples and conversion of O2 to
CO2 (the latter frozen out with liq. N2) is barely noticeable.

Is any one aware of a filter that can filter out N2 gas and let O2 .

Thanks in advance for any suggestions or comments

Ajaz Karim

- - - - - - - - - - - - - - - - - - - - - -
Ajaz Karim, Ph.D.
Department of Earth Sciences
University of Ottawa, Ottawa
Ontario, Canada K1N 6N5
Tel: (613) 562-5800 ext 6639
Fax: (613) 562-5192