Dear All,

Pier asked me whether I can have some comments on this problem. See also 
the message to isogeochem by Torsten Vennemann on 9 October 1996 for
the hydrogen-zinc problem.
As far as I remember the H2-glass exchange was shown to be important by 
the papers mentioned at high T, i.e. if the reaction tubes are kept in the
furnace too long.
When I started the dD business I used the stopcock-sealed tubes, but the N2
background was always unconveniently high (see Heaton and Chenery 1993, 
Chem. Geol. Isot. Geosci. Lett., 106:485-487.). So I use flame-sealed 6 mm
pyrex tubes now. This way there is no N2 in the sample gas. However, I don't
like storing the tubes containing Zn+ZnO and H2 for more than a couple of
days. Even if there is no obvious shift in dD values after a week, the 
reproducibility gets worse. Usually I prepare and reduce the water samples
one day and measure the dD values on the next day. This way for pure H2O 
the average reproducibility is about 0.5-0.7 permil. Why measuring next day.
I prepared a set of standards and started measuring them immediately after
the reduction. The dD values stabilized after ~2 hours, maybe due to the 
hydrogen-zinc reaction that needs time for equilibration, so anyway I would
wait for hours after the reaction.
Once I measured hydrogen gas stored together with the reacted zinc for ~2
years in flame-sealed tubes. There was no H2 in the tubes. If this is due to the
formation of zinc hydroxide, a solution for long-term storage would be 
absorption of H2 on activated charcoal in flame-sealed tubes. Also heating
the "zinc" to 350 oC  before the sample is let into the MS could eliminate the
problem.

Best regards,
Attila

Dr. Attila Demeny
Laboratory for Geochemical Research
Hungarian Academy of Sciences
Budapest, Budaorsi ut 45, H-1112, Hungary
tel/fax: 36-1-319-3145
e-mail: [log in to unmask]