Dear listmembers,
I have been storing hydrogen gas samples from duplicate runs (and thus
with established original delta-D values) for up to 6 years in sealed 9mm
o.d. Pyrex tubes, without any measurable isotope fractionation.  Storage
of these Pyrex tubes occurred at room temperature in dark boxes.  Contrary
to this comforting experience, our laboratory has observed that hydrogen
gas that was stored in sealed Pyrex ampules together with zinc (that is,
hydrogen derived from the reduction of water with zinc) does indeed change
its isotopic composition slowly over time.  Therefore, we always proceed
with the isotopic measurements immediately after the reduction of the
water samples.  I agree with other listmembers that fractionations over
time can be expected if stopcocks, O-rings, or even active charcoal or
other adsorbants are in contact with the stored hydrogen gas.
                              Arndt Schimmelmann