Dear Isogeochemers,

We have been using Zn reduction of water to prepare hydrogen gas for D/H
isotope ratio measurements for the past 6 years with good success, and
precisions on the order of +/- 1 per mille. Over this time I have noticed
that once prepared by cleaning with 1% HNO3, washing with distilled water
then vacuum drying at 300 degrees C the reagent has a finite shelf life and
after this it 'goes off'. This is first detected by consistently 'heavy'
results obtained from the laboratory standard. Once re-cleaned good results
are obtained again.

I suspect that this is consistent with other users experience. My question
is does anyone have an explanation for this behaviour? I have wondered
whether or not it is due to adsorption of atmospheric vapour onto the Zn as
the storage container is opened and that this adsorbed water is not removed
by subsequent evacuation leaving behind a heavy component.

I recall an earlier discussion in which someone reported that sealing water
into glass tubes with Zn and storing prior to reaction also yielded odd
results. Are these two effects related?

Any comments?

Paul