Dear zinc-users worldwide,

Frank Pawellek's frustrating experience with zinc seems to be the
exception rather than the rule.  I am personally using zinc in many
hundreds of delta-D determinations of waters every year - without any
significant problem.  If there are any "tricks of the trade" on my part,
then they are published in Analytical Chemistry 1993, vol. 65, p. 789-792.
Briefly, here is what I consider important in order to get good precision:
- start with clean, pre-annealed (500 to 550 deg C) Pyrex 6mm tubes that
are closed at one end.
- store your zinc under vacuum or in a dry, inert atmosphere.  Load about
100 mg of zinc for every 2 mg of water.
- after loading the zinc and before admitting any water, evacuate the
zinc-containing tube and heat the zinc to 350 deg C for at least 5 min in
vacuo.  Also, flame the glass above the zinc to desorb water.
- When admitting water (via syringe or capillary) be sure that the
entire sample is transferred (= typically frozen into the lower end of the
Zn-containing tube), or you will likely encounter irreproducible isotope
fractionation.
- after sealing the ampule, you should react the water with the zinc at
500 deg C for at least 30 min to quantitatively reduce water.  Be
consistent.
 I found that 2 methods work fine: (a) distribute the zinc shavings evenly
horizontally along the length of the Pyrex tube and place the tube
horizontally into an oven that reproducibly heats to 500 deg C. Caution:
Ovens with no forced convection may have a large thermal gradient from top
to bottom.  Check with a thermocouple. (b) If you prefer to stick your
Pyrex tubes into a heating block in a vertical fashion, let the top stick
out a few cm so that thermal convection causes mixing and quantitative
reaction.  In any case, you should do the water reduction on the same day
when you actually do the mass-spectrometry.

In my experience, one can expect a precision within plus minus one per mil
if all is done carefully in a standardized fashion.  Frank, I deeply
sympathize with you and truly wished that I could be of more help, but I
would need more information about your specific procedures.
                          Best regards,    Arndt Schimmelmann

********************
Biogeochemical Laboratories
Indiana University
Department of Geological Sciences
Bloomington, IN 47405-1403
U.S.A.
ph 812-855-0154 (to order zinc)
ph 812-855-7645 (to ask technical questions)
fax 812-855-7961
********************