Hello Dave and all users of Indiana Zinc,
Our group is responsible for the quality of Indiana Zinc. We are sure that
properly sealed zinc has an unlimited shelf life under vacuum. Indiana Zinc
will gradually deteriorate when in contact with moist air. Attila Demeny had
previously noted that zinc has a so-called "amount effect" (see earlier
postings by Attila on ISOGEOCHEM), which upon further testing in our lab
revealed itself as a hydrogen blank in zinc. The isotopic significance for
your D/H ratios depends on the weight ratio of zinc versus water during the
reduction step. Attila and I both noticed that some batches of zinc show less
of the "amount effect" than others. Therefore one should (a) keep the ratio of
zinc versus water constant, (b) avoid mixing batches, and (c) always run
internal water isotopic standards with the unknown water samples. We also
recommend that all glassware is pre-baked at 500 deg C before loading zinc,
because a hydrogen-containing contaminating film on the interior glass surface
can act as another source for a hydrogen blank. After loading the zinc
(typically 100 mg) and *before* admitting water (typically 2 microliter) to
the reaction ampule, the zinc should be outgassed for 5 minutes under a good
vacuum at 350 deg C, which in our experience reduces the hydrogen blank. A
weak metallic mirror should appear on the glass walls above the heated zone.
All our batches of zinc are rigorously tested (for the past 2 years also for
the severity of the "amount effect") in our lab before making them available to
the research community.
For outgassing of zinc under vacuum, we built a small, lightweight heating
sleeve (= coil of heating tape, with additional external insulation; an
integrated thermocouple permits monitoring of core temperature) that slips from
underneath over the end of the zinc-containing Pyrex ampule. The sleeve is fed
a constant voltage from a Variac transformer and always stays at a core
temperature of about 350 deg C.
The reaction of water with zinc occurs quantitatively at 500 deg C over 30
minutes, as long as one of the following options is used: (a) spread the zinc
over the lentire length of the glass ampule and place the ampule horizonatlly
into a 500 deg C oven. (b) with the zinc at the bottom, place the ampule
vertically into a heating block that is at 500 deg C. The upper part of the
ampule, which sticks out of the heating block, should be at about 300 deg C.
Convection will cause rapid mixing and will result in complete conversion of
water hydrogen to elemental hydrogen.
Indiana Zinc cannnot be regenerated after use.
For further details, please see the zinc section in mt home page (URL given in
my address below).
Best wishes, Arndt Schimmelmann
Dave Mattey wrote:
> Dear all
>
> We have been running D/H for the first time in several years using a 5 year
> old batch of Indiana zinc that has been stored in an evacuated flask. H
> conversion yields seem fine but D/H ratios correlate with sample size (20
> permil shift from 0.5 to 5 microlitres). All the obvious remedies have now
> been tried and I am forced into suspecting the zinc. Does anyone had a
> similar experienc using old zinc? Can/should it be pretreated/regenerated
> in any way after languishing for so long?
>
> Thanks and best wishes
>
> Dave
>
> Dave Mattey
>
> Geology Dept, Royal Holloway College, University of London, Egham Hill,
> Egham, Surrey, TW20 0EX
>
> Tel: 01784 443587 Office
> 443582 Secretary
> 443637 Prism lab
> 443629 Optima lab
>
> 471780 FAX
--
Arndt Schimmelmann, Ph.D.
Senior Scientist
Department of Geological Sciences
Biogeochemical Laboratories
Geology 129, Indiana University
1001 East 10th Street
Bloomington, IN 47405-1405
Ph (812) 855-7645
home (812) 339-3708
FAX (812) 855-7961
[log in to unmask]
http://www.indiana.edu/~geosci/people/faculty/schimmelmann.html
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