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Thu, 19 Jun 1997 07:12:42 -1000 |
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>Dear Isogeochemists
>
>We are interested in analyzing DIC from groundwaters by acidification with
>H3PO4. The water samples are commonly poisoned with HgCl2 for this method.
>Would anyone know if there is an alternative to the HgCl2?
>
Fellow Isotopists:
Just a note of warning. We have been using a saturated solution of HgCl2
to preserve samples of oligotrophic surface water (1 mL HgCl2 in 240 mL SW)
in a study of methane cycling in the surface ocean. Typically, we analyze
the carbon isotopic composition of CH4 within a few months of collection
(see methods in Popp et al, 1995 Anal Chem 67:405-411 and Sansone et al,
1997 Anal Chem 69:40-44). We recently analyzed a duplicate from cruise
samples collected and analyzed about 1 year ago. Some samples,
particularly those collected near the particle maximum, had much higher
[CH4] and lower d13C-CH4. A colleague incubated those samples in 14C and
confirmed bacterial activity, albiet low activity! I believe similar
examples exist in the literature. We are testing other preservation
methods.
We suspect that HgCl2 preservation is okay for short-term storage (a few to
6 months) and is probably okay for preservation of seawater samples for DIC
analyses, where the DIC pool is large. We have no experience with fresh
water. These results are a mixed blessing - HgCl2 may not stop *all*
bacterial activity but it is a good excuse to urge graduate students to
analyze the samples QUICKLY.
Brian
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Brian N. Popp, Associate Professor
University of Hawaii, Department of Geology and Geophysics
2525 Correa Road, Honolulu, Hawaii 96822
(808) 956-6206 Fax: (808) 956-7112
WWW Site: http://www.soest.hawaii.edu/GG/FACULTY/bpopp.html
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