Dear Clemente-

As many of us have found out, sulfate oxygen is a tough technique.  You've
received many good suggestions already.  While you didn't specify exactly,
I assume you are converting CO to CO2 using Pt plates and a high voltage,
low amperage supply.

The keys to good results in this tedious and difficult technique are:
(1)  pure BaSO4- no occluded water or carbonate or other anions, no solid
contaminant,
(2)  slow gradual careful heating of the finely ground BaSO4-C mix,
(3)  complete conversion of CO to CO2 and quantitative recombination with
the rest of the CO2,
(4)  no exchange of CO & CO2 with the quartz vessel (this is especially
important if the vessel is heated above room temperature), and
(5)  an extremely tight leak-free vacuum system (If any N2 leaks into the
system it may be converted to N2O, which will produce fractionation.  If
leakage is consistent, d18O results might also be consistent, but wrong.)

In our USGS stable isotope labs here in Denver, Bob Rye and Craig Johnson
are using the fluorination method originally developed by JIm Oneil for
sulfate oxygen.  It's much easier.

Pat Shanks


**********************************************************

W. C. Pat Shanks III
U.S. Geological Survey
973 Denver Federal Center
Denver, CO 80225

phone: 303.236.2497
fax: 303.236.3200
email: [log in to unmask]