David,

The description of your results sounds more like you may have contamination
from water.  Clearly you have a source of oxygen to explain, and this is not
likely
to come from nickel.  Having removed any traces of machine oil, etc. (you
will now
know that you should clean all parts for the machinists before they do the
welding),
passivation with BrF5 is the way to go, and this may take some time.  You
should plot
d18O vs. yield using a trustworthy standard to estimate the composition of
the contaminant,
and test whether it is, indeed, water.  Bromine deposits are notoriously
hygroscopic (as
is nickel fluoride), and may be found at the cool end of the vessel -- so
don't overlook
these.  Good Luck.

Bruce

> ----------
> From:         David L. Dettman
> Reply To:     Stable Isotope Geochemistry
> Sent:         Monday, April 6, 1998 12:33 PM
> To:   [log in to unmask]
> Subject:      info on nickel passivization
>
> Hello all,
>   I have been having problems with some of our new nickel tubes being used
> in a miniaturized conventional fluorination line for small (3mg) Qz
> samples. We have contamination problems, apparently from the machining and
> welding and I would like to know if users have followed a particular
> clean-up procedure. One that was suggested to me is 'nickel passivization'
> -- apparently a series of nitric acid treatments. This supposedly removes
> surface gunk and renders surfaces even more inert than machined nickel.
> Any
> observations about whether this is a good idea or not?
>   It seemed to me that the BrF5 would do a pretty good cleanup job after a
> standard solvent series, but I think the welding was done with a good deal
> of machine oil on the tube bore at the time. We now get high (>100%)
> yields
> in about 1/3 of our vessels and the d18-O values are too negative.
>
>   Many thanks for any suggestions.
>
> - David
>
>
>                David Dettman          [log in to unmask]
>                Dept. of Geosciences   ph. (520) 621-4618
>                Univ. of Arizona       FAX (520) 621-2672
>                Gould-Simpson Bldg.
>                Tucson, AZ 85721
>