Ed

Again, your comments on Ni oxidation are rather illuminating.

Our CO2 laser system measures isotopes on O2 gas, and frustratingly, we
have also been experiencing large fractionations with very small samples.
The 'oxidation' effect seems a likely explanation which we will be checking
out asap. I just hope that Au coating of the entire holder will eliminate
these fractionations in our system as well!

Is gold coating a long term solution? Can the holes be cleaned without
damaging the Au coat? Does any one go as far as using a solid gold tray? Is
SS316 a better (and cheaper) alternative to Ni?

What is the evidence for the 40oC oxidation threshold? This has
implications on the drying and storage of Ni trays in ovens or dessicators.

It is worth pointing out that the 'pressure effect' we described in Mattey
and Macpherson is an entirely different effect.... more a 'feature' of our
older system which uses a NdYAG laser and converts O2 to CO2 over hot
graphite. Here 18O *and* 13C vary as a function of samples size (or O2
pressure) for sub mg sized samples, and is largely a result of the O2-CO2
conversion process. This has been observed in some other systems but is by
no means prevalent. The 'oxidation' effect may be more universal but I
wonder if it is a more acute problem with CO2 lasers than with lasers
operating at shorter wavelengths?

Regards

Dave

Dave Mattey

Geology Department, Royal Holloway College, University of London, Egham
Hill, Egham, Surrey TW20 0EX

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