Dear Isogeochem readers,

We are using molecular seive 5A to freeze and transfer oxygen gas (O2) from
our extraction lines to our mass spectrometer for isotopic analysis.  We
measure masses 32, 33, and 34.  I am writing to ask about other people's
experiences with these or similar techniques.  I have listed specific
questions below.

1)  What types of sieve materials are being used?  Bead size?  For what
gases?  For what reasons?  (I have recently started experimenting with a
variety of molecular sieve materials (also silica gell) with variable
success.  My naive question is aimed at reducing my amount of reinventing
old techniques.)

2)  What types of pretreatment/outgasing procedures are being used to
prepare the sieve materials?  What temperature conditions, vacuum
conditions, and outgasing times do other people use?  (Mass interferences
resulting from contamination render my 33/32 and to a lesser extent my
34/32 unrepresentative of the oxygen that I am analyzing.  I have
successfully eliminated this source of contamination with a
high-temperature outgasing step at a vacuum of low 10e-5 Torr.)

3)  What freezing times do people use to freeze gases onto the sieves? 
What ultimate pressure of gas is maintained above the frozen sieve
material?

4)  What procedures do people use to thaw their sieves?  Thawing times? 
Thawing temperatures?  (I have noticed what appear to be systematic, small
(<~0.1 permil) fractionations for oxygen gas in contact with sieve material
relative to oxygen gas that is not in contact with sieve material.)  What
have people done to characterize these fractionations?  To minimize them?  

5)  Tyler Coplen and J.K. Bohlke suggested using liquid He, or an He
refrigerator.  Will anyone describe their own experiences using these
methods?

Thank you,

James Farquhar
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