Dear Steve and others,
A number of years ago we discovered that the Teflon stoppers in our hydrogen reaction vessels were out gassing (see Heaton, T.H.E., and Chenery, C.A.,
 1993.  Caution on the outgassing of polytetrafluorethylene valves used in the isotopic analysis of hydrogen.  Chem. Geol., 106, pp. 485-487.
).  The material used to make  valve stems and stoppers is made from compressed Teflon beads.  There is an inherent porosity which can lead to
trapping and later release of atmospheric (or other gas).   Not all Teflon is the same  there are different densities.  In addition to the hydrogen
stoppers I also looked at the potential for outgassing on some other vessels we had in the laboratory and  found that outgassing took place even with
valve stems that were not subjected to heating ( though not to the same extent).
Before you rush out and get new apparatus,  consider weather there is any contribution to your analytical results.  If there is the safest thing to do
is switch to glass, if not you might as well add the extra few minutes to your routine.
Good luck

Carolyn Chenery

NERC Isotope Geosciences Laboratory
Kingsley Dunham Centre
Keyworth, Nottingham UK NG12 5GG
Tel :   +44 (0)115 9363100 (Switchboard)
Tel :   +44 (0)115 9363406 (Direct Line)
FAX:  +44 (0)115 9363302
e-mail [log in to unmask]