The cautionary words of Clementi are very important. A plasma
discharge during RF heating can cause very large H-isotope fractionations
(up to 100 per mil or so, if I remember correctly, though often less). It
MUST be avoided. If you have a plasma problem and cannot change the
frequency of your RF generator, see Sheppard & Taylor (1974), Econ. Geol.,
vol. 69, p. 928. They discuss this problem and present a solution for
completely suppressing the discharge, thanks to a hint given to us by Dick
Armstrong in the library at Caltech where Hugh and I were discussing this
problem.
Simon Sheppard