Dear Albert,

We tried Ar as a carrier gas in a 252 and found that it sputters metal from
the lenses, which deposits on the ceramic insulators causing an increasing
leakage current.  This happens in a matter of days, and thus is not a
solution to the problem as it will wreck your optics.  The issue is that Ar
is mass 40 and thus is much more damaging to metal than He.  See Tobias,
Anal Chem 67, 2486 (1995).  Neon, a lower mass inert gas, is too expensive
to use as a carrier gas.  In our hands, a 2020 did effectively eliminate
the mass 4 tail.  The "special method" used by the other guys is an energy
filter similar to those on double focusing magnet sector molecular MS.  I
don't have much experience with that approach and can't comment.  Another
issue is the overall H3+ factor, which should be minimal to obtain
reasonable CF H results.  My guess is that is probably a more important
factor than any difference in 4-He tailing.

Tom Brenna


At 09:30 AM 3/15/2005 +1300, you wrote:
>With compound specific isotope analysis in mind, I would like to know from
>anybody who has tried CF-mode D/H measurements with a Geo2020 in a
>GC-Pyr-IRMS configuration, where helium is the carrier gas and thus ends
>up in the ion source. The detail I'd like to hear this community's opinion
>on whether the interference from the "low-energy" tail of the mass-4 He+
>peak underneath the mass-3 DH+ peak is very system dependent. Both the GVI
>Micromass and Thermo Finnigan systems have a special method in place to
>rid off this tail. Is it possible that the beam optics of the core Geo2020
>system effectively separates any remnant of the 4He+ peak from the DH+
>peak and therefore does not need an additional trick ? Did anybody try to
>replace the He by another noble gas to carry analyte through the various
>capillaries and plumbing ? I have to make a judgment call on an upgrade
>plan, and thus would greatly appreciate your thoughts.