Isotope Geochemists,

I recently started running CO & H2 in a MAT Delta+ xls in addition to N2
and CO2 and started to have serious tailing problems on all my N2 sample
peaks.

I changed the oxidation and reduction columns several times, changed the
water trap and then went to a larger water trap in the EA, baked out the GC
column two or three times, and finally pulled the source on the IRMS and
cleaned it and changed the filament, all without much success in
eliminating the tailing problem.  Another symptom was that the 28, 29 &
especially the 30 backgrounds would climb over the day, none of these
problems occurred when the IRMS was brand new (last year).

Finally I went to a research grade He carrier gas tank.  The IRMS
backgrounds stabilized and now are dropping, and the tailing problems are
decreasing every day.  I previously had been using UHP grade He for the
past 11 months as recommended by the manufacturer.

My question to the group is do other labs find it necessary to use research
grade He for CF-IRMS?  This is a bit pricey as a Research grade He tank
cost 3x to 4x what a UHP He tank costs.  Or do other groups employ traps to
clean up the UHP He, and if so what types of gas scrubbing traps do you
use?  This may be a simple problem to fix given other folks experiences,
but it has been a real headache for us over the past 4 months.  Are we just
getting contaminated UHP He from our supplier?

Thanks for the advice,

Bill Showers


----------------------------------------------------------
William J. Showers
Dept. of Marine, Earth, & Atm Sciences
North Carolina State University

(919) 515-7143 - voice
(919) 515-7802 - fax
[log in to unmask] - email

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