Bill:

We are currently running a delta plus which was installed right at
two years ago.  We have the Carlo-Erba NC 2500 elemental analyzer
which we use for carbon and nitrogen work.  We have always used ultra
high purity helium and have had no problems.  We use the helium as it
comes from the cylinder without any further treatment.  Our ultra
high purity helium has a purity of 99.999% and is certified by the
supplier.  Cost is $106.00 for the "J" size cylinder.  I would
suggest that you try another supplier if that is possible.  It
certainly sounds like you had a contaminated helium supply.  Good
Luck.

Bill Griffis

*******************************************************
William L. Griffis
Chemist
Integrated Stable Isotope Research Facility (ISIRF)
Terrestrial Plant Ecology Branch
Western Ecology Division
National Health and Environmental Effects
Research Laboratory
US Environmental Protection Agency
200 SW 35th Street
Corvallis, Oregon 97333
Telephone:  541-754-4761   FAX:  541-754-4799
E-mail:  [log in to unmask]
*******************************************************

>>> Bill Showers <[log in to unmask]> - 8/4/99 2:10 PM >>>
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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