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You should not have to use research grade He for CF-IRMS.  I have seen bad
batches of UHP He before, but that tailing is related to impure carrier
gas is quite odd.  Perhaps your GC column was affected by impurities in
your carrier gas.  You could bake it out to test this.

To guarantee pure He, we use Alltech "All-pure" He purifier, which is a
proprietary zeolite material, in line with UHP He.  This purifier
supposedly converts even 99.99% He to research grade (99.9999%).

Hope this helps,

John
******************************************************************************

John B. Miller
                                        also with,

Dept. of Chemistry and INSTAAR          Carbon Cycle Group
University of Colorado                  NOAA/CMDL, R/E/CG1
Boulder, CO, 80309-0450                 325 Broadway
303-492-7739 (CU office)                Boulder, CO, 80301
303-492-5495 (lab)                      303-497-7739 (NOAA office)
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On Wed, 4 Aug 1999, Bill Showers wrote:

> 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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