Wendy,

You may wish to try lowering your furnace temperature, in addition to
adding a trickle of O2 as Paul suggests.  I found the best d13C precision
for trihalomethanes using a CuO-based combustion furnace maintained at 850C
with a trickle of O2.  Perhaps you require 940C to get better combustion of
the alkanes.  But chloroform, bromoform, etc., higher temperatures resulted
in much poorer precision, a greater difference between offline and online
results, faster loss of O2 from CuO, and a significantly shorter quartz
combustion tube life (due to reaction of CuO with the quartz).  I
recondition the CuO overnight at 500C with great results.

Good luck.

Bryan
--------------------------------------------------
Dr. Bryan E. Bemis
U.S. Geological Survey
345 Middlefield Road, MS 434
Menlo Park, CA 94025
(650) 329-5603  Office
(650) 329-5590  Fax
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                      "Zhang,Wendy
                      [Ontario]"               To:       [log in to unmask]
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                      C.CA>                    Subject:  [ISOGEOCHEM] Combustion Furnace
                      Sent by: Stable
                      Isotope
                      Geochemistry
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                      05/01/2003 10:38
                      AM
                      Please respond to
                      Stable Isotope
                      Geochemistry






Hello everyone,

        I have run the stable carbon isotope composition of n-alkanes
standards (20ng/ul;n-C17 to n-C30)in GC-C-IRMS. I am using a Finnigan MAT
combustion furnace containing Cu, Ni, and Pt wires and maintained at a
temperature of 940 degree. I have some trouble with this kind of combustion
reactor since the offset between on-line and off-line was ~-0.5 to -1
permil
and the negative shift continued to increase gradually after 2 or 3 runs. I
was wondering if there is anyway to improve the furnace performace.(I
reoxidized the furnace @940 for ~20' everyday).
Any input would be greatly appreciated.
Thanks in advance,

Wendy Zhang


Air Quality Research Branch
Environment Canada
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Canada
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