Stable Isotope Geochemistry


Options: Use Forum View

Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Hilary Stuart-Williams <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Fri, 11 Apr 2003 12:50:24 +1000
text/plain (40 lines)
We have built a furnace for oxygen and hydrogen isotope analyses by
pyrolysis and continuous-flow IRMS.  The furnace consists of a ceramic outer
tube, a glassy carbon tube inside that (held away from the ceramic walls)
and a pack in the glassy carbon tube.  The ceramic outer tube (the casing)
is Degussit AL 23 by Friatec.

The system works fine at 1100 degrees and OK, but with a higher background,
at 1200 degrees.  By 1400 degrees the background has become intolerably
high, even when the system appears to be leak-free.  This background is
present even when samples are not being dropped.  Examination of the glassy
carbon inner tube after runs shows that the inside of the tube is shiny and
fresh, but the outside of the carbon tube is eroded and has what appear to
be fine tin crystals on it, probably from the sample cups.

I have come to the conclusion that the high mass 28 background is due to
atmosphere diffusing through the outer ceramic casing in the high
temperature zone, with the oxygen reacting with the glassy carbon tube and
forming CO.  It also appears that tin diffuses through the glassy carbon
tube, although this may not matter.  The fact that the inside of the glassy
carbon tube is not eroded shows that the carrier gas is probably not the

Could I ask the list members their thoughts on this?  In particular, what
are other people using as an outer casing?




Hilary Stuart-Williams PhD.
Research Officer in Stable Isotopes
Environmental Biology Group
Research School of Biological Sciences
Australian National University
Canberra, ACT, Australia
(02) 6125 2099
Personal home: