Stable Isotope Geochemistry


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Paul Brooks <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Tue, 24 Feb 2004 09:12:02 -0800
text/plain (54 lines)

I now back flush the GC column after every sample to get rid of these
strange trace interfering peaks for CO analysis.  I use a 2m by what looks
to be 6 mm OD molecular sieve 5A column from Costech Analytical
Technologies.   It is a rather large diameter but this diameter has to be
used for the flow rate of 100 mL min-1 I am using.  It separates N2 and CO
fine at 64 degrees C GC oven temperature.  Although the CO peak tails more
than I would like, the precision when injecting CO through the GC column is
plus or minus 0.06 delta units, which is not significantly different than I
get with ref gas injection, so apparently the trailing does not affect
precision significantly.


At 02:01 PM 2/24/04 +1100, you wrote:
>Thanks Toti
>Could I have a copy of the application notes?
>Now there's a good question about the CO2!
>Yes, it does come off, as rather a blurred peak at 1000 seconds under our
>conditions.  So, if you do a 600 second run, your CO2 comes off RIGHT in the
>tail of your CO.  And although the amplitude is very small after scrubbing,
>the dissociation in the source causes some wild values.  But even without
>the CO2 -> CO+O fractionation, the gas comes from the PREVIOUS sample.
>So you have to scrub VERY well or do long runs.  And the CO2 doesn't have
>enough amplitude (from our set up) to show on anything apart from the ratio
>traces.  And then only for about 1 in 20 samples ...

Paul D. Brooks,
Center for Stable Isotope Biogeochemistry,
Dept. Integrative Biology MC3140,
3060 Valley Life Sciences Building,
UC Berkeley, Ca. 94720-3140.

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