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Stable Isotope Geochemistry

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Subject:
From:
Paul Eby <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Wed, 17 Nov 2004 10:01:53 -0800
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List members,

Our lab has been working on hydrogen isotope measurements by GC.
Specifically, we have been measuring hydrocarbons anywhere from methane up
to C10, on a Delta Plus XL.

The problem is that extreme time shifts can be found between the mass 2 and
mass 3 signals: upwards of 2 seconds. This is using the same columns and
injection methods that we would for d13C, for which the time shift is
almost negligible.

I've been informed by Finnigan though that Isodat isn't capable of handling
time shifts that large. In order to stop the software from confusing
adjacent peaks as a time shift, they have imposed an artificial limit on
the time shift - it can only be as large as the integration time (default
is 0.25 seconds).

Finnigan is apparently working on a fix for this, but in the meantime I am
looking for other solutions. Increasing the integration time, for instance,
has limited usefulness as it also changes the integration and at 2 seconds
misses most of the peak.

Have others experienced this large time shift? Is it simply related to
chromatographic efficiency, or are there other factors that are involved?
Why do these large shifts appear when measuring hydrogen but not carbon?

Paul Eby

[log in to unmask]
Biogeochemistry Lab (E-Hut, room 102)
School of Earth and Ocean Science
University of Victoria

PO Box 3055
Victoria, B.C.
Canada
V8W 3P6

phone: 250-721-6183
fax:   250-472-4620


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