Paul,

When we run HD of C1-C5 on our GC/TC + MAT253 the time shifts are well
within 0.5 sec.
Are you using an exeptionally long column?

Rob Kreulen


Rob Kreulen
ISOLAB B.V.
1e Tieflaarsestraat 23
4182 PC Neerijnen, The Netherlands
tel. +31 (0)345 620 120
fax +31 (0)345 620 126


----- Original Message -----
From: "Paul Eby" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, November 17, 2004 7:01 PM
Subject: HD by GC-IRMS


> 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
>