thank you very much for your suggestions! It seems like a dirty
autosampler was the reason for the described pattern of alternating
accurate and non-accurate deltaD values. When cleaning the
autosampler, a trail of dirt along the "outer lane" of the autosampler
became visible. Since also a few samples of deuterated material had
been measured before the problem evolved, it is likely that traces of
deuterated material from a leaky capsule entered the reactor every
time a sample from the "outer lane" was dropped. The samples from the
"inner lane" were not affected.
It is always the obvious stuff that drives me crazy... :-)
Thanks a lot and best regards,
Zitat von Travis Horton <[log in to unmask]>:
> Hi Marc,
> Are you peak centering at the start of each sample?
> Have you run an IAEA-CH-7 with a long (i.e. extended) sample
> analysis time to see if there are any small peaks coming through
> well after the H2?
> Gas produced from one sample may carry over to the next sample in
> some systematically off-set way....perhaps arriving during peak
> centering, perhaps arriving during your reference gas injections,
> perhaps arriving at some other less consequential 'down time'.
> Timing of arrival for ALL gas phases coming off your sample is
> important to consider.....I have seen very small 'blips' in zoomed
> in spectra that won't have any effect - until they coincide with an
> important event - peak centering, reference gas peak start time,
> targeted sample gas peak. It is very easy to miss these small
> 'blip' peaks without zooming in your spectra. Just an idea for
> something easy to check for and modify in the sequence timing. My
> experience with this sort of thing has mostly been with a GasBench,
> but given the standard carrier gas + GC column set up, i don't see
> why similar things might be possible with other devices/set-ups.
> With Isodat you can always run a sample manually and use Instrument
> Control (running a scan on a specific magnet/gas configuration) to
> see what is coming off when relative to your sample inject/drop
> time....don't know what you're running but perhaps a way to have a
> look around for ghost peaks and timing related issues??
> Hope this helps,
> From: Stable Isotope Geochemistry [[log in to unmask]] on
> behalf of Marc Ruppenthal [[log in to unmask]]
> Sent: Sunday, March 03, 2013 2:10 AM
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] strange deltaD measurement results
> Dear isogeochemists,
> I am seeing very strange behavior of our PyroCube coupled to an
> isoprime IRMS:
> Systematically, EVERY SECOND deltaD value I measure for IAEA-CH-7 (at
> the moment used as a test for operational readiness of the machine) is
> accurate whereas one value in between two accurate ones is always off,
> sometimes by ten permil, sometimes by 20 or 30 permil (too positive).
> I am using a ceramics reactor filled with glassy C chips operated at
> 1360 °C (has always worked in the past). I have already checked the
> usual suspects for malfunction (temperature, flow, leaks) but could
> not find anything.
> Has anybody seen something like this before or does anybody have any
> suggestions what could cause such a pattern? I would be very happy to
> receive some suggestions because I have a hard time figuring out how
> this can be...it is weird...
> Thank you very much in advance for your help and effort!!!
> Best regards,
> Dipl.-Geograph Marc Ruppenthal
> AG Prof. Dr. Yvonne Oelmann
> Eberhard Karls Universitaet Tuebingen
> Forschungsbereich Geographie
> Ruemelinstraße 19-23
> 72070 Tuebingen
> Raum: W505
> Tel.: +49 (0) 7071 29 77527
> Mobil: +49 (0) 176 96810834
> E-Mail: [log in to unmask]
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Dipl.-Geograph Marc Ruppenthal
AG Prof. Dr. Yvonne Oelmann
Eberhard Karls Universitaet Tuebingen
Tel.: +49 (0) 7071 29 77527
Mobil: +49 (0) 176 96810834
E-Mail: [log in to unmask]