We had very high water backgrounds, but these have gone way with
time...Our typical background on our 15N cup is only ~ 200mv compared
to 1.5 V early on... I think it just takes awhile for the block to
get it all out and the inlet valves to degas. Our performance is much
better with low backgrounds, but was pretty good as long as the
background was stable, which was a problem..
At 02:33 PM 3/2/2009, you wrote:
>Hi Paul,
>
>
>In our lab the standard set-up on all IRMS instruments including a
>Delta V plus (without a GC interfaced yet) comprises the use of
>99.999% He that is piped through stainless steel tubing, passing
>through a Supelco High Capacity Gas Purifier (the thingy that looks
>like an HPLC column stuck in a heater) followed by a Supelco OMI-2
>indicating purifier tube; nothing else.
>
>Speaking of the Delta V plus, would you happen to know why the
>performance of the Delta V plus would or should degrade if the
>carrier gas is too dry? I am told the V plus has a improved ion
>source and I am happy to believe that since even without
>differential pumping the improvement in sensitivity is phenomenal
>(and probably bringing this 3kV instrument close to 10kV
>instruments). However I am still not sure what to make of this
>high-water-background-is-good-for-you claim other than being very
>cynical and to assume this unusually high level of water background
>level seen by all Delta V plus users is being talked up into the
>prerequisite for good instrument performance. The words emperor and
>new clothes spring to mind.
>
>Whatever happened to H2O plus CO2 resulting in formation of HCO2+
>(m/z 45) in the source and, hence to isobaric interference with 13CO2 (m/z 45)?
>
>And why would one want to have a 1,800 mV H2O background when one
>wishes to analyse for 2H (or 18O)?
>
>In my opinion, whoever wrote the manual of my Delta plus XP had the
>right idea; on the subject of water background this manual clearly
>states that this should not exceed 300 mV as measured on the centre
>cup. Incidentally, with the above set-up the water background on our
>plus XP is typically <150 mV (while on the V plus we get 1,800 mV; go figure).
>
>Then again, perhaps I am turning into a dinosaur and should be put
>out to grass.
>
>
>Best,
>
>Wolfram
>
>
>****************************************
>"Facts do not cease to exist because they are ignored."
>
>Aldous Huxley, "Proper Studies", 1927
>****************************************
>Dr W Meier-Augenstein, CChem, FRSC
>Principal Scientist - Stable Isotopes
>Stable Isotope Laboratory
>SCRI
>Invergowrie
>Dundee, DD2 5DA
>United Kingdom
>Tel.: +44 (0)1382 560025
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>
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>University of Dundee
>http://www.lifesci.dundee.ac.uk/CAHId/
><http://www.lifesci.dundee.ac.uk/CAHId/>
>
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>________________________________
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>From: Stable Isotope Geochemistry on behalf of stableisotopes
>Sent: Mon 02/03/2009 18:58
>To: [log in to unmask]
>Subject: Re: [ISOGEOCHEM] moisture traps for GC carrier gas?
>
>
>
>Everyone,
>
>We are attempting to analyze D/H in hydrocarbons with a Thermo
>GC-pyrolysis set up interfaced to a Delta V. The Thermo Scientific
>super clean Gas Filter kit baseplate has broken and Thermo have
>recommended just using an inline filter part number 60180-808 as a
>replacement. Before we switch I would like to hear others
>recommendations. My understanding is that if we remove too much water
>vapor the Delta V performance will degrade. Anyone else got
>recommendations on what to use?
>
>Thanks in advance, Paul Brooks.
>
>
>--
>Mr. Paul D. Brooks,
>Dept. Integrative Biology MC3140,
>3060 Valley Life Sciences Building,
>UC Berkeley, 94720-3140.
>
>[log in to unmask]
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>phone (510)643-1748
>FAX (510)643-1749
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