Dear Anta,

did you check for water contamination of your enriched sample?

17O measurements are very sensitve to the presence of any H
atom in the ionisation chamber. H will form HO2 which has a
mass 33.

joel


A 20:01 06/01/2005 +0530, Antra a écrit :
>dear list_members,
>
>I am trying to measure Oxygen isotope ratios (d17O and d18O)
>in oxygen gas using Europa GEO 20-20 mass spectrometer. I see
>that GEO gives correct d18O at high enrichment but  does not
>give correct d17O. The details of the problem are as follows:
>
>We know that in O2 gas abundance of mass 32 is  99.52%
>mass 33 is 0.08% and mass 34 is 0.4%. The mass spectrometer
>gives two ratios 33/32(0.0008) and 34/32(0.004) in sample and
>reference gases and finally d33O2 and d34O2 correspond to d17O
>and d18O respectively.
>
>We noted that during measurements, the mass spectrometer gives
>the correct zero enrichment values when the same O2 gas is put
>in both reference and sample sides.
>
>It also gives correct delta value(d17O and d18O near zero)  if
>one aliquot of reference gas is measured as sample.
>
>However, when we put enriched O2 sample gas of known composition
>it gives correct d18O value, but d17O is around 10-15 permil more
>than what is expected, i.e.,measured d17O value-real d17O value~ 10
>to 15 permil.
>
>The background for three amplifiers are:
>amp 1(major 32 beam)=1.44 *e-09
>amp 2(minor 33 beam)=2.68 *e-12
>amp 3(minor 34 beam)=2.68 *e-12
>
>The backgrounds are subtracted during analysis.
>
>Typical signals during the measuremnts are:
>
>beam1 =1.6 *e-08, beam 2(mass 33)= 1.13*e-11, beam 3(mass 34)=7.13*e-11
>
>
>The mismatch in mass 33 is independent of signal strength.
>
>It is a big puzzle to me i.e., why it doesn't give correct
>delta 17O in high enrichment(around 100 per mil) region.
>I do not have other oxygen gas yet to test the mismatch in 20
>to 30 per mil level.
>
>I suspect it could be a nonlinearity problem with amplifier 2
>which corresponds to mass 33.
>
>Any suggestion would be highly appreciated.
>
>regards,
>antra.
>
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Ms.Antra
>Research Fellow
>Stable Isotope Laboratory
>Planetary and Geosciences Division
>Physical Research Laboratory
>Navrangpura
>Ahmedabad-308 009
>India
>
>Phone: Office:   91-079-6302129-4270
>        Residence:91-079-6300505
>        Fax:      91-079-6301502
>email: [log in to unmask]
>        [log in to unmask]
>URL:http://www.prl.ernet.in/~antra
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Joel Savarino
Laboratoire de Glaciologie et Géophysique de l'Environnement/CNRS/UJF
54 rue Moliere, BP 96
38 402 St Martin d'Hères Cedex
France
tel: + 33 476 82 42 51
fax: + 33 476 82 42 01
Email: [log in to unmask]
Web: http://www-lgge.obs.ujf-grenoble.fr
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