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 >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ************************************************************************* 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 **************************************************************************