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From:
Bill Showers <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Wed, 24 May 2006 15:21:11 -0400
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Willi, Bruno, et al.,

The size non-linearity of CO is what caused us problems when we 
started using the TCEA several years ago.  We worked with Hairigh 
Avak at Bremen who helped us with this technical issue. Basically 
with liquids that are injected with an autosampler and a Hamilton 
micro-syringe you can inject about the same size sample every time 
into the TCEA and size linearity is not much of an issue between 
samples and standards. But with solids you can not weigh out exactly 
the same amount of oxygen in every sample, so your CO peaks from 
solid samples will encompass a range of sizes.

To get size linearity with CO, Hairigh had us basically detune the 
Delta to achieve maximum linearity.  This is best accomplished on our 
system by backing off on the extraction potential and then tuning 
everything else back up.  Maximum sensitivity for us is ~5.8 
extraction (out of 10), maximum linearity for CO is ~5.05 out of 10 
on the extraction setting.  We detune for for CO and O2, and have 
<0.1 per mil change between ~1-6 volts.

With a new filament and a cleaned source we can get good linearity 
for CO between 0.5 to 7 volts, but all solid samples need to be 
within this range to get good results. We then use tripicates of  3-4 
known isotopic standards that encompass the isotopic range of the 
samples we are running to calibrate each run for  the differences in 
slope for the isotopic response changes (with CF there is not a 1:1 
isotopic relationship between accepted isotopic values and measured 
values for standards due to instrument effects.  This linear 
correction then needs to be applied to the samples for each run).  I 
am not sure how you would correct for size and isotopic 
non-linearities in the same sample run.  I guess you would have to 
run low, medium and high isotopic standards at small, medium and 
large sizes and then have a multi-variate regression correction?

Detuning the extraction potential does decrease sensitivity, so there 
is a tradeoff.  Then there is the issue of what standards to use and 
what is the accepted isotopic value of these international 
standards.  Your corrected sample values can change depending upon 
what values you use to construct your linear regression (defined vs observed)

Since you can not run international standards every day in 
triplicate, you have to develop internal lab standards which are 
usually some sort of analytical grade reagent.  For CO we like sugars 
since they pyrolize so nicely, but they also have a limited isotopic 
range.  BaSO4s have low O-18 values and sucrose tends to be on the 
high side, and then there are plenty of compounds that are in the 
middle.  Soda nitre fertlizers from the Atacoma desert have very high 
O18 values because of their association with atmospheric deposition 
and you can order 5-20 lbs of that on line, enough for several 
careers.  Some folks have re-equilibrated reagent grade chemicals 
with waters of different O-18 compositions to get a wide range of 
isotopic compositions of the one particular compound they are 
running.  You still have to calibrate that with know standards.  But 
that is another discussion.  By detuning for size linearity and using 
a wide isotopic range of standards that bracket the sample isotopic 
values, we get consistent values run to run.  But we measure size 
linearity every day and throw out any sample values that fall out of 
that defined range.

Best,
Bill

At 02:03 PM 5/24/2006, you wrote:
>  Natalia, another thought...
>
>
>Ohlsson and Wallmark published a paper that presents an algorithm to 
>mitigate the mass effect on isotope ratio analyses.  That is, the 
>ratio changes as a function of material combusted-- which is why we 
>are fairly careful about weighing standards precisely. In a previous 
>life, I modified their algorithm because we found that isotope ratio 
>dropped as a function of Total N or C combusted.  It dropped in a 
>manner that could be approximated by a first order decay curve.  We 
>never adopted this in the lab because it requires analysis of 
>standards with a range of masses bracketing the masses of the 
>samples within every run.  However, trials showed that this improved 
>precision in analysis of standards by as much as an order of 
>magnitude.  If it is important for you to run samples of varying 
>sizes you might give this method a try.
>
>
>
>Reference    Ohlsson, K.E., and P.H. Wallmark. (1999) Novel 
>calibration with correction for drift and non-linear response for 
>continuous flow isotope ratio mass spectrometry applied to the 
>determination of (delta notation)15N, total nitrogen, (delta 
>notation)13C and total carbon in biological material.
>The Analyst 124:571-577.
>
>Good luck
>
>John
>
>John B. Cliff, Ph.D.
>Research Scientist
>Advanced Radioanalytical Chemistry Group
>National Security Division
>Pacific Northwest National Laboratory
>Richland,  WA   U.S.A.
>(509) 373-9003, fax (509) 376-5021
>([log in to unmask])
>
>
>
>
>-----Original Message-----
>From: Stable Isotope Geochemistry [mailto:[log in to unmask]] 
>On Behalf Of Dr. Bruno Glaser
>Sent: Tuesday, May 23, 2006 11:00 AM
>To: [log in to unmask]
>Subject: Re: [ISOGEOCHEM] calibration TC/EA for O2
>
>Dear Tiziano,
>
>amount dependence might also be due to the split dilution, 
>especially from the Conlfo II such effects are known and can only be 
>avoided when using samples with the same amount of analyte element 
>or in other words using samples yielding e.g. around 2 V signal intensity.
>
>Kind regards
>
>Bruno
>
>-----Original Message-----
>From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
>Behalf Of Tiziano Boschetti
>Sent: Dienstag, 23. Mai 2006 17:31
>To: [log in to unmask]
>Subject: Re: [ISOGEOCHEM] calibration TC/EA for O2
>
>
>Dear Willi and Natalia,
>during the analysis of O-isotope I've noted an increase of the 
>background mass 30 (14N16O, 12C18O), usually the twice of 28. 
>Probably this could be due to the low purity of CO-reference gas 
>(4.7 grade), in fact I've also noted an increase of the 32 mass 
>signal (impurity of oxygen in CO?).
>Low Purity of reference CO-gas could be resolved with a point-of-use 
>gas filtration or gas purifier ( 
>http://www.alltechweb.com/productinfo/technical/datasheets/4001u.pdf
>http://www.pall.com/microe_26234.asp?level1=0 ) What do you think?
>Ciao!
>*****
>Tiziano Boschetti
>University of Parma, Italy
>
>*****
>----- Original Message -----
>From: Willi A. Brand
>To: [log in to unmask]
>Sent: Tuesday, May 23, 2006 2:57 PM
>Subject: Re: [ISOGEOCHEM] calibration TC/EA for O2
>
>
>Dear Natalia,
>I have two comments to your question.
>1. With a perfectly tuned ion source (perfect linearity for CO2 and N2) we
>very consistently see a dependence of the 30/28 ratio upon signal size. It
>is surprizingly constant at +0.3 per mill / Volt (1V = 3 nA 28). We
>routinely correct for this, but we do not know the origin. One possibility
>would be formation of an extra ion current on m/z 30 much like the H3+. If
>small amounts of N2 are present in the reference gas, formation of NO can
>occur on the filament, giving rise to NO+ (m/z 30) during reference
>measurement. In this case, the correction would have to be applied to the
>reference gas only. However, we see the same dependence upon signal size on
>the sample peaks.
>2. We generally run our reduction at 1400$B!k(JC (tube-in-tube 
>arrangement with
>the He flow reversal installed) and we adjust the sample size to about 1 mg
>Oxygen to generate a larger ion current. We found this necessary in order to
>be somewhat less dependent upon background and memory effects.
>Regards   Willi
>
>Sevastyanov wrote:
>
>Hello!
>I am a post-graduate student of V.I. Vernadsky Institute of Geochemistry and
>Analytical Chemistry, Russian Academy of Science. I would like to ask some
>questions about our DeltaptusXP mass spectrometer.
>Two organic substances were analyzed for their $B&D(J18$B'0(J values using our
>pyrolysis EA-IRMS system: cellulose and benzoic acid (the reaction furnace
>is heated to 13500C, silver capsules were used). A various samples amount
>were examined, and our results revealed that there is a dependence of the
>$B&D(J18$B'0(J values upon peak amplitude. These changes are not 
>great when peak
>amplitudes exceed 2 V but increase rapidly when peak amplitudes become less
>than 2 V. Thus, in the range from 2 to 1 V $B&D(J18$B'0(J values 
>become about 5 $B"s(J
>smaller. We believe such phenomenon is not normal and we would like to know
>how this situation may be improved and where this phenomenon comes from?
>Where linearity area of our system ends? Is that true, that linearity bounds
>by 2V? If this is so, what is the nature of such phenomenon?
>
>
>Best regards,
>
>
>Natalia Sokolova
>post-graduate student
>V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry,
>Russian Academy of Science
>19, Kosigin st., Moscow, 119991, Russia
>
>
>
>
>
>--
>.....................................................................
>Willi A. Brand, Stable Isotope Laboratory      [log in to unmask]
>Max-Planck-Institute for Biogeochemistry (Beutenberg Campus)
>Hans-Knoell-Str. 10, 07745 Jena, Germany      Tel: +49-3641-576400
>P.O.Box 100164,      07701 Jena, Germany      Fax: +49-3641-577400
>http://www.bgc-jena.mpg.de/
>http://www.bgc-jena.mpg.de/service/iso_gas_lab/
>.....................................................................
>GASIR 2005 in Jena:
>http://www.bgc-jena.mpg.de/service/iso_gas_lab/gasir2005/index.shtml
>.....................................................................

William J. Showers
Dept of Marine, Earth & Atm Sciences
Box 8208
North Carolina State University
Raleigh NC 27695

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