Dear Bill,
I tried to find the article you mentioned, however, had some problems with
finding the reference (Anal Chem V74(5) has only pages of 900 or higher and
I can not find the A stuff). So please could you send me an electronic
reprint to [log in to unmask]
Many thanks
Bruno
********************************************
Dr. Bruno Glaser
Institute of Soil Science and Soil Geography
University of Bayreuth
D-95440 Bayreuth
Germany
Phone: ++49-921-552254
Fax: ++49-921-552246
Email: [log in to unmask]
Web: http://www.geo.uni-bayreuth.de/bodenkunde
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-----Ursprungliche Nachricht-----
Von: Bill Showers [SMTP:[log in to unmask]]
Gesendet am: Donnerstag, 23. Mai 2002 22:19
An: [log in to unmask]
Betreff: Re: [ISOGEOCHEM] 18-O of sulphate by TC/EA - IRMS
Neus,
We run BaSO4 and as well as AgPO4 and AgNO3 in the TC-EA for O-18 without
any of the types of problems that you report.
Please see our recent paper for standard calibrations in the TC-EA, we use
NBS 127, NBS 120C (phosphate rock prepared as AgPO4), and NIST 8542
(sucrose) as well as some internal standards. Sugars work really well. It
is important to run a wide range of O-18 standards. We did not have any
success running carbonates, there have been several papers that also report
problems with the pyrolysis of carbonates. If anyone is able to do this, I
would appreciate a copy of that paper. For more info see:
Showers W.J., Barrick, R., Genna, B. 2002 Isotopic Analysis of Dinosaur
Bones: New Techniques for Old Questions. Analytical Chemistry V74(5),143
A-150 A
I would suggest that you not use the graphite crucible, they can cause
clogging. It sounds like to me that you are putting too many samples into
the reactor and the tube is clogging with molten Ag. We found that we can
put about 70-80 samples in the reactor and this yields about 1.5 mg of
silver when the reactor cools, it would depend upon the size of your Ag
boats however. Any more samples and the silver ball touches the sides of
the reactor when it cools. The problem here is that the ball sticks to the
side of the reactor when you cool it and although you can knock out the
ball, your backgrounds will increase if the reactor is chipped. If you
have high backgrounds the only thing you can do is replace the glassy
carbon chips and/or the entire reactor and ceramic tube. We have not found
a good way to clean the tubes or the reactor that do not increase
backgrounds when the reactor is heated up again. Make sure to cool and
heat the SiC furnace slowly, or the coils will warp and eventually touch
the ceramic outer tube with disastrous results (melting, shorting out,
smoking, etc.).
Good luck,
Bill
At 07:00 PM 5/23/2002 +0200, you wrote:
>Hello
>
>We are setting up a TC/EA - IRMS for 18O analysis, we have started
>analysing sulphates.
>
>We are using a "tube in tube assembly" (glassy carbon in alumina tube),
the
>reactor at 1450oC and the GC ant 90oC, we run BaSO4 precipitates and we
>weight 0.15-0.30 mg.
>
>We have some questions:
>
>1) We have high backgrounds, even 24h after the reactor is at full
>temperature (1450oC), so we have started working with He dilution. Does
>everybody work with He dilution or someone does not?
>
>2) When we are running NBS-127 or BaSO4 samples precipitated from e.g.
>river water the peaks are correct (symmetric and nice shape), but when we
>are running BaSO4 samples precipitated from dissolved gypsum some of them
>have a tailing (asymmetric and even double-peak). Does anyone have such
>tailings in this kind of samples?
>
>3) We use as standards the NBS-127 and an internal standard (Ba SO4) with
>d18O = 18 per mil, but we have a problem with the internal standard, it is
>Ba SO4 from a dissolved gypsum and it shows this tailing. We are wondering
>if we can use nitrate internal standards or carbonate (NBS19) when
sulphate
>analyses are running. What are other labs using for standardisation?
>
>4) We observed a "memory effect" when we are running samples of different
>isotopic composition as 20 per mile and 10 per mile (we are running
>triplicates, e.g. 19.5 , 20.5 , 20.1 , 12.4 , 11.9 , 10.7 ). Have anyone
>observed such memory effects? How can we solve this problem? We have tried
>inserting a blank (no capsule) between the triplicates but it does not
seem
>to work (the total time of analysis is 8 min per sample).
>
>5) We have observed a yellowish crystalline substance on the outside of
the
>glassy carbon. Do you use nitric acid to clean it? (isogeochem messages,
>jan-01). Do you clean it every time that you change the crucible?
>
>Thanks a lot in advance.
>Neus
>
>_______________________________________
>Neus Otero Perez
>Grup de Mineralogia Aplicada i Medi Ambient
>Dept. Cristal.lografia, Mineralogia i Diposits Minerals
>Facultat de Geologia, Universitat de Barcelona
>Marti i Franques s/n
>08028 Barcelona, Spain
>Tel: 34 93 402 13 45 Fax: 34 93 402 13 40
>e-mail: [log in to unmask]
>_______________________________________
William J. Showers
Dept of Marine, Earth & Atm Sciences
Box 8208
North Carolina State University
Raleigh NC 27695
(919) 515 - 7143 Office
(919) 515 - 7802 Fax
(919) 515 - 3689 Lab
(919) 515 - 7911 Field Lab
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