Is anyone familiar with the sulfate 18O analysis technique of Van Stepvoort
and Krouse?  In the literature Ive found discussions about SiO2/V2O5
conversion to SO2.  Im assuming the oxygens are all exchanged during this
reaction and this is the means to get the 34S number, but its unclear where
the 18O numbers are coming from.  Im assuming its graphite reduction to CO2
or CO but I can not find that explicitly stated in any of the papers.  Are
there any references to the 18O aspect other than Van Stepvoort's PhD thesis?
Greg Michalski


Greg,

I'm not familiar with the Van Stepvoort and Krouse technique, but I'd echo earlier observations that SO2 is very likely to exchange O isotopes with quartz at high temperature.  I have a paper coming out shortly in Rapid Commun. Mass Spectrom. (2005, v.19, p.1-3) that looks at this issue with regards to running d33S and d34S analyses using an elemental analyser technique. My interest was more towards the contribution of d18O to delta66 and d34S, but I think it's relevant to your question too (I've pasted the abstract below).

You'll probably want to look at the following papers too, which are germane to the topic:

Kim A. Baublys, Suzanne D. Golding, Elisa Young and Balz S. Kamber (2004)
Rapid Commun. Mass Spectrom. 2004; 18: 2765-2769
Simultaneous determination of d 33SV-CDT and d34SV-CDT using masses 48, 49 and 50 on a continuous flow isotope ratio mass spectrometer

Fry B., Silva S.R., Kendall C. and Anderson R.K. (2002) Oxygen isotope corrections for online d34S analysis. 
Rapid Communications in Mass Spectrometry, 16: 854-858.

I hope this helps!

Cheers,
Simon




The effect of sulfate-d18O upon on-line sulfate-d34S analysis, and implications for measurements of d33S and D33S

Simon R. Poulson
Dept. of Geological Sciences MS-172, University of Nevada-Reno, Reno, NV 89557, USA
E-mail: [log in to unmask]

On-line d34S analysis of sulfate using an elemental analyzer has a number of advantages vs. conventional off-line techniques, such as ease of operation, rapidity, and the requirement for small amounts of material.  Although the analyses are performed by converting sulfate to SO2 gas, the effect of sulfate-d18O composition upon the SO2-d18O  composition and the value of d66 during elemental analysis, and ultimately the calculated sulfate-d34S  composition, has rarely been addressed.  Three BaSO4 samples have been prepared with known identical d34S compositions, but with a wide range of d18O compositions.  d18O values were shown to range over 40, but conventional on-line d34S analyses verified that the sulfate-d34S  compositions were identical.  These results indicate that conventional on-line analysis of sulfate-d34S  is unaffected by the value of sulfate-d18O, and suggest that sulfide d34S standards can be used to calibrate sulfate d34S analyses (and vice versa).  Moreover, these results suggest that it may be possible to use on-line sulfur isotope analysis of SO2 to measure d33S and D33S in addition to d34S, as a faster and safer alternative to the SF6 technique currently utilized, and hence promote further study of mass-independent sulfur isotope fractionation effects.
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Simon Poulson
Research Assistant Professor
Dept. of Geological Sciences MS-172
1664 N. Virginia St.
University of Nevada - Reno
Reno, NV 89557-0138. USA
(775) 784-1104
Fax: (775) 784-1833
[log in to unmask]
http://www.mines.unr.edu/isotope/simonhome.html
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