Hi Bill,
We're using NBS19.  I'm assuming at these large sample sizes (100ug, ~5-8 grains) that homogeneity would not be a problem. 
----- Original Message -----
From: [log in to unmask] href="mailto:[log in to unmask]">Bill Showers
To: [log in to unmask] href="mailto:[log in to unmask]">[log in to unmask]
Sent: Friday, June 10, 2005 7:31 AM
Subject: Re: [ISOGEOCHEM] Isoprime advice requested


What carbonate standard are you using?  We get different results with different standards, are you sure your standard is homogeneous?


At 12:47 PM 6/8/2005, you wrote:
Good Morning Everybody,
We're struggling to get good precision with our GV Isoprime/Dual Inlet.  We are running 100ug carbonate samples/standards.  We've achieved the specified +/-0.05 d18O (1 sd) on some runs, but it doesn't last.  Our last 4 runs resulted in +/- 0.07, 0.12, 0.22, 0.09 d18O (1 sd).
The mass spec seems to be working beautifully as indicated by the peak shape, all open analysis, and single cold finger volume anlaysis with ref gas on both sides of the dual inlet. 
We learned some techiques from our esteemed and helpful GV service engineers regarding the carbonate prep line. These include replacing the prep line and heating the prep line between runs.  We saw improvements in performance, especially after heating the line.  Unfortunately, the good data quality deteriorates quickly (several runs).  We've experimented with heating the prep line continuously, heating between samples, and not heating at all.  Our latest experiments involve replacing the 1/16" ss prep line with fat tubing (1/4" convoluted ss tubing).  The fat tubing showed promising results at first, but alas, has deteriorated again (above data collected with fat tubing). 
I like the rapid pumpdown and definitive leak checking that the fat tubing allows.  My thought was that the increased conductance and lower surface area to volume ratio of the fat tubing would be a great benefit.
When using 1/16" tubing, it is standard 316 ss tubing that is cut with a dremel tool.
Any ideas or suggestions for further testing would be greatly appreciated.
Take care,
Georges L. Paradis
ICPMS Lab Manager
Department of Geological Sciences, Marine Science Institute
Unviversity of California Santa Barbara
Santa Barbara, CA 93106
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William J. Showers
Dept of Marine, Earth & Atm Sciences
Box 8208
North Carolina State University
Raleigh NC 27695

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