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
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North Carolina State University
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