A couple of thoughts.
Your first three runs got steadily worse and then the fourth is better.
What did you do between runs?
Is your chiller working well? If not you will get water through to the
DI. Look at the water background before and after a run. Also look at
the water in a sample. Gross water contamination stands out like a sore
thumb but I think minor contamination could be harder to see. Does
baking the inlet help?
Are you getting phosphoric acid residues into the s/s transfer tubing?
Check the tip of your needle under a low-power microscope. The tip
wears away, connecting the inner tube and outer sleeve, and allowing
acid to be sucked into the tubing.
With the sample and ref beams balanced the ion gauge should read the
same for both beams. If it is higher on the sample side you have a
contaminant in you sample gas, possibly from a leak. Of course, given
the precision of the IG, small contaminats may not be seen.
Georges L. Paradis 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
> 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
> [log in to unmask] <mailto:[log in to unmask]>
School of Earth and Environmental Sciences
University of Wollongong,
Wollongong, 2522, NSW
Phone 02 4221 4586 (office)
02 4226 5903 (lab)
Fax 02 4221 4250
E-mail [log in to unmask]
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