Charlotte,
I can't say I've EVER gotten a slope on my standard curve that was
exactly 1.0, usually it is a little lower - though maybe not 0.93.
This I don't see as a problem, however, provided that you are running
multiple in-house standards that cover a wide range of d13C whenever
you do analyses. When we run carbonates, we use three standards that
cover our entire expected range for values. All of these are
carefully calibrated to the NIST standards NBS-19, NBS-18, and L-SVEC
at least once every 6 months. This calibration usually has a slope
less than one. By doing this, we have had excellent long-term
reproducibility and no problems with samples that are especially
enriched or depleted in 13C.
I never concern myself with what the actual isotopic value is for my
reference tank. I set it to zero in ISODAT and do all my corrections
later using my other standards and LIMS for light stable isotopes.
The value for the reference tank gas is irrelevant. It sounds like
your system is working great, so all I can suggest is to be sure
you're running more than one in-house standard with your analyses, or
if you have to use the reference tank gas, have at least one other
in-house standard to run with the analyses.
Take care,
~Penny
At 10:52 AM 2/5/2007, you wrote:
>I recently ran NIST standards to develop a new standard curve for d13C
>bulk isotopes on our Delta V and am wondering to what to attribute the
>fact that the slope is less than 1.0. This time it was 0.93, resulting
>in enriched values that are much more enriched than they were six
>months ago. We have a new tank of Scientific CO2 for the reference
>gas, and it appears to have a d13C value of about -39.4 per mil
>(whereas our previous tank from the same supplier was -22 per mil).
>Any suggestions?
>
>Also, and I don't know if this is contributing to the above, we have a
>fair amount of water hanging around as evidenced by Mass 18 values in
>the several thousand mV range, with no indication of leaks (Mass 40 is
>low). I just discovered that the gas purifier on the GC-GCC III side
>had a blown fuse. But the problem is evident on the EA-Conflo III side
>of things and we have never used a gas purifier there and not had this
>problem before to my recollection. Heaters are on at the valves and
>the source. We already replaced the needle valves with Nupro valves.
>
>Spent some time running diagnostics last week: focus is good;
>linearity checks out. Amplifier test passes (although I can't pick the
>gas configuration to use because it only sees "CO" which we do not
>routinely use. Peak shape and flatness are good. System stability is
>excellent. Signal stability (on CO2) seems to be 2.5 times what the
>manual says it should be; not sure what that means or how to correct
>it...
>
>I'll be interested to hear any ideas!
>--
>Charlotte Lehmann
>Research Technician
>Bates College
>Department of Geology
>206A Carnegie Science Building
>44 Campus Avenue
>Lewiston, ME 04240
>Phone: 207-786-6485
>FAX: 207-786-8334
*******************************************************************
Dr. Pennilyn Higgins
Research Associate
"SIREAL"
Stable Isotope Ratios in the Environment Analytical Laboratory
Department of Earth and Environmental Sciences
University of Rochester
227 Hutchison Hall
Rochester, NY 14627
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Office: 209b Hutchison Hall Lab: 209 Hutchison Hall
Voice : (585) 275-0601 Outer lab: (585) 273-1405
FAX : (585) 244-5689 Inner lab: (585) 273-1397
http://www.earth.rochester.edu/SIREAL/index.html
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