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