Dear All,
Just a short comment on the linearity diagnostic test in isodat.  In 
this diagnosis method, the background values are determined after a 
very short delay when you start the script.  The script does a peak 
center just before the test commences so the reference port is open 
just before the background values are determined.  The isodat delay 
is too short and the background values are a good bit higher than 
they would be if the delay was twice or three times isodats.  The 
background cup values are subtracted from each measurement so they 
affect the low-voltage ratios more than the high-voltage ratios. 
Thus, you can have poor "isodat" linearity but good instrument 
linearity and vice-versa.  My workaround is to determine the 
background values myself, export the cup voltages from the linearity 
test and calculate the linearity myself in excel.  Sounds hard but is 
actually quite easy.

Nitpicking isodat to death,
Pratigya Polissar

>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


-- 
Pratigya Polissar

411 Deike Building
Department of Geosciences
Pennsylvania State University
University Park, PA 16802
(814)863-9903 office
(814)863-8673 mass spectrometry lab
(814)865-5808 organic geochemistry lab