HI Greg,

Sometimes I'll try tuning on CO2 then use that as the starting point for 
N2 and vice versa.  I'll also try tuning on low then high voltages.  Fix 
the trap and ee at low settings and try from there (then medium the 
high, for lack of better terms).  The peak centering is fine?  And are 
you seeing the same problem with linearity tested via ref gas pulses?  
That is, no other funkiness in the EA and sample burn. The problem with 
trying to save O2 is that you have to remember to turn it on again ;)  
(ok, maybe that's just me).  Otherwise if might be "time to bake the 
donu..." ahhh, clean the source.

good luck...

gerry




On 6/2/2011 10:04 AM, kpesil wrote:
> Hello Isotopers,
>
> I'm curious as to what kind of tuning folks are going through before running their MAT253s for d15N analysis. I seem to be going through a hit and miss in terms of obtaining consistent d15N values for a variable weight standard, when at other times I've had no problems. I've just switched over from our gasbench to the EA, and I'm unable to obtain the "sweet" spot I used to have with d15N results on a given standard. I've spent the past week or so simply doing variable weight standard analyses and I'm seeing a very apparent logarithmic isotope effect as I lower the weight on this one typically very consistent standard (and no the standard is not contaminated). My experience with tuning and speaking with thermo engineers is:
>
> i) tune for max sensitivity
> ii) then lower the extraction (a tradeoff occurs where you give up voltage for improved linearity)
> iii) if that doesn't work, start adjusting the electron energy.
>
> Thoughts?
>
> Greg
>
> Gregory Cane
> KPESIL-Keck Paleoenvironmental&  Environmental Stable Isotope Laboratory
> University of Kansas, Dept. of Geology
> Multidisciplinary Research Building
> 2030 Becker Drive, RM 130
> Lawrence, KS
> 66047
> Tel: 785-864-7750
> Fax: 785-864-1906
>