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.
KPESIL-Keck Paleoenvironmental & Environmental Stable Isotope Laboratory
University of Kansas, Dept. of Geology
Multidisciplinary Research Building
2030 Becker Drive, RM 130