Hi,

The Box/Trap differences are usually caused by a dirty source. On our
Delta V the sensitivity beginns to drop at 1.1/0.4. Don't know about
MAT253. Did you try to tune the lenses and stuff allready?

Regards

Kov

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Jamie Elsila
Sent: Montag, 29. Oktober 2007 16:52
To: [log in to unmask]
Subject: [ISOGEOCHEM] Another sensitivity drop question

We're experiencing a sensitivity drop with our MAT253 and I was hoping
for some advice.  I know other people have recently asked about similar
problems, and I've found the responses helpful, but we still have a
problem.

It's a relatively new instrument and we just changed the filament last
week for the first time.  We had to reseat the source a couple of times
after that because of a short in one of the wires.  Now, we're getting a
signal intensity that's about half of what it should be, both with the
dual inlet and through the Conflo.

We're a little worried because we think that at least one of the times
we put the source in, we didn't have the pin and notch lined up
properly, but we tightened it down anyway.  Our Thermo technician says
this could have bent the support rods, but that if that were the case
we'd most likely be unable to get it to seat correctly now and would be
getting almost no signal, rather than just a 50% drop.

We've checked the new filament and it looks correctly centered.  Our box
and trap are at 0.9 and 0.6. I know they should be closer to 0.8/0.7.
What could be causing this, and could it be the cause of our sensitivity
drop? 
We've done a lot of checking of the tuning, both using the autofocus and
manual focus.  All of the settings seem to respond, and we've checked
the leads several times, so we think they're okay.

We know that the position of the source is important.  At one point, we
know we had the pin and notch lined up correctly, but were getting about
a 90% drop in sensitivity.  After reseating the source, we got to the
point we are now.  We could keep trying to reposition the source, but at
some point the numerous iterations start to feel futile.  I'm willing to
keep trying, though.

Any ideas would be greatly appreciated.  Thanks!