In <[log in to unmask]>, on 07/16/01
at 09:49 AM, Nicolas ARNAUD <[log in to unmask]> said:
>Hi everyone
>I have the opportunity of buying a MM1200 at low price, but I wonder how
>good it is theoritically for 40/39. Has anyone the experience with that
>model of machine ?
Nick:
I have worked with both an MAP215 and 2 different MM1200's. They both are
good for 40/39 work. In fact, the first of the modern automated laser
40/39 systems was attached to an MM1200.
It's important to find out a bit more about the mass spec itself. There
were, I believe, 3 models with Nier sources (A, B, and C) and one with a
modified magnet, flight tube, and a Bauer-Signer source (model S). My
experience has been only with the S versions (they also are used in
Toronto, Kiel, and USGS Menlo Park). There used to be one at Princeton,
but some telephone installers decided to walk on the flight tube.
I think early 1200's only had Faraday's (not sure), but later ones could
come with dual detectors. The Toronto machine has a mechanically
retractable Faraday, which means for all practical purposes, all runs have
to be done on the multiplier. The Michigan machine (which began life in
Cambridge) has a newer arrangement which might have spawned the 3600
detector design. It has a Daly detector running in analog mode and a
Faraday in the straight through position (the original window is too slow
for ion counting, but newer scintillator windows might allow for this on a
retrofit). There is a significant Faraday transient from turning on or off
the Daly knob, but I've found that there's also a problem at the Faraday
in the 215 when there's a cloud of electrons coming out of the multiplier
when it has a big signal.
The Nier type 1200's have better mass resolution than the 1200S, but the
latter (which has no electron trap, but controls using total emission) has
a souce which gets more ions per electron emitted from the filament. Even
the 1200S has plenty of mass resolution for argon work, including samples
with 40/39 ratios in the many hundreds. It is important to tune the source
according to the instructions (they really work!). My experience with
source optmization with the 215 and the 1200S suggests that even though
the 1200S has more plates, it is easier to optimize. The 215 has a very
long and bumpy optimization "ridge" in parameter space. Also, putting a
1200S source into the source block is much easier than installing a 215
source. There are no source magnets and no electron trap plate to turn
into an accordion, and the fit is much simpler.
As for performance they are pretty similar. The source electronics box for
a 1200S is rather complicated, but it's probably the best bit of
electronic design that VG ever did. I had lots of grief with the 215
source electronics, which had (including the spares!) a bad batch of pnp
transistors in the source darlington pair. It could spontaneously go
closed circuit and kill the filament. We lost 3 filaments that way until
we replaced the transistor with a "work alike".
So if it's got a decent source (Bauer-Signer is fine, but Ian at ANU
really likes his 1200 Nier source) and a multiplier or Daly, it can be as
good as any mass spec around. In fact, if I were designing a purely argon
machine from scratch, it would be smaller than a 215, more like a 1200.
--
Chris M. Hall
Assistant Research Scientist
Dept. of Geological Sciences
University of Michigan
2534 C.C. Little Building
425 E. University Ave.
Ann Arbor, MI 48109-2534
Thank you Andre, I'll have the veal piccate.
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