I don't have "big magnets, just a 200 and 300 (unshielded) in a room
next to the mass spec where I have a large high resolution double
focusing sector instrument and an ion trap system -- there is no
effect. I suspect you would have no problems with shielded larger
magnets laterally, but vertically would be another story unless the
building floors were poured on a steel pan which might deflect the
fields. an unshielded 300 I think has a similar lateral field to a
shielded 600. My 300 nmr magnet is about 8 feet from the mass spec
console through a block wall, and the mass spec is on the other side
of the console and is 12 feet long, leaving the source end which is
closest to the console about 14 feet from the magnet. The Bruker trap
is at about the same distance.
High voltage cables in the slab (sector mass specs are usually in
labs with slab on grade floors) because of the weight. Vendors come
in and measure the fields in a room prior to installation and if you
don't satisfy their field specs they can claim they can't meet their
mass spec specs. However even significant 60 Hz fields from 600 volt
cables though significant are completely shielded out by the frame of
the mass spec below the flight tube. The nmr field is DC and easier
to tune out if a problem.
It is not as bad as when I was postdocing 38 years ago in Cambridge
England and when we cycled the magnet of the old Varian HR40 we
shifted the spectra on the permanent magnet 60MHz PE machine by
several ppm.-- most disconcerting to people in the middle of scans.
How many of us are left that remember cyclng old iron magnets and
then shimming initially with a 3 foot ratchet lever bending a 1 inch
steel plate attached to the yoke!
--
Prof. Jack M. Miller,
Associate Vice-President, Research and
Dean of Graduate Studies,
Professor of Chemistry,
Brock University,
St. Catharines, Ont.,
Canada, L2S 3A1.
Phone (905) 688 5550, ext 3789
FAX (905) 684 2277
e-mail jmiller_at_brocku.ca
http://chemiris.labs.brocku.ca/~chemweb/faculty/miller/
Received on Tue Feb 26 2002 - 18:47:14 MST