I recieved 18 replies plus several addenda to replies that were split
evenly (10/8) between academic and commercial sites. The definitive
experiment was run by Ron Brown who has measured the performance of tables
and legs, and finds the table to be superior to leg stablization by an
order of magnitude, particularly for low (~2Hz) frequency noise. This was
due to the large mass of the table relative to the mass of the magnet.
Curiously, the number of labs using leg stabilization is the same as the
number of labs using tables.
In the long run, almost all high field systems used tables (that is 500 MHz
and above). A major drawback to the table is probe changing and the need
for extra head space above the magnet for helium fills. Doug Lowman at
Kodak solved the former problem by constructing a wooden platform above the
table, while others used a plastic stool with the steel wheels removed to
gain better access to underneath the magnet. The latter situation (lack of
head space) is obviously a limitation that cannot be overcome, although I
have heard of companies digging pits to lower the magnet; this seems a bit
extreme and is clearly out of the question for a second floor operation.
The labs with leg stabilization recommended these highly as well. The
systems ran from 500-300 MHz. The legs are less expensive than the table
and can be installed on an energized magnet, further lowering the overall
cost.
Lastly, several labs weighed in with the doughnuts. These are inflatable
tires that fit under the ringstand (if you will) of the magnet. Although
far less expensive, the doughnuts must be fitted with a manifold to ensure
that each is expanded evenly. This is probably necessary for any system,
when I think about it. If given a choice, most of these users would opt
for anything else.
The bottom line?
Tables are best, but more expensive and somewhat problematical. Probably
necessary for high (>=500MHz) field NMRs.
Legs are a great alternative, particularly for 400 MHz systems (ie just
like mine!) but will have problems damping low frequency noise. Less
expensive as well.
Doughnuts are also an option particularly if there is little, yet
noticeable, vibrational noise and your budget is tight.
There was one recommended vendor, TMC as well as your favourite (sic) NMR
dealer.
Thanks once again for all your helpful comments.
R. Fronko
Microcide Pharmaceuticals, Inc.
(415)428-3516
rfronko@microcide.com