This is certainly not a "dumb" question as there most assuredly are changes
in shimming and ease of spinning associated with VT operation. I haven't
used anything but Bruker spectrometers for VT operation so I can't comment
about other brands.
With respect to shimming, the fundamental problem arises from the fact
that the hot/cold gas is not dewared as it exits the probe. In fact it is
dumped directly onto the aluminum that holds the shim coils. The problem
can be substantially alleviated by the addition of an extra flange at the
top of the stack that allows you to introduce room temperature air that
flows around the shim coils. Not only is the problem with drifting shims
alleviated but you are much less likely to quench your magnet because you
have either cooked or frozen the O-rings in the dewar during extended
periods of operation at high or low temperature. I have added these flanges
myself many years ago and we do a lot of VT for extended periods. The
flange is (or at least used to be) standard on the wide bore magnets but not
all installation engineers hook it up since they don't know what it is for.
Henry Luhrs at Bruker in Billerica, Massachusetts, says they now have these
flanges for narrow bore magnets (he showed me one) and they even have a
part number for ordering. They are not trivial to install since it requires
that you re-adjust the height of your shim stack to accommodate for the
extra length. They are however definitely worth it if you do much VT. I
believe it would be a great help if potential customers were aware of this
possibility prior to installation since that is certainly the easiest time
to put it on.
With respect to spinning, the main problem is that a fairly high flow
rate is required to achieve extreme temperatures. The use of the ceramic
spinner helps because it is heavier. We have sometimes put two spinners on
the sample for extra weight. You can also use gentle vacuum to increase the
flow without blowing the sample out of the probe. This problem is pretty
probe dependent and does seem to be substantially better on the later
version of the shim stack.
As far as the concerns over plastic tubing, in our systems the cooling
gas remains at room temperature until it gets to the probe so it is not an
issue.
Jane Strouse
mjs@argon.chem.ucla.edu