Several people made the point that it is not very desirable to have a
high
current in the Z0 shim coil, as it is not entirely orthogonal to the
other shim coils, and will therefore affect field homogeneity. While
this seems quite reasonable, the change in current must be slow over
time, and I would guess that day to day shimming would compensate for
this effect w/out any extra effort. Also, it seems preferable to
have the option of a large range of Z0 compensation for field drift,
even if one decides not to use it in light of the effect on the other
shims.
Many people with Varian spectrometers which had drift rates comparable
to ours
(1-2 Hz/hour) reported having to reset the spectrometer frequency at
least
once a year. While this is easy to do, it does turn out that one has to
rereference any spectra which are recorded before but processed after
the
frequency change. Personally, I find it disconcerting that I can not
rely
on a particular proton in my sample having the same resonance frequency
from one year to the next, even though one can obviously work with this.
Many people said they only have to change their spectrometer frequencies
every 5 years or less, but this seems to be due in most cases to very
low
field drift rates (~0.1 Hz/hour), not to differences in the hardware.
However, I had very few responses from Bruker users at all.
I obtained specs for the Z0 shim from both Varian and Bruker directly,
and found out that the varian Z0 power supply will put out up to 21
milliamps
corresponding to 11 kHz (proton), whereas the bruker shim is spec'd to
run
at up to 170 milliamps, correponding to 160 kHz. This probably explains
why
I had never heard of a spectrometer frequency having to be changed
during
my years as a bruker user, and probably explains why I didn't hear from
many bruker users.
David Eliezer