>The problem is that we have a custom made probe which seems to be
>7-8 mm too 'short'. (I.e., will we have to re-build the probe or not?)
>73, Peter (et al)
The answer depends on two major factors and some minor ones. Assuming
the coil is 7-8 mm too low in the magnet, three additional pieces of
information are necessary to "guess" if this will be a problem.
1. What is the length of the probe's coil? A typical coil length
is between 12 and 18 mm for a 5mm diameter probe. This means
you are about 1/2 a coil length away from center. More than
1/4 a coil length has a high potential of problems.
2. Where is the "inversion point" of the RT shims? RT shims create
their correction field over a region specified by their geometry.
Outside that region they can create the opposite effect. The
point of zero correction is called the "inversion point". Your
RT shims will have an inverion point a certain distance above
and a similiar distance below the center of your RT shims. The
inversion point can be different for each gradient of the
shim set. The inversion point of wide bore shims is farther
out than the inversion point of narrow bore shims. Typical Z
axis narrow bore (54mm) shims work well over a length of 2 to
3 cm and wde bore shims work well over 3 to 5cm. If the sum
of your coil length and coil displacement brings the ends of
your coil's active region near these limits you will have
problems shimming.
3. Lock pull can result from coil displacement. Since the center
of the coil and the center of the shims are not the same, when
the Z1 ( and others ) gradient is adjusted the NMR signal will
move to a slightly different field. To see this, start unlocked
with Z1 adjusted for best resolution. Change Z1 by x units where
x is enough to see a linewidth of about 100 Hz. This should
be, after FT and phasing, a square wave type plot. Now change
the Z1 to -x units and take another spectrum. The two spectra
should look similar, have about the same width and be in the
same position. If they are displaced, as they will be if the
center of the probe coil and the center of the RT shims are
displaced, the amount of displacement shows the amount of
field shift per Z1 correction. Many NMR systems have slow
responding locks and the lock has trouble following the field
shifts as the shims are adjusted. If your NMR system has a
slow lock and you shim on the lock signal, this slow interaction
can be a MAJOR pain when shimming.
These are the considerations when deciding what to do, but the main
consideration is "try it". If it is less of a pain to shim than a
pain to rebuild then use it as is!
=========================================
Virginia W. Miner gina@acornnmr.com
Woodrow W. Conover woody@acornnmr.com
Sam Le Chien sam@acornnmr.com
Acorn NMR Voice (510) 683-8595
46560 Fremont Blvd., #418 FAX (510) 683-6784
Fremont, CA 94538-6491