Deuterium Gradient Shimming
By Bill Stevens as taught by Gary Lueker
Signal routing: Use quarter-wave cable for 45-65 MHz on broadband preamp.
Bypass lock-reject filter so transmitter goes into xmit port of preamp. Route
preamp in/out to lock BNC on probe. Turn off sample spinning and lock.
Routine gradient shimming: typing gmapsys will call in user’s previous shim
map and parameters. One need only make sure nt and d1 are appropriate for the
solvent in use. D1=5 for CDCl3 or D2O, d1=10 for acetone, nt=4 for D2O or
acetone, nt=8 for chloroform. With gzsize=4, gzlvl=2000, autoshim on Z.
When shims converge, usually within two iterations, recable for conventional
operation. If sample spinning is desired, minor adjustment of z1 may be required.
Making a shim map: If d1 and nt are appropriate, the crucial remaining
parameters are the d3 array values, the parameter gzwin, and tof. For proton
shimming d3=0,0.05 is fine but for deuterium, d3=0,0.16 is best. If the second d3
value is too short, a very noisy and unusable map will result..
Click on set params to check that everything is correct or to select a nucleus and
solvent if needed. Clicking go dssh will display two profiles so that one can
evaluate whether there is sufficient signal. Clicking find tof will set tof and
recommend a gzwin, but not set it. Either enter the recommended gzwin setting,
somewhere in the 40-45% range, or determine the useful gzwin from the ds(2)
profile. If gzwin encompasses too much noise, the ends of the shim map will be
poorly determined.
Preliminary map: Use the doped H2O/D2O sample or the autotest sample to
make the map. To see whether success is likely, set gzsize=2, dg to see
at=0.016, np=320, sw=10,000, tpwr=42, pw=220, nt=4, d1=5. Set gzsize=2, click
make shim map and, when finished, return, display, display shim map. Z1
should be linear and z2 a smooth curve. If noisy, d3 is too short or nt is
insufficient. If the ends are frayed, gzwin is badly defined.
Final map: Set gzsize=4, use old values of z5 and z6, make shim map. Or try
the “Karpovich-Stevens” method: set gzsize=6, gmap_z1z4=’y’, make shim
map.
Vnmr1 should place good shim maps in the global directory.