In high resolution NMR and to a lesser extent solid state NMR the spectroscopist
always strives to obtain the narrowest line possible, which for an ensemble of spins
in a 5 mm tube is really a function of having as uniform a magnetic field as possible
so that all the spins in the sample have the same larmor frequency. Since the perfect
magnet has not been built (and even if it had the magnetic susceptibility of the
sample and tube would distort the field) a mechanism is need to correct for imperfections
in the main field and for introduced perturbations. This process is called shimming
and is enacted by virtue of resistive room-temperature coils (the RT shim tube)
which carry electric current and produce correction fields concentrated at the point
of the probe and sample. These localized magnetic fields are designed to cancel
out spatial inhomogeneities in the main field. A detailed discussion on the design
and implementation of shim coils is beyond the scope of this workshop, however the
shims break down into two basic types, axial (developing a correction along the
main B0 axis) and radial (developing corrections in the transverse directions).
The other main point is that the higher the number of shims the greater the ability
to correct for higher order distortions, i.e. it is better to have 28 shims on a
500 MHz magnet than 13 shims.
To investigate what these shims produce in terms of correction fields and what affects
the imperfections have on the distribution of larmor frequencies the following simulations
was performed for a completely homogeneous field (using a Lorentzian line with 10
Hz width at half height):
If the Z1 shim is offset by the maximum current value we find the following effect:
The result is a linear gradient across the sample which has the effect of broadening
the line immensely (interestingly this effect is the basis for all imaging experiments
as well as field gradient based shimming methods).
A similar treatment of the Z2 shim gives the following plots:
A stylized schematic of the z-axis shim corrections is shown in the following diagram:
At this point several things about the shapes should be obvious. In addition the
effect on the lines for an offset of only one shim has certain tell-tale signs.
What is not so obvious is when more than one shim is offset. to see what happens
under these circumstances the Z1 and Z3 shims were offset to give the following
As you can see it is possible for a singlet to really be a doublet!
Overall shimming can be performed in 4 basic ways. The first is shimming on the
intensity of the deuterium lock signal. This is by far the most common / popular
method. Second is shimming on the FID which, while tedious, can result in superior
lineshape than lock level shimming. With the advent of faster computers has come
shimming on the actual spectrum. This is a modification of FID shimming in which
the FID is fourier transformed on the fly to give an NMR line to use as a shimming
target. The last method is actually fast becoming the most powerful and in many
ways easiest method to use. This method is pulse field gradient shimming.
For sake of brevity we wil concern ourselves primarily with lock level shimming
although we can take advantage of the PFG equipment on the 500 for a introduction
to field gradient mapping and shimming.
Since the NMR coil is the method by which all of these methods are detected it should
be no surprise that the sample prep is integrally connected with the shimming process.
The NMR coil wants to see an infinite cylinder of solvent so as to minimize magnetic
susceptibility issues of solvent-glass-air transitions. In order to approximate
an inifinite cylinder of solvent one need to have a length of solvent below the
coil and above the coil equal to the coil length. Most of todays high resolution
probes employ coils between 15 and 18 mm of lenth. Skimping on solvent will inevitably
end up in more work on the shimming process as well as inferior results!
By far the easiest method for shimming is described in the next section.
Shimming the sample
If your sample conforms to the 4.0 to 4.5 cm column length of solvnet then shimming
is EZ! From the acqi lock screen shown in figure 5 click the button labeled SHIM.
The following screen should appear:
Start the shimming process by checking that the lock level is below 100% (best range
is 70 - 90%). If the level is greater than 100% then you must lower the lock gain
by clicking on the -1 button at the bottom of the shim screen labeled lock gain.
Once the lock level is below the 100% level begin by clicking the Z1C -1+ button
with either the left or right mouse button. The lock level as shown by the thermometer
bars should move either up or down. If the level moves down then click the button
in the opposite direction untill the lock level maximizes and begins to go back
down. Note that there are two thermometer bars. The top one is the "coarse"
or overall level while the bottom is the "fine" level which is really
a magnification of the tip of the coarse thermometer bar. The fine bar changes color
in a continuous scroll when the level gets 2% higher or lower thus giving you an
idea of the change. There is also a numerical readout which shows the "starting"
level and the "current" level so that you have an idea if you are getting
better or worse. At any point you can start over by clicking the "first"
button or go to your best setting by clicking the "best" button.
Once Z1C has been optimized you click the Z2C -1+ button in the opposite direction
to the Z1C correction. The bars should move slightly up. Keep clicking untill Z2C
is maximized and begins to drop back down. Return to Z1C and adjust -1+ to re-maximize
the level. Repeat Z2C to maximize. Progress to the Z1 -4+ button and click in the
opposite direction to the last Z2C adjustment. If the level begins to fall reverse
the direction untill Z1 is maximized. Maximize Z2 in an analogous fashion. At this
point the shims should be adjusted well enough to obtain a good quality spectrum.
If the shims were left in an unadjustable state by the previous user you can load
a set of "standard" shims which are determined on the ethylbenzene standard
on a monthly basis. To load these starter shims just click the Shims button found
on the Main Menu bar. If you do not see such a button click on Main Menu button
followed by the Shims button.
The fine axial ("Z" direction) shims can be accessed by holding the right
mouse button down on the
arrow in the button box above. This button box denotes a menu is under which can
be pulled open with the right mouse button to show screens for the fine Z shims
as well as the non-spin shims.
The shimming process can be closed by clicking on the close button at the top of
the shim screen.