Locking and Shimming 3/4/94

Any shim may be changed during your session, as long as you change the shims back to their origional values when you are done. See Recalling and Saving Shim Values. Lock and shim values are changed by placing the mouse pointer on a numerical button and clicking the left or right mouse button. The left mouse button subtracts from the current parameter value, and the right button adds to the value. The mouse pointer can also be placed within the slide bar, and the left mouse button can be clicked to change the parameter value.

The lock and shims can be accessed by typing acqi to display the VNMR ACQUISITION window, and clicking on the connect button of the vnmr acquisition window. To leave the vnmr acquisition window, click on disconnect.

Shimming on the Lock

1. Lock on deuterium:

A. Click the lock button in the vnmr acquisition window.

B. Click the lock menu to off.

C. Adjust Z0 to the "on resonance" condition.

D. Click the lock menu to auto or on.

E. Adjust the lock power so that the on-resonance lock signal is observable, BUT NOT SATURATED. A saturated signal will spontaneously change in amplitude.

2. Click the shim button. Change the shims to the standard shim values listed at the spectrometer. The standard shims for the standard sample are stored as StdShims. See Recalling and Saving Shim Values.

If you are interactively optimizing Z1 and Z2 and their values are changing in a consistent direction as you switch from one shim to another, "overshim" Z1 by passing through the value that maximizes the lock and setting Z1 beyond this optimal value. Continue to optimize Z1 and Z2.

2. Spin the sample at 20 Hz and adjust Z1 and Z2 interactively to produce the largest lock signal. If you change Z2 by a large value, readjust the field and phase of the lock.

3. Collect a spectrum (nt=1, ss=0, np=16448, rd=0, lb=0.3). If the spectral lines are narrow and symmetric with small spinning sidebands, the shims are OK.

4. If the spectral lines have a large hump, change Z2 until the lock level is degraded by 3-5%, optimize Z1, and collect a quick spectrum. If there is no improvement, change Z2 the other way, optimize Z1, and collect a quick spectrum. If there is still no improvement, see Shimming on the FID. When finished, reoptimize the lock.

Steps 5 through 8 are NOT recommended. WRITE DOWN THE ORIGIONAL SHIM VALUES OF ANY SHIM THAT YOU CHANGE AFTER THIS POINT, or store the shims in a file, before attempting these steps. See Recalling and Saving Shim Values.

5. If the spectral lines are wide at the base, optimize Z3, then optimize Z1 and Z2 interactively, and collect a quick spectrum. If the base is still wide, change Z3 enough to degrade the lock level by 5-10%, optimize Z1 and Z2, and collect a quick spectrum. If there is no improvement, change Z3 the other way, optimize Z1 and Z2 interactively, and collect a quick spectrum. If there is still no improvement, see Shiming on the FID.

6. If the spectral line has a hump near the base, optimize Z4, then optimize Z1 and Z2 interactively, and collect a quick spectrum. If the base still has a hump, change Z4 until the lock level is degraded by 5-10%, optimize Z1 and Z2, and collect a quick spectrum. If there is no improvement, change Z4 the other way, optimize Z1 and Z2 interactively, and collect a quick spectrum. If there is still no improvement, see Shimming on the FID. If there is an imporvement, optimize Z3, then Z1 and Z2 interactively, then reoptimize the lock.

7. If spinning sidebands are >3% of line height, record the lock level, then turn off the spinner. If the lock decreases more than 10%, adjust X and ZX interactively for the tallest lock signal, then adjust Y and ZY interactively for the tallest lock signal, turn spinner on, then collect a quick spectrum. If the lock doesn't drop upon turning off the spinner, start again at step 1.

8. If there are two sets of spinning sidebands on each side of the peak, record the lock level, then turn off the spinner. If the lock decreases more than 10%, then adjust XY and X²-Y² for the tallest lock signal, turn spinner on, then collect a quick spectrum. If the lock doesn't drop upon turning off the spinner, start again at step 1.

Recalling and Saving Shim Values

1. Optimized shims for teh standard sample are taped to the table in front of the spectrometer.

2. To recall shim values during the same shimming session, click on first to recall the origional shims, click on best to recall the best set of shims during the session. Once you exit the shimming window, this method is lost.

3. To recall the standard shims, type rts('StdShims'). These shims are also listed at the console.

4. To save your shim values, type svs('filename') . To retrieve your shim values, type rts('filename') . Shims are stored in /home/Research/vnmrsys/shims. Shim files older than one month should not be used. Shim files are frequently deleted and are not archived.

Shimming on the FID

Shimming on the FID works best if the sample has only a few very large resonances.

It can be difficult to remove the beating pattern when the shims are almost optimized. Change a shim by 100 points and move it back to the optimum value.

1. Set up parameters for a single-pulse experiment (np=16384, sw=4400, d1=0). Set carrier frequency at least 1 ppm from the largest peaks.

2. Type gf and connect to the acqi window. Click on grid.

3. If the FID has a beating pattern, adjust z2 and/or z4 until the FID has a smooth decay.

4. Adjust z3 and z3 until the FID has the slowest decay.

5. Disconnect from the acqi window to stop shimming. Shim values are saved. lockpower and lockgain changes are NOT saved.

6. Collect a quick spectrum. If peaks are wide or non-lorentzian, start again at step 2.

Autoshimming

Interactive autoshimming should not be used as a replacement for manual shimming. Autoshimming is fast and efficient only when the shims are near optimum. Autoshimming only on Z1 and Z2 is recommended. Do not autoshim on shims other than Z1, Z2, and Z3.

1. In the vnmr acquisition shim window, click on auto.

2. Select m>m to "start with medium and end with medium" shimming quality.

3. Select the set of shims to be used by the autoshimming routine.

4. Click on start. The start button will change to stop. The autoshim is finished when the stop button changes to start.

Use background autoshimming for long data acquisitions EXCEPT for ¹⁵N and spin-lock experiments:

1. While setting up your experiment parameters, type method('z1z2') for samples > 0.5 ml, type method('z1z2z3') for samples £ 0.5 ml.

2. After the experiment has started, type shim.

Table 1 lists the room temperature shims. If you change a shim, you should reoptimize the other shims which interacts with the changed shim. Shims which start with 'Z' (the spinning shims) should be optimized with the spinner ON, while shims that start with 'X' or 'Y' (the non-spinning shims) should be optimized with the spinner OFF.

Shims (ethylbenzene in chloroform): Linewidth = ______ sidebands = _____
Shim	Interacts With	Value	Shim	Interacts With	Value
Lock	none		X1	none
Z1C	none		Y1	none
Z2C	Z1		XZ	X1
Z1	none		YZ	Y1
Z2	Z1		XY	X1,Y1
Z3	Z1,Z2		X2Y2	X1,Y1
Z4	Z1,Z2		X3	X1,Y1
			Y3	X1,Y1