III. Shimming, Linewidths and Lineshapes - II

It is important to have a basic understanding of lineshape to be able to judge when:

(1) The shimming is off, and

(2) More time should be spent in shimming the sample. The best way to avoid problems is to establish a procedure using the guidelines below.

1. Always load a shim file when you sit down at the instrument. You should never assume the previous user left the instrument with a standard shim file loaded. Without reloading standard shims, you will have to start where the last person stopped --- and that might include someone who shimmed for a short sample, a “bad” tube, a viscous sample, etc.

2. Be aware of lock parameters, especially if you only shim on the lock display. Establish lock transmitter power and gain levels that work for most of your samples. If you encounter a sample that seems to require an unusually high power or gain setting, there is a problem with your sample and/or the instrument, and shimming on the lock level may be difficult or impossible.

3. Shimming problems are confirmed only if the “problem” is visible on every peak in your spectrum. If only one peak is doubled, the “problem” is sample related, and can’t be shimmed away. Remember, anomalies close to the base of intense single lines may not be visible on less intense peaks unless the vertical scale is increased.

4. Establish a shimming method. Shimming is an art form that requires patience and practice. You should always approach shimming with some method that works for you to give acceptable results. Example: load a shim file, adjust lock level to a maximum with Z1, then Z2, then Z1, then Z3, then Z1.

5. Spinning sidebands should always be below 2%. If spinning sidebands are above 2%, turn off the spinner air, optimize X and Y, then turn the spinner air back on and re-optimize Z1, Z2, and Z3. If this does not solve the problem, consider transferring your sample to another tube.