III. Shimming, Linewidths and Lineshapes -III

Knowledge of correct lineshape can help in correcting problems such as those shown in Figure III-3. Although the peak in Figure III-3b may have a linewidth at half-height (LW_1/2) that is less than 0.50 Hz, the checkout requirement, it is obviously poorly shimmed. You should never accept a poorly shimmed lineshape as is shown in Figure III-3b, where a single line is expected.

On the pages that follow are some lineshape defects and the shims that should be adjusted to correct the problem. In general, odd-order shims (Z1, Z3) affect the lineshape symmetrically while eve-order shims (Z2, Z4) cause a non-symmetrical lineshape. The higher the order (Z4 is higher order than Z2), the lower (i.e., closer to the base of the peaks) the problem is observed.