Shimming, Part III
Symptoms of Inhomogeneity
An examination of the NMR signal lineshape can often reveal which shim needs the most attention. In general, even-order spinning shims (Z2 and Z4) create asymmetric lineshapes when misadjusted. The odd-order spinning shims (Zl, Z3, and Z5) create symmetrical lineshape symptoms when misadjusted. Also, in general, the higher order the shim causing the problem, the farther down the peak the symptom will appear. The lineshape problems to be expected are summarized in the following figures, which were created using the SAM subroutine within NUTS.
The most common observable symptoms of misadjustment of the nonspin shims are spinning sidebands. First-order spinning sidebands are derived from field inhomogeneities the sample experiences once each rotation (X, Y, ZX, and ZY). Second-order spinning sidebands are derived from field inhomogeneities which the sample experiences twice per sample rotation (XY and X2-Y2).
The third-order nonspin shims, Z2X, Z2Y, ZXY, Z(X2-Y2), X3, and Y3, can create both spinning sideband problems and lineshape problems. The lineshape problems are usually very low at the base of the peak. An especially interesting symptom from two of these shims can be seen while adjusting the spinning shims. When the adjustment of Z3 changes the observed NMR signal from a broad-based signal with no spinning sidebands to a narrow-based signal with spinning sidebands, then the responsible shim is most likely Z2X or Z2Y.
As the NMR spectroscopist endeavors to find more sensitivity, one method he has historically turned to is longer coils in the NMR probe. As the coils become longer and see more sample, it becomes much harder to optimize the field over the entire length of the coil. Also, the higher-order shim gradients become much more important with longer coils, making shimming more complex. Usually some degradation of lineshape is inevitable when longer coils are used. Conversely, if better resolution and lineshape are required, a probe with a shorter coil should be tried.
The probe coil also contributes to resolution and lineshape problems by means of the magnetic susceptibility of the materials of construction. All parts of the probe have a magnetic susceptibility to some degree. This magnetic susceptibility has a tendency to bend the magnetic field nearby. These bent field lines distort the observed lineshape and the effect is greatest close to the source of the magnetic susceptibility. As the probe filling factors become better and the coil parts move closer to the sample, the coil’s magnetic susceptibility has a greater effect on lineshape and resolution. A typical probe induces magnetic gradients on the sample which look a lot like a Z4 inhomogeneity symptom. The process of creating these gradients and how to shim them is discussed in depth in the chapter about Probes and Shimming.
Return to Part I, Introduction
Return to Part II, Basics
Proceed to Part IV, The Z1 Profile
Skip to Part V, Effect of Sample and Coil
Last updated: 01/22/03