Computerized Shimming with the Tuning Algorithm

Introduction The efficiency of automation with large numbers of individual samples and the quality of the acquired spectra depend heavily on the efficiency and quality of adjustments made to the B0 field homogeneity (shimming) after each sample change. The adjustments required depend mainly on sample properties such as bulk magnetic susceptibility of the solvent, volume, and position within the rf coil. The influence of these parameters increases as the volume is reduced and B0 is increased. Part I of this report presents some basic information and practical advice which should help to minimize the effort and increase the quality and consistency of field homogeneity adjustment for high sample throughput. In particular, the computerized shimming algorithms called Tuning, which is available with XWIN-NMR and AVANCE spectrometers will be explained in some detail. These methods were originally developed for AM and AC spectrometers with the Aspect 3000 data system (TX subroutine) so that much of the information in this article will be useful for spectroscopists who still have an “oldtimer” in the laboratory. In Part II of this article (in the next SpinReport) the Simplex iteration method and the field mapping procedure GradShim, which uses gradient echoes to actually measure the inhomogeneity of B0, will be examined in detail.