Computer assisted optimization of magnetic field homogeneity in nuclear magnetic resonance spectroscopy



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A historical review of experimental measures taken to improve magnetic field homogeneity is presented, followed by a rationale for the present approach to improving field homogeneity. An electronic instrument has been constructed which can be operated by computer control to improve the field homogeneity of a Varian V-7405 15-inch electromagnet, used with the University of Houston Varian XL-IOO nuclear magnetic resonance spectrometer. This new unit replaces the Varian 924029 homogeneity control unit which can only be operated manually by turning potentiometers. The performance of the unit is demonstrated for both proton and carbon-13 nuclear magnetic resonance spectroscopy. Resolution closely approaching the best resolution seen in the published proton n.m.r. spectra of orthodichlorobenzene has been achieved. The advantage of a computer controlled electronic homogeneity control unit is that the field homogeneity can be regulated more precisely and can be kept in continual tune during long signal averaging runs, effectively increasing the signal to noise ratio for a given period of signal averaging.