A method (10) insures that the characteristics of the magnetic field used in a magnetic resonance check weighing system (20) for samples in vials (22) on a production line track deviations from the resonant frequency of the sample. The method (10) includes the steps of obtaining (50) a free induction decay signal from a magnetic resonance measurement of the sample, monitoring (52) from the free induction decay signal the deviation of the resonance frequency of the magnetic resonance measurement from a preselected resonance frequency; and adjusting (62) the magnetic field to maintain the preselected resonance frequency.