Magnetic resonance methods are provided for determining the mass of samples in vials (1) in a production line, each sample having a net magnetisation capability, including the steps of applying a first magnetic field in a first direction in an interrogation zone (25) for creating a net magnetisation within a sample located within the interrogation zone (25), applying an alternating magnetic field in a second direction in the interrogation zone (25) for temporarily changing the net magnetisation of the sample located within the interrogation zone (25), monitoring energy emitted by the sample as the net magnetisation of the sample returns to its original state and generating an output signal whose current amplitude is proportional to the energy emitted, comparing the current amplitude with like data obtained from at least one similar sample of known mass, and determining the mass of the sample. The step of applying an alternating magnetic field includes the step of triggering application of the alternating magnetic field when the sample is located at a position (P.sub.M) within the interrogation zone (25) with the current amplitude at its maximum.