Methods and apparatus for detecting substances containing nuclei of a first and second kind

A method for detecting the presence of a substance in a heterogeneous sample or article (i.e., a contraband substance) in a magnetic field includes establishing a level-crossing condition (BLC) for the substance to be detected for a set time (tb), and subsequently measuring nuclear magnetic resonance (NMR) signals to detect some consequence of the level-crossing condition. A saturating signal is applied at the begining of each cyclic test. Apparatus for detecting the presence of predetermined substances is also disclosed.