Patent US 4,134,058

Selected orders of multiple quantum transitions may be detected by preparing a statistical non-equilibrium state of an assembly of gyromagnetic resonators of selected phase with respect to a subsequent mixing pulse. After an evolution period, t.sub.1, a 90.degree. mixing pulse is applied and the free induction decay is sampled as a function of time, t.sub.2. The signal functions S.sub.i (t.sub.1, t.sub.2, .phi..sub.i) is developed for systematically varied values of t.sub.1 and specially selected values of .phi..sub.i. Linear combinations of the signal functions S.sub.i yield, after Fourier transformation a two dimensional spectrum of multiple quantum transitions limited by the choice of phase and linear combination. In another embodiment, a magnetic gradient pulse is applied during the evolution period and resulting multiple quantum transitions yield spectral peaks of width dependent upon the order of such transitions. In particular, all spectral peaks may be removed from resulting spectra save only zero order transitions.