Nonlinear Effects in NMR

Analytical applications of NMR exploit the widely assumed linearity of the NMR response with respect to the spin concentration. Deviations of this linearity are undesired and in this review some current approaches to tackle this problem are given. However, from a physics point of view, the nonlinearity is a more complex affair. Indeed, NMR as a primary representative of coherent spectroscopy can be viewed as an inherently nonlinear method, in particular with respect to the excitation amplitude. Even excluding this intrinsic property, amplitude response is not always found to be linear and several distinct fundamental nonlinearity or nonadditivity effects can be caused by spin interactions, which can either be internal to the specimen, or owed to feedback from the observation electronics as summarized here with references to the latest literature. Ways to avoid nonlinear response for quantitative NMR applications are summarized briefly.