Off-resonance irradiation effect in steady-state NMR saturation transfer

The off-resonance irradiation effect (spill-over effect), occurring in steady-state NMR saturation-transfer experiments, is studied theoretically and experimentally for a two-spin system in chemical exchange, when a contralateral irradiation is applied to record the reference spectrum. The relevant parameter chosen for this study is the saturation-transfer ratio. It is defined as the ratio between the value of one exchanging magnetization, obtained when saturating the other, and that of the same magnetization measured when applying a contralateral irradiation. The theoretical study is carried out via a model based on the Bloch equations modified for chemical exchange and expressed in a doubly tilted single rotating frame. The saturation-transfer ratio is expressed as a function of the saturating RF field magnitude. It is shown that the RF field applied off-resonance during the acquisition of the reference spectrum does not correct the experimental saturation-transfer ratio for the spill-over effect. In fact, the saturation-transfer ratio increases with the magnitude of this field. This result is qualitatively explained by the consequence of the effective magnetic fields' relative orientations upon the amount of exchanged magnetization. The validity of the theoretical description is tested experimentally with a solution of N,N-dimethylacetamide in which chemical exchange arises from internal hindered rotation. An experimental protocol is proposed to detect spill-over and correct it when necessary. The way to describe spill-over theoretically when more than two spins interact by chemical exchange and/or dipolar coupling is also given.