Insensitive nucleus excitation

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

Other features of nuclei that may differ and cause macroscopic substance-like properties to be different include the difference of ortho- and para-hydrogen the para-modification is completely insensitive to the presence of a magnetic field. In addition there are nuclear isomers in which a metastable state of an atomic nucleus is brought about by the excitation of one or more of its nucleons. ... 

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