Spi selection

Phenolic and etherified softwood lignin model compounds were also successfully crosslinked at the meta positions. Special care was taken to assign the structure of the dimer (22) (Fig. 4C). With NMR-SPI (selective population inversion) techniques it was unevocably proved that the methylene linkage was situated at the 6-positions (13). 

The physics behind SPI (Selective Polarization Inversion)77 or SPT (Selective Polarization Transfer)78,79 experiments is described and explained in every current NMR textbook, since it provides a nice introduction to understanding some of the more common current experiments (e.g. INEPT or 2D homo- and heteronuclear correlation experiments). The two names, SPI and SPT, are used indiscriminately for the same experiment, although in general SPI might be considered a special case of an SPT experiment with maximum polarization transfer achieved by inversion78. 

Hayashi, H., Kondo, H., Masumura, K., Shindo, Y, and Nohmi, T. (2003). Novel transgenic rat for in vivo genotoxicity assays using 6-thioguanine and Spi" selection. Environ Mol Mutagen 41, 253-259. 

Nohmi, T., Suzuki, M., Masumura, K., Yamada, M., Matsui, K., Ueda, O., Suzuki, H., Katoh, M., Ikeda, H., and Sofuni, T. (1999). Spi(-) selection An efficient method to detect gamma-ray-induced deletions in transgenic mice. Environ Mol Mutagen 34, 9-15. 

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. 

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