In its most basic form diffusion ordered NMR spectroscopy (DOSY) relies on the differences in the hydrodynamic radius to distinguish species. If these species differ significantly in size then the method can be very effective, however in the case of isomers it often fails.
By introducing a co-solute which interacts to a greater degree with one or more of the isomers they can be resolved in the diffusion domain and identified by virtue of an effective change in hydrodynamic radius of the species in solution. This method is known as Matrix Assisted Diffusion-Ordered NMR Spectroscopy (MAD).
Matrix Assisted Diffusion-Ordered NMR Spectroscopy exploits chemically specific interactions to change the diffusion behaviour of analytes in a mixture, for example using interactions with surfactant micelles to resolve the signals of isomers that would normally have the same diffusion coefficient.
Tormena, CF, Evans R, Haiber S, Nilsson M, Morris GA, 2010, Matrix-assisted diffusion-ordered spectroscopy: mixture resolution by NMR using SDS micelles. Magnetic Resonance in Chemistry. 48(7):550-553.
Evans, R, Haiber S, Nilsson M, Morris GA, 2009. Isomer Resolution by Micelle-Assisted Diffusion-Ordered Spectroscopy. Analytical Chemistry. 81(11):4548-4550