Biexponential fitting of diffusion-ordered NMR data: practicalities and limitations.

High resolution diffusion-ordered NMR spectroscopy (HR-DOSY) generally uses monoexponential fitting of the diffusional attenuation of pulsed-field gradient stimulated echo spectra and can, thus, only give proper separation of signals in the diffusion domain where the individual peaks are well-resolved in the spectral domain. In principle, it should be possible to resolve the decays of coincident spectral peaks by multiexponential fitting, but the well-known fundamental difficulties in such a separation are exacerbated by instrumental distortions of the signal decay. The limitations imposed on biexponential fitting by finite signal-to-noise ratio and by the distortion of the theoretical form of the signal attenuation as a result of pulsed-field gradient nonuniformity are explored, and the improvement in DOSY spectra produced using biexponential fitting afforded by compensating for the latter are illustrated.