Transcending\nResolution Limits in HPLC and Diffusion\nNMR

Mixture analysis\nis crucial in many areas of chemistry, and a wide\nvariety of separation methods are in use. A common method for physical\nseparation is high-performance liquid chromatography (HPLC), but resolution\nis a problem: chemically similar species coelute. An alternative approach\nis diffusion-ordered NMR spectroscopy (DOSY), in which the signals\nof mixture components are separated according to the diffusion coefficient.\nAgain, separation is limited if species diffuse similarly or have\noverlap in their NMR spectra. Using the two techniques in combination\ncan resolve both NMR spectra and the elution profiles of individual\ncomponents, even where both techniques fail when used in isolation.\nRecording diffusion NMR data as a function of HPLC retention time\ngives a three-dimensional (3D) data set that can be analyzed using\nmultiway statistical methods. PARAFAC analysis of diffusion NMR data\nmeasured from HPLC eluate for commercial “monoacetin”\n(a mixture of glycerol and its mono-, di-, and triacetates) yielded\nfully resolved and quantitative NMR spectra and elution profiles for\nall four components, whereas neither HPLC nor diffusion NMR applied\nindependently was able to resolve the components.