Diffusion-ordered spectroscopy (DOSY) is a powerful NMR method for the analysis of mixtures. In DOSY, signals in the NMR spectrum are resolved according to the measured diffusion coefficient for each signal, yielding a 2D spectrum which has chemical shift along the x-axis and diffusion coefficient along the y-axis.
Unfortunately, overlap between signals in the NMR spectrum can significantly limit the resolution in the diffusion dimension of DOSY spectra.
This is a particular problem in 1H spectra, where broader lines due to J-splittings mean that overlap is common. Fig. 1 shows a 1H spectrum of L-menthol and R-(+)-limonene. The region of 1.3-2.2 ppm (red box), in particular, is heavily overlapped Recently, a new class of so-called "Pure shift" experiments has emerged that greatly reduces signal overlap by removing J-splittings in the spectrum to give single lines instead of broad multiplets for each peak. Fig. 2 shows the region of 1.3-2.2 ppm collected using a standard 1H experiment (top) and a Pure shift (PSYCHE) experiment (bottom).
Pure shift and DOSY methods can be combined to enable the measurement of Pure shift DOSY spectra with much higher resolution in the diffusion dimension than conventional DOSY spectra. Fig. 3 shows the Pure shift DOSY spectrum of the same mixture. The 2D spectrum is shown on the left and the 1D slices of each component on the right. The signals of L-menthol and R-(+)-limonene have been completely resolved.
Fig. 1: 1H spectrum of L-menthol and R-(+)-limonene
Fig. 2: Application of PSYCHE Pure shift method significantly reduces signal overlap
Fig. 3: Pure shift DOSY spectrum of L-menthol and R-(+)-limonene
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