Evaluation of pure shift NMR methods for the analysis of complex metabolite mixtures with a benchtop NMR spectrometer†

Abstract

Compact, low-cost and cryogen-free benchtop nuclear magnetic resonance (NMR) spectrometers have become an attractive option for analysing complex biological mixtures. The implementation of spectral simplification methods such as pure shift spectroscopy is particularly important to circumvent the ubiquitous peak overlaps that occur due to the limited resolution of compact instruments. Pure shift (PS) strategies consist of eliminating the signal multiplicity induced by the homonuclear J-couplings observed in 1D ¹H NMR spectra, thereby simplifying the spectral information. This paper provides an analytical evaluation of optimised pure shift spectroscopy pulse sequences for the analysis of metabolite mixtures on a recent benchtop NMR spectrometer. Six PS pulse sequences were carefully evaluated, based on three families of PS techniques: 1D projections of 2D J-resolved spectra, Zangger–Sterk (ZS) and PSYCHE. The methods were evaluated in terms of resolution, sensitivity, spectral purity and repeatability. Among the strategies we explored, 1D projections of 2D J-resolved double-echo (J-RES DE) spectra, combined with an improved processing strategy, appeared to offer the best compromise based on these analytical criteria. The potential of this method was illustrated on a complex sample from the food industry and enabled key metabolites to be detected with improved resolution and sensitivity, showing that PS NMR could be used for a rapid (22 minutes) profiling of complex metabolite mixtures on a benchtop NMR spectrometer.