The combined use of constrained total-line-shape 1H NMR and LC–MS/MS for quantitative analysis of bioactive components in yellow onion

Onions (Allium cepa L.) are an important source of bioactive compounds including flavonoids, fructo-oligosaccharides (FOS), thiosulfinates and other sulfur compounds, and many of these compounds have potential beneficial properties for human health. Onion metabolites have traditionally been analyzed separately using high-performance liquid chromatography (HPLC) and mass spectrometry (MS). However, it has recently been found that nuclear magnetic resonance (NMR) spectroscopy represents a powerful tool for the simultaneous analysis of a large number of compounds from plants. Although the NMR quantification method provides excellent quantification of compounds with separated spectral lines, the method often produces overlapping signals, and an improved quantification method is needed. Here, we used a constrained total-line-shape (CTLS) approach to quantify overlapping metabolites from onion proton nuclear magnetic resonance (1H NMR) spectra. This method permitted the identification and quantification of amino acids, organic acids, and sugars; however, aromatic compounds excluding aromatic amino acids were difficult to analyze by NMR spectroscopy because of the small signal intensities produced by flavonols and their sugar derivatives. Metabolites present at low concentrations were successfully analyzed using the developed mass spectrometry method operating in the positive mode. The 1H NMR method developed here is an efficient technique for use in simultaneously analyzing compounds in a food matrix.