Preface: Natural deep eutectic solvents: A third liquid phase in living organisms? Discovery, theory, biology, and applications

About 20 years ago the concept of metabolomics was introduced with the aim of qualitative and quantitative chemical analysis of all compounds present in an organism or any biological material. This approach should ideally give a total picture of all small molecules in an organism, and complement the data obtained by proteomics, transcriptomics, and genomics. The methods applied are mass spectroscopy (MS) and nuclear magnetic resonance-spectroscopy (NMR), either stand alone or coupled with a chromatographic system (LC-MS, GC-MS, and LC-NMR) (Schripsema & Verpoorte, 1991; Verpoorte, Choi, & Kim, 2007; Wolfender, Rudaz, Choi, & Kim, 2013). These methods are capable of analyzing complex mixtures, and in combination with various chemometric methods new insights can be obtained from the large data sets generated in various experiments. The omics were the beginning of a paradigm shift of doing research, moving from a hypothesis-based strategy to a systems biology approach. The latter is based on as many observations as possible and analyzing the huge data sets by chemometric methods, e.g., to identify which genes, proteins, and compounds correlate with the resistance of a plant against pests and diseases (Kim, Choi, & Verpoorte, 2010; Leiss, Choi, Verpoorte, & Klinkhamer, 2011; Mouden, Klinkhamer, Choi, & Leiss, 2017).