###### Abstract
At the other end of the chain, wherever possible, the amount of chemical(s) used to achieve a given effect should be decreased by a factor of two every five years [6], according to Moore's Law for Chemistry (MLFC) [7]. In this scenario, scientists are thus encouraged to develop or adopt new green analytical methods that (1) contribute to the reduction of pollution, including real-time analysis of pollutants; (2) avoid sample pretreatment or involve greener approaches for sample pretreatment, including safe solvents and auxiliaries; (3) use miniaturization and automation, including flow analysis and microfluidics and (4) use green separation techniques. Based on these aspects, it is now possible to answer the second question: how can NMR-based metabolomics play a key role in this scenario? The High Resolution (HR)-NMR is a successful and functional research technology because of both its peculiarities which are listed in pillar 1 above. HR-NMR spectroscopy is one of the main analytical technologies that operates following the green chemistry guidelines in sample preparation [8]. [PERSON], et al. [9] recognized the NMR as a green method as it requires for metabolomics studies only 10% D\({}_{2}\)O (deuterated water) in 90% of distilled water (H\({}_{2}\)O\({}^{\text{dd}}\)). The solution has the characteristic of being versatile as it can be used in different studies. For example, this aqueous solution has been adopted for urine and serum fluid analysis, as shown by [PERSON], et al. [10] and [PERSON], et al. [11]. The authors used simply a phosphate-buffered saline (PBS) which also includes sodium azide (NaN\({}_{3}\)) as an antibacterial agent and a 20 mM 2-chloropyrimidine-5-carboxylic acid (2 CLPYR5 CA) as the reference standard. Also, in the case of food metabolomics analysis, the NMR spectroscopic method requires simplicity in the analysis of polar metabolites. Their extraction can be performed by using a solution of trichloroacetic acid (TCA) 7% including 10% D\({}_{2}\)O [12; 13]. This application has been described in the work of [PERSON], et al. [14], where the authors show how the NMR satisfies, for the quantification of trimethylamine (TMA) content in fish, all the validation requirements at the same level as the most frequently used methods (as HPLC). Furthermore, the technique has the advantage of being faster and more repeatable, avoiding the use of solvents, such as toluene and formaldehyde, or dangerous reagents, such as picric acid. In addition to the lower environmental impacts in sample preparations, other peculiar characteristics make NMR spectroscopic methods very attractive for metabolomics analysis. Several papers highlight its capability to offer qualitative, as well as quantitative, knowledge about complex biological samples [15] and their high reproducibility. At the same level the NMR weaknesses, such as limited sensitivity and resolution of the spectra, are largely documented [9; 16; 17; 18], and summarized in Table 1 in which we compared the two most important platforms used for metabolomics analysis, such as Mass Spectroscopy (MS) and NMR as well. Anyhow, significant developments have been made to enhance the NMR sensitivity, including microprobes, cryogenically cooled probes [19], and the dynamic nuclear polarization (DNP) approach [20]. Another critical point about this technique regards the huge amounts of cryogens (helium and nitrogen) required that are impacting the environment quite deeply. The increasing growth of cryogen consumption and their limited resources are the challenges that industries will face in the future [21]. Anyhow, the present review is focused on the green aspect concerning NMR-based metabolomics samples' preparation so the cryogens matter has not been considered, even if their reduction is still a big deal.