Benchtop NMR spectroscopy has known a major growth over the last decade, thanks to the design of permanent, compact magnets in the 1-2 T range, that provide remarkable performance in terms of resolution and sensitivity. Although resulting spectra are more limited than their high-field counterparts, the achievable structural and quantitative information can be maximized by a clever use of pulse sequences -in particular those involving gradient pulses-and by advanced data processing algorithms. In this chapter, we describe the main characteristics of benchtop NMR spectroscopy in 2020, both in terms of hardware and typical performance. We highlight the most recent methodological improvements in the field, in terms of pulse sequences, hyperpolarization and data processing, which have significantly improved the resolution and sensitivity of benchtop NMR spectrometers. Finally, we discuss major applications of benchtop NMR spectroscopy, for reaction and process monitoring, but also for quality control and profiling. The number of papers in this field in the last few years undoubtedly highlights the major role that benchtop NMR has to play for applications in areas such as food and pharmaceutical industry, flow chemistry, and profiling of complex samples.
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New psychoactive substances (NPS) have become a serious threat for public health in Europe due to their ability to be sold in the street or on the darknet. Regulating NPS is an urgent priority but comes with a number of analytical challenges since they are structurally close to legal products. A number of analytical techniques can be used for identifying NPS, among which NMR spectroscopy is in the limelight. High field NMR is typically used for structural elucidation in combination with others techniques like GC-MS, Infrared spectroscopy, together with databases. In addition to their strong ability to elucidate molecular structures, high field NMR techniques are the gold standard for quantification without any physical isolation procedure and with a single internal standard. However, high field NMR remains expensive and emerging "benchtop" NMR apparatus which are cheaper and transportable can be considered as valuable alternatives to high field NMR. Indeed, benchtop NMR, which has emerged about ten years ago, makes it possible to carry out structural elucidation and quantification of new psychoactive substances despite the gap in resolution and sensitivity as compared to high field NMR. This review describes recent advances in the field of NMR applied to the characterization of NPS. High-field NMR methods are first described in view of their complementarity with other analytical methods, focusing on both structural and quantitative aspects. The second part of the review highlights how emerging benchtop NMR approaches could act as a game changer in the field of forensics.
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