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Benchtop NMRs – Bringing NMR Spectroscopy within Reach By Dr Cameron Chai and Peter Airey, AXT PTY LTD NMR (Nuclear Magnetic Resonance) spectrometers have traditionally been huge instruments that require cryogenic cooling. This makes them expensive to acquire and run, requiring specialist technicians and often difficult to access. In the last six years, benchtop NMR spectrometers have become available. During this time, they have grown rapidly in capability and performance so that they can now perform a significant part of the analytical tasks normally carried out using high field (superconducting) instruments.
What is NMR Spectroscopy? NMR spectroscopy is an analytical chemistry technique. It is used for determining the composition, purity and molecular structure of a sample. It can be used for quantitative analysis of known mixtures and compounds, as well as identifying unknowns, with or without referencing against spectral libraries.
The peak intensity is proportional to the number of nuclei that are resonating at that frequency.
The Evolution of Benchtop NMRs Traditional NMR spectrometers relied on large superconducting magnets, weighing hundreds of kilograms and requiring cryogenic cooling, to generate large fields. These massive magnets were seen as necessary, as bigger magnets provided greater resolution. In the 2000s, advanced in permanent magnet design and technology (samarium-cobalt and neodymium) facilitated the development of benchtop NMRs which can operate at room temperature with no cooling requirements.
Current Benchtop NMRs The X-Pulse by Oxford Instruments is the only broadband instrument allowing the measurement of 1H, 19F, 13C, 31P, 7Li, 29Si, 11B and 23Na on a single probe. It is capable of a wide range of 1D and 2D measurements, for example, experiments as a function of time or temperature and offers amongst the highest resolution (<0.35 Hz / 10Hz) of any benchtop NMR. Its compact size, combined with capabilities and affordability make the instrument and the technology a reality for commercial, industrial and teaching labs.
Applications With the advent of benchtop NMRs, the applicability of the technology has been growing. As a chemical analysis technique, it has found applications both in materials science and other fields for chemical analysis, reaction monitoring, as well as quality control and quality assurance in commercial production environments. Some examples include: Stacked plot of 19F 1D spectra of a sample of Li[PF6] in an organic
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Electrolyte from a Li-ion battery acquired over three hours to monitor progression of the decomposition reactions.
• Battery research • Polymers • Textiles • Construction materials • Chemical engineering
• Agriculture and food • Drugs and illicit substances • Pharmaceuticals/ biopharmaceuticals • Geology, mining and minerals • Fuels • Oil and gas
How does NMR work? When placed inside a powerful magnet, the nuclei of some atoms begin to behave as tiny magnets. When subjected to a broad range of radio waves, the nuclei will resonate with certain frequencies. When an NMR spectra is examined, the frequency at which the nuclei resonates, or where a peak is centred, provides information about the surroundings of the atom in question, that is, neighbouring atoms and their relative locations. Atoms in close proximity to one another can also cause each other to resonate. Looking at these cross peaks allows the determination of 3D structures. 32 | DECEMBER 2021
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Summary Benchtop NMR is now a viable alternative to much larger and expensive systems. With the improved affordability, performance, and no requirements for special infrastructure, benchtop NMRs, such as the X-pulse, can be easily located alongside related instrumentation, resulting in improved accessibility and convenience, increasing the appeal and applicability of NMR spectroscopy. WWW.MATERIALSAUSTRALIA.COM.AU
