Nuclear magnetic resonance (NMR) spectroscopy enables scientists to study the structure, properties, and dynamics of molecules by measuring the response of nuclei to a magnetic field. It is non-invasive and widely used in a range of areas, including medical, pharmaceutical, food, environmental, and energy. This versatile analytical technique is rapidly advancing, and its effectiveness is often enhanced by coupling with various chromatography methods.
How NMR spectroscopy works
NMR spectroscopy is based on the discovery that certain atomic nuclei behave like tiny magnets as a result of nuclear spinning. When placed in an applied magnetic field, these nuclei align parallel or antiparallel to the field. The energy required to change this alignment is proportional to the strength of the magnetic field.
An NMR spectrometer comprises three major parts: a magnet, a radio frequency (RF) transmitter, and an RF receiver. A sample is placed in a tube or probe and inserted into the magnet. The magnet produces a stable, uniform magnetic field, which causes the sample nuclei to align in a specific direction. The RF transmitter generates a pulse that excites the nuclei and flips their alignment. While returning to their original alignment, the nuclei emit electromagnetic radiation, which is detected by the RF receiver and converted into an NMR spectrum.
The NMR spectrum plots the signal intensity with respect to RF frequency. The difference between a peak’s corresponding RF frequency and the reference frequency is known as the chemical shift. Each peak corresponds to a particular nucleus, and the chemical shift enables identification of the nucleus and its chemical environment, for example, the types of bonds it is involved in or the solvent surrounding it.
NMR spectroscopy also reveals information about the dynamics of a compound, such as the mobility of constituent atoms and the rates of chemical reactions. These findings require techniques such as relaxation measurements—determining the rate at which the nuclei relax back to their original alignment—or studying the nuclear Overhauser effect, which involves measuring the interaction between different nuclei.
Advantages and limitations of NMR spectroscopy
NMR spectroscopy has a broad range of applications. Common uses include determining the structure of drugs, proteins, nucleic acids, and other molecules and studying the dynamics of chemical reactions. And because NMR spectroscopy provides information about an atom’s chemical environment, it is useful for studying catalytic processes. This technique also serves as the basis for magnetic resonance imaging (MRI), a widely used diagnostic tool. Meanwhile, material scientists use NMR spectroscopy to investigate the properties and structures of polymers, ceramics, and more.
As with all techniques, NMR spectroscopy has its limitations. Primarily, it is restricted to measuring nuclei with an odd number of protons or neutrons, although samples can be enriched with detectable isotopes, such as carbon-13 and nitrogen-15. In addition, NMR spectroscopy has relatively low sensitivity, and experiments can be expensive and time-consuming.
So, what are some of the key advantages of using this analytical technique over others? NMR spectroscopy can provide qualitative and quantitative data in a single run. What’s more, this technique is non-destructive, can handle a range of sample types, and requires minimal sample preparation. It is also highly reproducible and offers the opportunity for a high level of automation. Overall, this is a powerful technique that provides pivotal insights in many fields.