NMR Spectroscopy: A Brief Guide
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Nuclear magnetic resonance spectroscopy, or NMR as it is more frequently called, is a non-destructive analytical technique that enables interrogation of the nature and structure of organic compounds.
How Does NMR Spectroscopy Work?
NMR exploits the magnetic properties, also referred to as “spin”, of certain atomic nuclei to provide information about their immediate environment. Hydrogen nuclei are a popular choice due to their simple single proton and subsequent sensitivity, this is termed proton NMR or 1H-NMR. The hydrogen nucleus, being positively charged, can be aligned to an external magnetic field. The nucleus can be made to flip to oppose the direction of the magnetic field, but this is a less stable state and so, much like a compass needle, will preferentially revert to realign to the magnetic field. However, the application of just the right amount of energy (normally around the energy of a radio wave at 60-100 MHz) will enable the flip, and the precise point at which it occurred can be recorded and plotted. This is called the resonance condition. However, the environment of that nucleus is important, the presence of electrons and protons on neighbouring atoms and their interactions affect the magnetic field and thus the energy required to flip the nucleus. The magnetic field required is therefore a useful guide as to the arrangement of the hydrogen atom and its surrounding environment.
From this data, sample-specific spectra can then be generated, like the example shown, that provide information on:
• Types of atoms present
• Relative amounts of atoms present
• Specific environments of atoms within a molecule
• Purity and composition of a sample
• Structural information about a molecule (constitutional and conformational isomerisation).
An example 1H-NMR spectrum generated from analysis of ethyl acetate. Peak labels are colour coded to correspond to the hydrogen atoms in the molecule they represent.
NMR Spectroscopy is an Important Tool for Food Provenance
NMR spectroscopy can be applied to any sample containing nuclei that have spin, but most frequently it is used by chemists and biochemists to investigate organic molecules. Applications include the chemical manufacturing, pharmaceutical, agrochemical, polymer and food industries, as well as the biological and biochemical research areas. With the increasing legalisation of cannabis and its derivatives, cannabis analysis also represents a growing area of usage.
An exciting area of NMR that is currently under development is its application to food provenance determination as Dr David Ellis, Scientific Officer in NMR spectroscopy at Heriot-Watt University, tells us. “The interest here is not so much detailed assignments, rather trying to spot characteristic ‘fingerprints’ from an extract of a particular food or drink brand that seem to be associated with a specific country or area of origin. For example, at the University of Edinburgh, UK, chemists have recently embarked on a detailed NMR analysis of Scotch Whisky. Whisky is almost all water and ethanol, but with the careful design of experiments, signals due to these can be suppressed. Professor Dusan Uhrin and his student Will Kew (in collaboration with the Scotch Whisky Research Institute) have consequently been able to examine the trace products within different types of Scotch Whisky and are attempting to correlate these with specific areas of production, and even the type of cask used for maturation.” The technique could play an important role in battling the ever-growing problem of food fraud, which is responsible for economic losses and potentially hazardous contaminants entering the food chain.
Advances in NMR Spectroscopy Set to Improve Analysis Speed and Sensitivity
When asked about advances in NMR, Dr Ellis commented “One of the major downsides of NMR is its perceived lack of sensitivity. For example, an almost invisible smear in a flask is likely to provide a respectable infra-red spectrum in a few seconds, whereas NMR work, even the study of 1H NMR the most sensitive nucleus, often requires milligrams of material, and non-trivial experiment times.
A number of approaches have been adopted in order to try and ‘speed-up’ the acquisition of NMR, and/or reduce the amount of sample required. One issue is the relatively poor polarisation of nuclear spins, restricting the absolute number of nuclear spin transitions and therefore the intensity of the signal produced. One technique, called Dynamic Nuclear Polarisation or DNP has attempted to increase the amount of signal produced, by coupling nuclear spins with electrons, the latter exhibiting far greater polarisation. In practice this is not straightforward but may potentially bring benefits moving forward.
A more recent development has provided scope for ‘speeding-up’ NMR dramatically. In Ultra-Fast NMR, the sample (liquid –phase experiments are typically carried out by dissolving the analyte in a deuterated solvent and syringing into a glass tube of 5mm diameter) is ‘divided’ into slices and data collected on each slice, before combining to create the final result. By such means, experiment times may be reduced from hours to seconds.”
With modern instruments, good quality data is beginning to be obtained from samples weighing less than a milligram. Minimal sample processing and preparation also makes the technique a favourable analytical tool in many industrial, commercial and research settings. It therefore seems likely that NMR will continue to feature as an important analytical tool in the food and chemicals industries.