The World of Nuclear Science

Scientific Analysis Techniques

Nuclear magnetic resonance (NMR) was a phenomenon first discovered in 1946, and was refined in the 1950s by the Swiss-born United States physicist Felix Bloch. Since then, it has been applied in sciences ranging from physics to biochemistry, as well as applications such as medicine and forensic science. It is a technique used for analysis of various substances.

Nuclear magnetic resonance is the vibration or resonance of a nucleus, when it is placed in a magnetic field - hence the name nuclear magnetic resonance. Particularly in medicine, it is also known as magnetic resonance imaging. The word 'nuclear' in a medical procedure frightens some people, hence the alternative name! However, nuclear magnetic resonance has nothing to do with radiation or radioactivity.

Nuclei begin to resonate when placed in strong magnetic fields and targeted with specific radio waves. (Energy of the radio waves is absorbed by the nuclei, making it resonate.) This resonance occurs because at certain radio frequencies nuclei will absorb the wave energy, becoming excited from a low energy state to a higher energy state. These different energy states are called magnetic moments. Without the magnetic field, nuclei do not have the magnetic moments: they cannot exist in different energy states, and will therefore not be able to absorb any energy from the radio waves. This is why a magnetic field is used.

If an energy level diagram is drawn for the hydrogen nucleus, its energy can be split into two different levels in the presence of a magnetic field. The change between the levels (moments) requires a certain amount of energy.

in the presence of a magnetic field, some nuclei can have more than one energy state. The difference between these states can be calculated to give values for the moments, based on the frequency of the radio wave.

The energy difference between the two states marked on the graph can be calculated. Physicists have found that energy of radio waves (E) is equal to a constant h (Planck's constant, 6.626 × 10^-34) multiplied by the frequency of the wave f. In other words,

Thus, for the frequencies at which a nucleus will resonate, the corresponding energy can be calculated to give relative values for the magnetic moments.

The specific frequencies at which nuclei resonate can be used to identify them - as a type of "fingerprint". Measurements have been taken by scientists for the frequencies and magnetic moments of many atoms - forming a database that can be used to identify unknown samples.

See MRI in medicine.

Spectroscopy is the analysis of different compounds. Scientists use NMR spectroscopy to study the bonds and structures of molecules.

Hydrogen atoms not bonded to other atoms are called free hydrogen atoms. These absorb energy at the radio wave frequency 42.58 MHz (in the presence of a magnetic field). However, this frequency varies slightly if the hydrogen atom is bonded to other atoms (for example, when hydrogen is in a compound). This variance can be used to determine the structure of an unknown compound.

The atoms surrounding the hydrogen atom in a molecule are collectively called the neighbourhood of atoms. Different neighbourhoods affect the absorbed frequency of the hydrogen atom in different ways, giving rise to a fingerprint for each neighbourhood. Variations in the frequency absorbed due to different neighbourhoods are called chemical shifts. Because of this variation, shifts can be used (in conjunction with other analytical data) to identify a given neighbourhood.

sample NMR spectroscopy output. The graph can be interpreted to deduce the structure of the analysed molecule.

The specific hydrogen atom analysed with this technique is hydrogen-1, because it is the most abundant (over 99% of all hydrogen atoms in existence are of the hydrogen-1 isotope) and it can take on different magnetic moments. However, carbon-13 can also be analysed although this is not common since the abundance of it is very small, and equipment sensitivity to it is about 10,000 weaker than for hydrogen-1.

NMR spectroscopy produces a graph which is then analysed to identify the compound tested. The technique is used to identify unknown compounds, analysing samples obtained from oil exploration, and forensic science. It is a widely used technique in organic chemistry, and has also been used to help establish the chemical structure of benzene - a structure that had eluded scientists for many years.

Radioisotopes emit radiation, and this radiation can be detected using devices including Geiger meters and scintillation counters. The radiation that is produced can be used to identify where the radioisotope is located, without actually "seeing" it.

This technique, known as tracing is similar to following a radioisotope's "footprints" of radiation as it passes through an environment. When the radioisotopes are tracked this way, they are known as tracers.

Often, the technique is utilised to find out how a compound is used up in the bodies of plants or animals. This is done by administering or feeding the subject to be tested with a radioisotope. For example, if scientists were studying how an animal digested a certain food, they would feed it a sample of that food which contained a radioisotope. The simplicity means that the process is easy to use since the experimental results are unaffected by the experiment itself. Bodies of plants or animals cannot distinguish between a radioisotope or a stable isotope. Therefore the food does not need to be "special" - it just needs to contain a radioisotope.

As the radioisotope passes through the body, the trail of radiation that is emitted can be recorded using networks of detectors to show how the radioisotope is consumed.
Alternatively, in a process called autoradiography used often in studying plants, the plant is exposed to a radioactive isotope (for example, placed in an atmosphere filled with carbon dioxide containing the carbon-14 radioisotope). After absorbing the radioactive carbon dioxide, the plant has some of the radioisotope within it. It is then placed on a photographic film. The regions of the plant where the radioisotope has mostly concentrated darkens the film; the regions where the radioisotope does not exist does not affect the film. Overall a map of the location of the radioisotopes is generated. This technique is useful in the study of how plants absorb nutrients, and also has been used by scientists to study DNA replication.

a sample of an autoradiography film darkened by a plant exposed to radioactive carbon dioxide. The white areas indicate where the carbon-14 radioisotope is located.

See Also
Radioactive Dating Techniques