Atoms have nuclei (red) of protons and neutrons surrounded by electrons (green).
In nuclear magnetic resonance (NMR), magnetic nuclei are placed in a static (unchanging) magnetic field and then subjected to electromagnetic (EM) radiation. If the EM radiation is at the characteristic (resonance) frequency, the magnetic nuclei will absorb and re-emit EM radiation at this frequency. Detailed analysis of the various resonance frequencies observed for a NMR sample can be used as a diagnostic tool in chemistry. It is also the basis for the magnetic resonance imaging (MRI) scans done in hospitals.
Particle Magnetic Fields
The strength of a magnetic field and the pattern of its magnetic field lines are described scientifically by giving the magnetic-moment characteristics of the field. Protons, neutrons and electrons all have a measurable magnetic dipole moment. This means they are surrounded by a magnetic field similar to that produced by a simple bar magnet with a north and south pole. The particle's magnetic moment points in the same direction as an arrow (or vector) pointing from the particle's "south" to its "north" pole and is used to describe its orientation.
Magnetic Energy States
In a non-magnetic environment, the magnetic-moment vectors of a group of particles have no preferred orientation and randomly point in any direction. If the particles are placed in an external magnetic field, the particles will align their magnetic-moment vectors to point in either the same or the opposite direction as the field. Particles aligned with the magnetic field have a slightly lower energy than those anti-aligned. The energy difference between aligned and non-aligned states depends on the strength of the external field. The stronger the field is, the larger the energy difference will be.
Energy Transitions
Particles in an atom can move to a higher (or lower) energy state by absorbing (or emitting) electromagnetic radiation whose energy exactly matches the energy change of the particle. The energy of EM radiation depends only on its frequency, so a given energy change for a particle corresponds to a very specific characteristic frequency of the associated EM radiation.
NMR Basics
The simplest atomic nucleus is hydrogen, which consists of a single proton. When NMR is used to analyze organic compounds or for medical imaging, it is usually the hydrogen signal that is being analyzed, so this is the example used here. When a sample containing hydrogen atoms is placed in an external magnetic field, the sample polarizes as the protons align with or against the field. The sample is then subjected to an electromagnetic field. If the frequency of the EM field matches the characteristic frequency of the energy needed to flip the proton orientation, many of the lower-energy protons will absorb EM radiation and move to the higher-energy state. When the EM field is turned off, protons will emit EM radiation at this same characteristic frequency, returning to their lower-energy state.
NMR Spectroscopy
The characteristic NMR frequency of a hydrogen nucleus in a sample is also affected by the number and types of other atoms surrounding that particular hydrogen. Thus, the NMR frequencies of the emitted EM radiation can be analyzed to determine the structure of organic compounds or to map the density of tissues for medical imaging using magnetic resonance imaging (MRI). The EM frequencies absorbed by hydrogen atoms in organic molecules are in the radio frequency (RF) region. This is the low-energy end of the EM spectrum, making MRI a non-invasive and low-risk medical procedure.
Tags: magnetic field, characteristic frequency, magnetic resonance, change particle, electromagnetic radiation
