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An analytic technique concerned with the measurement of the interaction (usually the absorption or the emission) of radiant energy with matter, with the instruments necessary to make such measurements, and with the interpretation of the interaction both at the fundamental level and for practical analysis.

A display of such data is called a spectrum, that is, a plot of the intensity of emitted or transmitted radiant energy (or some function of the intensity) versus the energy of that light. Spectra due to the emission of radiant energy are produced as energy is emitted from matter, after some form of excitation, then collimated by passage through a slit, then separated into components of different energy by transmission through a prism (refraction) or by reflection from a ruled grating or a crystalline solid (diffraction), and finally detected. Spectra due to the absorption of radiant energy are produced when radiant energy from a stable source, collimated and separated into its components in a monochromator, passes through the sample whose absorption spectrum is to be measured, and is detected. Instruments which produce spectra are variously called spectroscopes, spectrometers, spectrographs, and spectrophotometers. See Spectrum

Interpretation of spectra provides fundamental information on atomic and molecular energy levels, the distribution of species within those levels, the nature of processes involving change from one level to another, molecular geometries, chemical bonding, and interaction of molecules in solution. At the practical level, comparisons of spectra provide a basis for the determination of qualitative chemical composition and chemical structure, and for quantitative chemical analysis.

Origin of spectra

Atoms, ions, and molecules emit or absorb characteristically; only certain energies of these species are possible; the energy of the photon (quantum of radiant energy) emitted or absorbed corresponds to the difference between two permitted values of the energy of the species, or energy levels. (If the flux of photons incident upon the species is great enough, simultaneous absorption of two or more photons may occur.) Thus the energy levels may be studied by observing the differences between them. The absorption of radiant energy is accompanied by the promotion of the species from a lower to a higher energy level; the emission of radiant energy is accompanied by falling from a higher to a lower state; and if both processes occur together, the condition is called resonance.


Spectroscopic methods involve a number of instruments designed for specialized applications.

An optical instrument consisting of a slit, collimator lens, prism or grating, and a telescope or objective lens which produces a spectrum for visual observation is called a spectroscope.

If a spectroscope is provided with a photographic camera or other device for recording the spectrum, the instrument is called a spectrograph.

A spectroscope that is provided with a calibrated scale either for measurement of wavelength or for measurement of refractive indices of transparent prism materials is called a spectrometer.

A spectrophotometer consists basically of a radiant-energy source, monochromator, sample holder, and detector. It is used for measurement of radiant flux as a function of wavelength and for measurement of absorption spectra.

An interferometer is an optical device that measures differences of geometric path when two beams travel in the same medium, or the difference of refractive index when the geometric paths are equal. Interferometers are employed for high-resolution measurements and for precise determination of relative wavelengths. See Interferometry

Methods and applications

Since the early methods of spectroscopy there has been a proliferation of techniques, often incorporating sophisticated technology.

Acoustic spectroscopy uses modulated radiant energy that is absorbed by a sample. The loss of that excess produces a temperature increase that can be monitored around the sample by using a microphone transducer. This is the optoacoustic effect.

In astronomical spectroscopy, the radiant energy emitted by celestial objects is studied by combined spectroscopic and telescopic techniques to obtain information about their chemical composition, temperature, pressure, density, magnetic fields, electric forces, and radial velocity.

Atomic absorption and fluorescence spectroscopy is a branch of electronic spectroscopy that uses line spectra from atomized samples to give quantitative analysis for selected elements at levels down to parts per million, on the average.

Attenuated total reflectance spectroscopy is the study of spectra of substances in thin films or on surfaces obtained by the technique of attenuated total reflectance or by a closely related technique called frustrated multiple internal reflection. In either method the radiant-energy beam penetrates only a few micrometers of the sample. The technique is employed primarily in infrared spectroscopy for qualitative analysis of coatings and of opaque liquids.

Electron spectroscopy includes a number of subdivisions, all of which are associated with electronic energy levels. The outermost or valence levels are studied in photoelectron spectroscopy. Electron impact spectroscopy uses low-energy electrons (0–100 eV).

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), and Auger spectroscopy use x-ray photons to remove inner-shell electrons. Ion neutralization spectroscopy uses protons or other charged particles instead of photons. See Auger effect, Surface physics

Fourier transform spectroscopy is a technique that has been applied to infrared spectrometry and nuclear magnetic resonance spectrometry to allow the acquisition of spectra from smaller samples in less time, with high resolution and wavelength accuracy.

Gamma-ray spectroscopy employs the techniques of activation analysis and Mössbauer spectroscopy. See Mössbauer effect, Neutron spectrometry

Information on processes which occur on a picosecond time scale can be obtained by making use of the coherent properties of laser radiation, as in coherent anti-Stokes-Raman spectroscopy. Laser fluorescence spectroscopy provides the lowest detection limits for many materials of interest in biochemistry and biotechnology. Ultrafast laser spectroscopy may be used to study some aspects of chemical reactions, such as transition states of elementary reactions and orientations in bimolecular reactions. See Laser spectroscopy

In mass spectrometry, the source of the spectrometer produces ions, often from a gas, but also in some instruments from a liquid, a solid, or a material absorbed on a surface. The dispersive unit provides either temporal or spatial dispersion of ions according to their mass-to-charge ratio.

In multiplex or frequency-modulated spectroscopy, each optical wavelength exiting the spectrometer output is encoded or modulated with an audio frequency that contains the optical wavelength information. Use of a wavelength analyzer then allows recovery of the original optical spectrum.

When a beam of light passes through a sample, a small fraction of the light exits the sample at a different angle. If the wavelength of the scattered light is different than the original wavelength, it is called Raman scattering. Raman spectroscopy is used in structural chemistry and is a valuable tool for surface analysis. A related process, resonance Raman spectroscopy, makes use of the fact that Raman probabilities are greatly increased when the exciting radiation has an energy which approaches the energy of an allowed electronic absorption. See Raman effect

In x-ray spectroscopy, the excitation of inner electrons in atoms is manifested as x-ray absorption; emission of a photon as an electron falls from a higher level into the vacancy thus created is x-ray fluorescence. The techniques are used for chemical analysis.


(spek-tros -kŏ-pee) In general, the production and interpretation of spectra. The application of spectroscopy to the study of the light of celestial bodies began in the late 19th century. Astronomical spectroscopy is now used over the whole range of electromagnetic radiation from radio waves to gamma rays. It is the main source of information on the composition, temperature, and nature of celestial bodies.

The lines and bands in emission and absorption spectra are characteristic of the atoms, molecules, and ions producing them, and spectral analysis leads to the identification of these components in planetary atmospheres, comets, stars, nebulae, galaxies, and in the interstellar medium. Measurements of the intensities of spectral lines (spectrophotometry) can give quantitative information on the chemical composition.

In addition, spectroscopy yields information on the physical conditions and the processes occurring in celestial bodies. For instance, temperature may be measured by the vibrational excitation of molecules (if present) through analysis of the intensities of the individual lines in their band spectra. The degree of ionization can be used to measure higher temperatures. The width and shape of spectral lines indicates temperature, movement, pressure, and the presence of magnetic fields (see line broadening; Zeeman effect). The processes occurring are often directly responsible for the production of the spectra, as in the recombination of ions and electrons in H II regions or synchrotron emission from electrons in magnetic fields. See also Doppler effect; redshift.



the branch of physics that studies the spectra of electromagnetic radiation. Spectroscopic methods are used to investigate the energy levels of atoms, molecules, and macroscopic systems formed from atoms and molecules and to study quantum transitions between the energy levels. These methods provide important information on the structure and properties of matter. Spectrum analysis and astrophysics are important areas of application of spectroscopy.

The origin of spectroscopy can be traced to 1666, when I. Newton first decomposed sunlight into a spectrum. In the early 19th century, Fraunhofer lines, which are absorption lines in the solar spectrum, were discovered and investigated. In 1859, G. R. Kirchhoff and R. Bunsen established a connection between emission spectra and absorption spectra; on the basis of this connection they developed spectrum analysis. Spectrum analysis made possible for the first time determination of the composition of astronomical objects, such as the sun, stars, and nebulas. From the second half of the 19th to the early 20th century, spectroscopy continued to develop as an empirical science; an enormous amount of data was accumulated on the optical spectra of atoms and molecules, and regularities in the arrangement of spectral lines and bands were established. In 1913, N. Bohr explained these regularities on the basis of the quantum theory, according to which spectra of electromagnetic radiation arise in quantum transitions between energy levels of atomic systems in accordance with the Bohr postulates (seeATOMIC PHYSICS). Spectroscopy subsequently played an important role in the creation of quantum mechanics and quantum electrodynamics, which in turn became the theoretical basis for modern spectroscopy.

The field of spectroscopy can be subdivided according to various criteria. The following branches are distinguished according to the ranges of wavelengths or frequencies of the electromagnetic waves: radio-frequency spectroscopy, which encompasses the entire region of radio waves; optical spectroscopy, which studies optical spectra and includes infrared spectroscopy, the spectroscopy of visible radiation, and ultraviolet spectroscopy; X-ray spectroscopy; and gamma-ray spectroscopy. The specific features of each branch are based on the properties of the electromagnetic waves of the corresponding range and on the methods of producing and studying the waves. Radio-frequency spectroscopy makes use of radio-engineering methods, X-ray spectroscopy involves the use of techniques for the production and study of X rays, gamma-ray spectroscopy makes use of experimental methods of nuclear physics, and optical spectroscopy employs optical methods in conjunction with methods of modern electronics. The term “spectroscopy” is often understood to mean only optical spectroscopy.

Branches of spectroscopy are also distinguished according to the different experimental methods used. In optical spectroscopy, for example, a distinction is made between interference spectroscopy, which is based on the use of interference and interferometers, vacuum spectroscopy, Fourier spectroscopy, and laser spectroscopy, which is based on the use of lasers. One of the branches of ultraviolet and X-ray spectroscopy is photoelectron spectroscopy, which is based on analysis of the energies of electrons ejected from a substance during the absorption of ultraviolet and X-ray photons.

According to the types of systems studied, spectroscopy is divided into atomic spectroscopy, which studies atomic spectra; molecular spectroscopy, which studies molecular spectra; and spectroscopy of substances in the condensed state (in particular, crystal spectroscopy).

According to the forms of motion in the molecule (electronic, vibrational, or rotational), molecular spectroscopy is divided into electronic, vibrational, and rotational spectroscopy. Similarly, electronic and vibrational crystal spectroscopy are distinguished. Optical, X-ray, and radio-frequency methods are used in atomic, molecular, and crystal spectroscopy.

A special area of research is nuclear spectroscopy, which includes gamma-ray, alpha-ray, and beta-ray spectroscopy; of these three, only gamma-ray spectroscopy has to do with spectroscopy of electromagnetic radiation.


El’iashevich, M. A. Atomnaia i molekuliarnaia spektroskopiia. Moscow, 1962.
Herzberg, G. Spektry i stroenieprostykh svobodnykh radikalov. Moscow, 1974. (Translated from English.)



The branch of physics concerned with the production, measurement, and interpretation of electromagnetic spectra arising from either emission or absorption of radiant energy by various substances.


The study of electromagnetic radiation.
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