Mass Spectrometry(redirected from Isotopic distribution)
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mass spectrometry[′mas spek′träm·ə·trē]
a method of investigating a substance by determining the mass (more often, the ratios of the mass of the ions to their charge) and quantity of its ions. The set of the values of the masses and their relative content is called the mass spectrum.
The separation of ions of different masses in a vacuum under the influence of electric and magnetic fields is used in mass spectrometry. Therefore, the substance under study above all undergoes ionization. The ionization process is omitted in the study of the ion composition of gases that have already been ionized—for example, in an electric discharge or in the ionospheres of the planets. In the case of liquids and solids, they are first vaporized and then ionized, or surface ionization is used. Positive ions are more often studied, since current methods of ionization make it possible to obtain them in simpler ways and in greater quantities than negative ions. However, negative ions are also studied in many cases.
Mass spectra were first obtained in Great Britain by J. J. Thomson (1910) and later by F. Aston (1919). They led to the discovery of stable isotopes. Mass spectrometry was originally used chiefly to determine the isotopic composition of elements and for precise measurement of atomic mass. Mass spectrometry remains one of the basic methods used to obtain data on nuclear mass and the atomic mass of elements. The variations of the isotopic composition of the elements can be determined with a relative error of ± 10-2 percent, and the masses of nuclei, with a relative error of ±10-5 percent for light elements and ±10-4 percent for heavy elements.
The high accuracy and sensitivity of mass spectrometry as a method of isotopic analysis have led to its application in fields where knowledge of the isotopic elements is essential, above all in nuclear technology. In geology and geochemistry the massspectrometric determination of the isotopic composition of a number of elements (such as lead and argon) underlies the methods used to determine the age of rocks and ore formations. Mass spectrometry is widely used in chemistry for elemental and molecular structural analysis. The first applications of mass spectrometry in chemistry were associated with the works of V. N. Kondrat’ev (1923).
Mass-spectrometric analysis of the elemental composition of a substance is particularly accurate when the substance is vaporized in the form of the original undecayed molecules and an appreciable fraction of the molecules do not decay in the ion source of the mass spectrometer. Then, by using high-resolution mass spectrometers, it is possible, for example, unambiguously to determine the number of atoms of carbon, hydrogen, oxygen, and other elements in a molecule of organic matter from the mass of the molecular ions. lonization by the vacuumspark method is used to analyze the elemental composition of poorly volatile substances. Here high sensitivity (~10-5-10-7percent) and versatility are achieved with moderate accuracy in the determination of component content (from several percent to tenths of a percent). The qualitative molecular mass-spectrometric analysis of mixtures is based on the fact that the mass spectra of molecules of different structure are different, and quantitative analysis, on the fact that the ion fluxes from the components of the mixture are proportional to the content of the components.
At best, the accuracy of quantitative molecular analysis equals the accuracy of isotopic analysis, but quantitative molecular analysis is often difficult because of the coincidence of the masses of various ions that are formed upon ordinary and dissociative ionization of various substances. To overcome this difficulty, “soft” methods of ionization, which produce few ion fragments, are used in mass spectrometers, or mass spectrometry is combined with other methods of analysis, frequently gas chromatography.
Mass-spectrometric molecular structural analysis is based on the fact that, upon ionization of a substance, some fraction of the molecules are converted to ions without being destroyed, and in the process a certain fraction decay into fragments (dissociative ionization, or fragmentation). Measurement of the mass and relative content of molecular ions and ion fragments (the molecular mass spectrum) provides information not only on molecular mass but also on the structure of the molecule.
In the most frequently used method of ionization, by electron collision (involving electrons whose energy is several times greater than the ionization energy), the theory of mass-spectrometric molecular structural analysis is based on the concept that an excited molecular ion that then decays with scission of the weaker bonds in the molecule is formed in such a collision. The state of the theory does not yet allow quantitative predictions of the mass spectrum of a molecule and the sensitivity indexes of the instrument for different substances that are required for quantitative analysis. Therefore, correlation data on the mass spectra of substances of various classes are used to determine the unknown structure of a molecule from its mass spectrum, and also for qualitative analysis; the virtually linear relation between the total probability of ionization and the molecular mass for molecules of one homologous series that are not overly heavy is used for a rough estimate of the sensitivity index. Therefore, in molecular mass-spectrometric analysis the instrument is calibrated, whenever possible, for known substances or mixtures of known composition (in determining isotopic composition, analysis is sometimes possible without calibration for mixtures of known composition because of the relatively small difference in the probabilities of ionization or dissociation of the particles being compared).
In physicochemical research, mass spectrometry is used in studying processes of ionization, the excitation of particles, and other problems of physical and chemical kinetics, and also to determine ionization potentials, the heat of vaporization, and the binding energy of atoms in molecules. Measurements of the neutral and ionic composition of the earth’s upper atmosphere have been made by mass spectrometry (similar measurements of the atmospheric composition of other planets are possible). The use of mass spectrometry in medicine as a speedy method of gas analysis is beginning. The principles of mass spectrometry underlie the design of highly sensitive leak detectors. The high absolute sensitivity of the mass-spectrometric method permits its use for analysis of a very small quantity of a substance (~10-13 g).
V. L. TAL’ROZE