MASS SPECTROMETRY

also called mass spectroscopy, analytic technique by which chemical substances are identified by the sorting of gaseous ions in electric and magnetic fields according to their mass-to-charge ratios. The instruments used in such studies are called mass spectrometers and mass spectrographs, and they operate on the principle that moving ions may be deflected by electric and magnetic fields. The two instruments differ only in the way in which the sorted charged particles are detected. In the mass spectrometer they are detected electrically, in the mass spectrograph by photographic or other nonelectrical means; the term mass spectroscope is used to include both kinds of devices. Since electrical detectors are now most commonly used, the field is typically referred to as mass spectrometry. Mass spectroscopes consist of five basic parts: a high vacuum system; a sample handling system, through which the sample to be investigated can be introduced; an ion source, in which a beam of charged particles characteristic of the sample can be produced; an analyzer, in which the beam can be separated into its components; and a detector or receiver by means of which the separated ion beams can be observed or collected. Many investigations have been conducted with the help of mass spectrometry. These include the identification of the isotopes of the chemical elements and determination of their precise masses and relative abundances, the dating of geologic samples, the analysis of inorganic and organic chemicals especially for small amounts of impurities, structural formula determination of complex organic substances, the strengths of chemical bonds and energies necessary to produce particular ions, the identification of products of ion decomposition, and the analysis of unknown materials, such as lunar samples, for their chemical and isotopic constituents. Mass spectroscopes also are employed to separate isotopes and to measure the abundance of concentrated isotopes when used as tracers in chemistry, biology, and medicine. also called mass spectroscopy analytic technique by which chemical substances are identified by the sorting of gaseous ions in electric and magnetic fields. A device that performs this operation and uses electrical means to detect the sorted ions is called a mass spectrometer; one that uses photographic or other nonelectrical means is called a mass spectrograph; either may be called a mass spectroscope. Using mass spectrometry with a suitable choice of experimental conditions, it is possible to measure precisely the mass of ions, to show the presence of different isotopes, and to measure the relative abundance of ions in a mixture. Organic chemicals can be made to produce a spectrum of ions from the fragmenting of the parent molecule; by identifying the fragments according to their masses and relative abundances, the structure of the original molecule can be established. Mass spectrometry developed from experiments conducted by J.J. Thomson and others on the behaviour of charged particles in electrical and magnetic fields. Thomson built a form of mass spectrometer known as a parabola spectrograph in 1913. With such a device F.W. Aston demonstrated in 1919 the existence of isotopes by showing that ions of mass 22 found in samples of air were in fact a heavy form of neon (thitherto thought of as mass 20). Mass spectrometers, which operate under high vacuum, consist of four basic parts: a handling system to introduce the unknown sample into the equipment; an ion source, in which a beam of particles characteristic of the sample is produced; an analyzer, in which the particles in the beam are separated according to mass; and a detector, in which the separated ion components are collected and characterized. The most widely used ionization method is electron bombardment, in which electrons striking the sample molecules supply the energy needed to convert them to ions. Separation of the ions is accomplished by mass analyzers, including the magnetic, time-of-flight, and quadrupole analyzers. In magnetic analysis, the ions are accelerated by an electric field and passed into a magnetic field. A charged particle traveling at high speed in a magnetic field follows a curved path, the radius of which depends on the speed of the particle and on its mass-to-charge ratio (m/z). By changing the accelerating voltage (hence the speed of the particle) or the magnetic field strength, ions of different m/z can be collected and measured, yielding a plot of numbers of ions at various masses, or a mass spectrum. A powerful method for the analysis of mixtures containing unknown constituents is the use of mass spectrometry to analyze products of a separation accomplished by liquid or gas chromatography (q.v.). Mass spectrometry is widely used to measure the masses and relative abundances of different isotopes and to determine their relative abundances in various natural or enriched samples. Many compounds are available in which the molecules have an enhanced proportion of a particular isotope, notably the heavy isotopes 2H, 13C, 15N, 17O, and 18O. These are used to label substances involved in biological processes, making possible precise chemical studies of such complex reactions as metabolism, photosynthesis, plant respiration, enzymatic reactions, phosphate-transfer reactions, and the direct application of oxygen in physiological oxidation. The products of such processes are analyzed by mass spectrometry, and the details of the metabolic pathways involved can be worked out from the distribution of heavy isotopes in the resulting molecular fragments. Mass spectroscopy is also used in gas analysis. In particular, the method is widely used for hydrocarbon gases; with the addition of automatic recording, continuous gas analysis is possible for process control in chemical plants. Mass spectrometry can be used as a sensitive method for testing vacuum tightness in high-vacuum equipment. Apparatus under test is connected to a mass spectrometer tuned to detect a particular tracer gas, and this gas (usually helium) is then applied to the apparatus; the spectrometer reading shows where leakages occur. Another mass spectroscopic technique can be used to measure the geologic age of minerals. Since radioactive disintegration of uranium and thorium results in the formation of different lead isotopes, analysis of the proportions of the latter makes possible accurate estimates of the age of the minerals in which they occur. In accelerator mass spectrometry, high-energy particle accelerators are coupled with electrostatic and magnetic mass analyzers to measure rare, low-abundance isotopes. This method has greatly improved the range of radiocarbon dating, enabling the use of much smaller sample sizes. Additional reading F.W. Aston, Mass-spectra and Isotopes, 2nd ed. (1942), is a comprehensive account of the early work that laid the foundations of accurate mass and isotopic abundance measurements. H.E. Duckworth, R.C. Barber, and V.S. Venkatasubramanian, Mass Spectroscopy, 2nd ed. (1986), provides good general coverage of techniques and applications. Ian Howe, Dudley H. Williams, and Richard D. Bowen, Mass Spectrometry: Principles and Applications, 2nd ed. (1981), emphasizes applications to molecular reactions. F. Adams, R. Gijbels, and R. van Grieken (eds.), Inorganic Mass Spectrometry (1988), surveys the most recent experimental developments. J.R. Chapman, Practical Organic Mass Spectrometry (1985), is a simple, up-to-date text. Gordon M. Message, Practical Aspects of Gas Chromatography/Mass Spectrometry (1984), contains a general treatment of practice and application. Peter H. Dawson (ed.), Quadrupole Mass Spectrometry and Its Applications (1976), gives an extensive treatment of theory. Alan G. Marshall and Francis R. Verdun, Fourier Transforms in NMR, Optical, and Mass Spectrometry (1990), very lucidly explains this technique. David Elmore and Fred M. Phillips, Accelerator Mass Spectrometry for Measurement of Long-lived Radioisotopes, Science, 236(4801):543550 (May 1, 1987), surveys this method. Louis Brown