- Also called:
- mass spectroscopy
- On the Web:
- CiteSeerX - Mass spectrometry - A review (PDF) (Apr. 19, 2025)
The development of electronic techniques for television during the 1930s yielded a device of extraordinary sensitivity for measuring small electron beams—namely, the secondary electron multiplier. Although originally invented for the amplification of the tiny currents from a photocathode, it soon proved to be an excellent detector for ion beams with a sensitivity sufficient to record the arrival of single ions. The fundamental principle of the multiplier is, as the name suggests, a multiplication of the number of electrons emerging from an electrode as compared with the number incident upon it. Electrodes, called dynodes, are so arranged that each succeeding generation of electrons is attracted to the next dynode. For example, if 4 electrons are released at the first dynode, then 16 will emerge from the second and so forth. Gains of as much as one million are easily attained; the noise is limited to the currents originating from the few electrons leaving the first dynode as a result of thermal electron emission. Multipliers were originally constructed with discrete dynodes, a form still in wide use. Continuous dynode multipliers, which use a semiconducting glass to provide the distribution of electrostatic potential, are smaller and perform equally well in most applications. A multiplier can be employed in an analog mode, in which the output current is measured with an electrometer as is any small current, or in a pulse-counting mode, in which individual ions are counted.
The mass spectrum of osmium (Os), obtained using an electron multiplier detector, is shown in . It is a recorder trace of the electrometer output from an electron multiplier detecting OsO3− taken as the field of the analyzing magnet was steadily increased. Owing to their small sizes, the leftmost and two rightmost peaks were recorded with an electrometer gain 100 times what was used for the other peaks; the change in gain is marked by a change in the position of the baseline. The osmium isotopes observed, from left to right, are 184, 186, 187, 188, 189, 190, and 192. The oxygen (O) in the ions results in very small satellite peaks caused by the low abundance isotopes 17O and 18O; the satellite peaks of 192Os are at the right. This trace is typical of machines used in geochronology, where flat-topped peaks are desired rather than high resolution. The irregular signal of the three weak isotopes results from the low rate at which these ions are detected.
Daly detector
In 1960 N.R. Daly introduced a form of detector with properties superior to the electron multipliers described above. In this design the incident ions are attracted to a rounded electrode of a few centimetres in dimension that is held at 10,000 to 20,000 volts negative. The ions strike the “door knob” and release a few secondary electrons for each incident ion; these electrons are then accelerated from the high negative potential to a scintillation crystal mounted on a photomultiplier at ground potential. The electrons generate a light signal in the scintillation crystal that is amplified by the photomultiplier. The output is then treated just as the output of an electron multiplier. The advantage of this more complicated device is an almost complete independence of the signal size with the position of the ion beam in the defining slit that precedes the detector. This is an important property for accurate measurement of isotopic ratios because the invariable instability of the analyzing magnet and the ion-source voltage cause the beam to drift within the limits set by the slit, and so a nonuniform response gives rise to errors in the measured ratios. Electron multipliers require various adjustments, not always satisfactory, to produce a signal size independent of beam position. The Daly detector cannot be used with negative ions.
Multiple detectors
In instances in which the ratio of two or more isotopes is the experimental goal with a magnetic sector analyzer, there are advantages in measuring the currents of the beams simultaneously in multiple detectors rather than switching the magnetic field. This is relatively simple for light elements that have widely spaced beams at the detector location and is often employed in such cases. For heavy elements the small spacing necessitates more difficult techniques that came into routine use only in the 1980s.
Important technical adjuncts
Vacuum
In the devices heretofore described, the presence of a good vacuum system has been assumed. Mass spectroscopy originated at about the time that high vacuum was first attained in the laboratory. High vacuum refers to a pressure low enough that the mean free path (the distance traveled between collisions) of molecules in the residual gas is greater than the dimensions of the vacuum vessel. Mass spectroscopists invariably seek conditions of improved vacuum. The properties that render low pressures desirable include a reduction in the scattering of the beam in the analyzer, which causes interfering background effects and a reduction in the production of spurious beams out of the residual gases, particularly from the organic compounds that are present. The history of vacuum techniques is varied and great and has provided present mass spectrometrists with pressures that are routinely four to five orders of magnitude lower than those first used by Thomson, Aston, and Dempster. The invention of the diffusion pump by the German physicist Wolfgang Gaede in 1915, with important improvements by the American chemist Irving Langmuir shortly thereafter, freed mass spectroscopy from the severe limitations of poor vacuum. During the 1960s diffusion pumps began to be replaced by ion-getter pumps, with turbomolecular pumps becoming common in the 1980s.
Electronics
The operation of a mass spectrometer depends on elaborate electronic equipment: ion sources require extremely stable power supplies, magnets need instruments for measuring the magnetic field and controlling the current supply for the coils, detectors use a variety of power supplies and amplifiers, and general operation requires electronic auxiliary equipment. The rapid increase in the use of mass spectrometers following World War II can likely be attributed in part to the large number of physicists who had gained electronic training during the war, many of whom had utilized mass spectroscopy during that conflict to monitor uranium isotope separation and to analyze aviation gasoline.
Computers
The introduction of small computers for laboratory work during the 1960s altered entirely the manner in which mass spectrometry was performed and widened its applications to an extraordinary degree. Computers were interfaced with spectrometers, making it possible to repeat a measurement schedule on a steady basis and record the data acquired. In organic analysis the computer was programmed to store the spectra of thousands of compounds, allowing rapid identification of the substance under study. Users soon devised ways by which the answers to their questions came within minutes after the conclusion of the analysis.
Applications
Atomic
Atomic masses
The discovery of isotopes with the first mass spectrograph answered the question about the integer value of atoms only to a crude level of accuracy and made all too clear the need for more accurate mass determinations. These were first undertaken by Aston and repeated with increasing precision by succeeding generations. The first data showed slight deviations from an integer law but also showed a quasi-systematic variation as a function of atomic number. In the early 1930s these data explained the energies of nuclear reactions then being observed through the mass-energy relation that had been given two decades earlier by the special theory of relativity. Since that time mass spectroscopy and nuclear physics have combined to determine isotopic masses to a high degree of accuracy. The mass unit now used is defined so that the mass of the carbon-12 isotope is exactly 12 atomic mass units (amu). Nuclear theory is continually tested in its ability to reproduce the observed values. Indeed, these masses provide an early and critical test of nuclear models. Mass spectrometry has been closely associated with nuclear physics since its beginning. A mass spectrometer is frequently found in some form as a part of nuclear experiments to identify reaction products. Large mass spectrometers are employed as isotope separators and are capable of producing weighable amounts of selected stable isotopes that have valuable analytical applications. They are used for labeling compounds so that they can be traced through various chemical, physical, and environmental processes without the problems created by radioisotope tracers. Analysis is, of course, performed by mass spectrometer. In addition, nuclear reactions often produce extremely small amounts of radioactive products, which cannot easily be manipulated by normal chemical procedures. In such cases, stable isotopes of the product are added to the radioactive element, thereby increasing the concentration so that it falls within the range of the analytical method. These enriched isotopes are indispensable for research in nuclear physics.
Geochronology and geochemistry
The early studies of the radioactive decay of uranium and thorium into lead caused the British physicist Ernest Rutherford to suggest that this process could be used to determine the age of rocks and consequently of the Earth by observing the amount of helium retained by a rock relative to its uranium and thorium contents. Mass spectrometers capable of measuring isotopic ratios allow the composition of elements to be determined in which one or more isotopes result from radioactive decay. The age of the rock from which the element has been obtained can be determined if the amount of the parent element can be measured and certain requirements on the environmental history of the rock are met (see dating: Absolute dating).
The Earth’s crust is generally richer in oxygen-18 (18O) than is the mantle, as a result of the reaction of these upper-layer rocks with the hydrosphere and atmosphere. This fact allows oxygen-18 to be used to assess the degree to which ascending magmas have incorporated crustal rocks as they rise to the surface. The use of isotopes has proved especially valuable in understanding the origin and nature of the solar system. A great body of evidence now suggests that meteorites are objects that solidified very early in the history of the solar system. Extinct radioactivities of elements with various half-lives have been identified that set limits on the time between the synthesis of the elements and their condensation. (Extinct radioactivities are nuclides that have nearly completely decayed into their daughter elements.) An example is the excess of magnesium-26 (26Mg) found in primitive meteorites that resulted from the decay of aluminum-26 (26Al), which has a 720,000-year half-life.
