A History of the Mass Spectrometer

Keith A. Nier
from the "Instruments of Science: A Historical Encyclopedia." Robert Bud and Deborah Jean Warner, editors. 1998. New York & London: The Science Museum, London, and The National Museum of American History, Smithsonian Institution, in association with Garland Publishing, Inc. Pages 552-56. Contributed on November 15, 1999

Mass spectrometers constitute a large, very diverse, and widely employed and produced class or family of instruments. It is likely that no other type of complex instrument has been as important for so many fields of science in the twentieth century. The defining characteristics of this vast range of devices are operational (or functional) and hypothetical in nature, more than material or structural. As Cooks, Busch, and Glish stated in 1983; "Instruments that go by the name of mass spectrometer are appearing in ever-increasing variety with an astonishing range of applications." A mass spectrometer is whatever operates by process that could be used to produce a mass spectrum, no matter how different its design and processes may be from those of any other mass spectrometer. The mass spectrum of any substance or mixture is a record of the distribution of materials of different masses that can be found when a sample is ionized. The instruments were first created as experiments early in the twentieth century, and proliferated most dramatically both in terms of new types developed and number of machines in use during the last third of the century.

The heart of any mass spectrometer is its analyzer. This is a region of high vacuum through which ions extracted from sample substances are made to move by some kind or kinds of static or oscillating electromagnetic field. Ions of different mass, velocity, and charge are moved differently by the field(s). Taking account of velocity and charge effects, ions of specific masses can be separately collected if the field is carefully controlled, and their numbers and masses precisely determined. Mass spectrometers accordingly must have vacuum systems and arrangements to produce, control, and vary the analyzing field(s), as well as ways to introduce samples, to create ions from the sample and get them into the analyzer, to collect analyzed ions, and to display or record the results. Within these shared requirements, different types, arrangements, and uses of analyzers, different sources of samples, and different modes of ionization have resulted in a remarkably extensive set of different types of mass spectrometers, each of which could warrant its own entry.

"Mass spectrometry," (MS) refers to these machines, to information about them, and to development of techniques of employing them in the several branches of natural science and industry; it labels as well a range of research areas in various fields of chemistry that have been intensively developed with these devices. Some significant instruments earlier in the century that used photographic plates to collect ions were called mass "spectrographs," as distinct from "spectrometers," while "mass spectroscope" and "mass spectroscopy" served as inclusive labels, but that distinction and terminology have become uncommon except in reference works.

The origin of all mass spectrometers was the turn-of-the-century research on kanalstrahlen, the streams of positive ions formed from residual gases in cathode ray tubs, initially found coming through channels cut in the cathode plate. Local magnetic and electrostatic fields differentially deflected these positive rays depending on their mass; they made diverging traces on a photographic plate. J. J. Thomson was the crucial experimenter, and the first evidence for the existence of stable (nonradioactive) isotopes was the most dramatic result. Since World War I, a rough cycle of five recurrent, overlapping processes, phases, or stages of development of mass spectrometers is discernible. These can conveniently be called demonstration, familiarization, routinization, radiation, and diversification. In the first, a mass spectrometer was the experiment; instead of an integrated unit, an instrument used to do work defined in other terms, it was the arrangement of equipment composing essentially the whole experiment. Successful experiments provided particular results, but also demonstrated that apparatus of this design could be made to work. An experimental arrangement that was successfully copied for use in further research (and named) made more scientists familiar with its potentials. When a design was standardized and treated as a reliable entity by more people in more contexts, it became a routine instrument. The instruments were spread in wider areas of applications where their basic viability was not at issue. Inevitable limitations spurred trials of alternative components and designs, generating diverse types of equipment that in turn needed demonstration of viability, founding new lines of instrument development.

The clear demonstration of mass spectrometry came at the end of World War I by Francis W. Aston (who has helped design Thomson's equipment and make it work) and Arthur J. Dempster, in Cambridge and Chicago, respectively. Dempster used a magnetic analyzer that focused ions into an electrical collector, while Aston used both electrostatic and magnetic fields to focus ions on a photographic plate. Their continued work, along with that of Joseph H. E. Mattauch, R. F. K. Herzog, Kenneth T. Bainbridge, and Alfred O. C. Nier, among others, produced major results in atomic and nuclear physics, including discovery of the existence, and measurement of the abundances and masses, of numerous isotopes and the determination of their nuclear stabilities and energies. Such work led to Aston's Nobel Prize (in chemistry) and created some familiarity with such equipment in the 1930s.

Stabilizing the esoteric and "touchy" experimental apparatus into routine instruments and applying them to new kinds of tasks required extensive effort. Most influential in the first cycle were the papers, devices, and students of Alfred Nier (often cooperating with various others) from the end of the 1930s to the early 1950s. Nier incorporated recent developments in vacuum technologies and electronics for power supplies, ion detection, and so on, while providing the foundation for determining the age of the earth. His work significantly improved magnetic focusing instruments, establishing that good results could be obtained with ions being sent merely through a comparatively modest wedge-shaped sector magnetic field rather than having as their entire path a semicircle within an analyzer completely confined between the poles of a massive magnet (prior standard practice). A more practical electron-bombardment ion sources, along with several other crucial aspects of construction and technique, also brought improvements in performance, convenience, and costs. Double-focusing machines, attaining greater precision by adding an electrostatic analyzer, were also greatly refined. During these years machines and expertise spread outward from a few physics laboratories, founding precise geochronology and cosmochronology, facilitating isotopic tracer studies, providing the analytic and vacuum controls that made the Manhattan Project's uranium enrichment facilities workable, and making the instruments common in the petroleum industry. Commercial production of mass spectrometers began in the 1940s.

By 1953, convenient handbooks of design and practice had appeared in the United States, the U. K., Germany, and Russia, and annual conferences of mass spectrometrists began. Instrument designs created for isotopic analysis soon were applied to analysis of complex organic molecules. Meanwhile, the demonstration of quite different instrument types was well underway. Analyzers based on the different times of flight over a set path for different accelerated ions, and others using various types and combinations of fields had some success. In 1953 Wolfgang Paul and his colleagues initiated development of what became the most common type of those mass spectrometers having no magnetic field, the quadrupole mass analyzer (and ion trap), using crossed radio-frequency and electrostatic fields. This eventuated in Paul's Nobel Prize (in physics). The middle and later 1950s saw the first wave of effort to concatenate mass spectrometer with other significant types of instruments, in this case creating various types of gas chromatograph mass spectrometers, which have become the most widely sold of all mass spectrometer instrumentation.

Despite the growing familiarity of these and other types, the great majority of mass spectrometers in service well into the 1960s were magnetic sector or double-focusing machines, mostly producing ions by electron bombardment. A score of companies marketed the standard models, costing thousands and tens of thousands of dollars. The spread of mass spectrometers from physics into geology, chemistry, physiology, and other industries continued. They performed gas analysis in venues as disparate as hospital operating rooms and rockets in the upper atmosphere. From the perspective of earlier decades, their numbers and impact seemed to be growing quite rapidly, but by comparison with later decades, the expansion of numbers, uses, and types had hardly started.

Since then a host of workers have transformed mass spectrometers, and founded an array of new families of machines of vastly enhanced scope and precision, based particularly on different approaches to ion production but also on other modes of analysis. Cooks, Busch, and Glish rightly noted that "latitude in methodology, and characteristics of the hardware which demand interaction with the equipment, have made mass spectroscopists likely to modify instrumentation or to develop entirely new instruments." Each new type has grown (despite the substantial cost of the machines), being commercialized in turn and approaching or surpassing the total growth of the 1950s; three score companies were in the market early in the 1990s.

In the 1960s, chemical ionization mass spectrometry (or CIMS) and field desorption MS (FDMS) emerged, and several mass spectrometry journals began publication. In the 1970s, secondary ionization MS, Fourier transform MS, plasma desorption MS, electrohydrodynamic MS, laser desorption MS, thermal desorption MS, spark source MS, and glow discharge MS were invented or developed significantly, and additional journals were started. At the same time, the scale of the machines went to opposite extremes, with the quest for high performance and the development of tandem MS (in which machines are combined, one serving as a source for the next) leading to large "grand scale" instruments, with particle accelerators being used as new forms of mass spectrometer, with instruments being shrunk for portable medical uses, and others miniaturized for missions to Mars, Venus, Halley's comet, and beyond.

Clearly identifiable developments of the 1980s, beyond still more new journals, included laser resonance ionization MS, matrix-assisted laser desorption MS, fast atom bombardment MS and its continuous flow transformation, the astonishingly sudden development of ion trap MS, a dramatic advance in electrospray MS, as well as a considerable development of liquid chromatography MS. These lists are hardly complete, nor do they include combinations. One review found over ten new types of machine per year appearing in the literature at the start of the 1990s. Yet earlier designs have not been dropped; what are called "Nier-machines" and quadrupoles remain very numerous and productive. And in a crucial sense all these diversified instruments still compose a single class, sharing their fundamental functional characteristic; they all sort ions by mass. Although the inferences drawn from their data vary enormously, they are many different ways of producing the same basic kind of information.

Originally designed to work with atomic isotopes or comparatively light gases, mass spectrometers now deal with a range of substances that is almost unlimited. By the early 1960s the ever-increasing precision in measurement of isotopic masses and abundances drove physicists and chemists to new international standards for atomic mass and weight. Capable of precision to a part in a billion in dealing with the mass of an atom, the instruments also provide extremely precise measurements when dealing with ever larger molecules, along with a flood of structural information, all on the basis of minuscule samples. In the early 1980s it seemed amazing that mass spectrometers could handle ions even with a molecular weight of over 10,000, yet by the 1990s the proven range extended to several hundred thousand, and no material seemed so involatile or unstable as to be beyond the capabilities of all the techniques.

Several applications of mass spectrometers have been mentioned above. To be comprehensive about the uses to which these instruments have been put and the results achieved, at even the most superficial level, requires little less than a survey of the full range of natural scientific endeavor in the twentieth century and some account as well of a wide range of medical, industrial, and governmental concerns. A partial list is all that is suitable here.

Analyses by mass spectrometers are crucial for astronomical studies of the components of our solar system, for all geochronology (including the histories of climates and of life's evolution), for isotope archaeology, and for much else in geophysics and geochemistry as well. No less than the geological fields, chemistry is "awash" in mass spectrometers, for their use constitutes both one of the most precise modes of experimentation and most powerful methods of chemical analysis. The same can be said increasingly of their biological, biochemical, and medical uses. Mass spectrometry is employed in the identification of complex natural products and of metabolic pathways. The capabilities of detecting and identifying mere trace presences in tiny samples have led to use of mass spectrometers in toxicology, drug abuse diagnosis, environmental pollution monitoring, and elsewhere. These instruments have long played a significant role in materials analysis and process monitoring in the petroleum, chemical, and pharmaceutical industries, and they are being used in food processing and electronics industries. Mass spectrometers are the key to non-invasive (thus politically viable) international monitoring of nuclear facilities. They are even returning to some prominence in physics, where they had become less central since determining the masses of stable and unstable nuclides. They are becoming important tools in studies of surface phenomena, and of the solid state several atoms deep, which may well lead to further industrial applications. Finally, mass spectrometers are the key to coping with next to nothing at all, as leak detectors and as the most sensitive gauges for the most extreme vacua we can produce.

Despite this importance, mass spectrometers have received hardly any attention from historians of science and technology, and remain almost totally unknown among the generally educated public. A notable chronicle literature exists, but the history remains largely unexplored. Even in as laudatory a work as Claude Allègre's survey From Stone to Star, in which almost all the growth of our understanding of how the solar system, the earth, the atmosphere, and the biosphere all developed is ascribed to accurate mass spectrometry, the instruments themselves are left highly praised yet essentially undiscussed and invisible.

Bibliography

1. Cooks R.G., Busch K.L., Glish G.L., Mass Spectrometry: Analytical Capabilities and Potentials, Science, 1983, 222, p.273-291
2. Cornides I., Mass Spectrometric Analysis of Inorganic Solids-- The Historical Background Inorganic Mass Spectrometry, New York: Wiley 1988, edited by Adams F., Gijbels R., Van Grieken R., 1-15
3. Falconer I., J.J. Thomson's Work on Positive Rays, 1906-1914, Historical Studies in the Physical and Biological Sciences, 1988, 18, p.265-310
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