Since its beginnings about 100 years ago, mass spectrometry (MS) has become a virtually ubiquitous research tool. “Scientific breakthroughs made possible by MS have included the discovery of isotopes, the exact determination of atomic weights, the characterization of new elements, quantitative gas analysis, stable isotope labeling, fast identification of trace pollutants and drugs, and the characterization of molecular structure,” says chemistry professor Fred W. McLafferty of Cornell University. This article covers key developments in MS techniques and instrumentation.
The history of MS began with Sir J. J. Thomson of the Cavendish Laboratory of the University of Cambridge, whose studies on electrical discharges in gases led to the discovery of the electron in 1897. In the first decade of the 20th century, Thomson went on to construct the first mass spectrometer (then called a parabola spectrograph) for the determination of mass-to-charge ratios of ions. In this instrument, ions generated in discharge tubes were passed into electric and magnetic fields, which made the ions move through parabolic trajectories. The rays were then detected on a fluorescent screen or photographic plate. Thomson received the 1906 Nobel Prize in Physics “in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases.”
Thomson's protégé, Francis W. Aston of the University of Cambridge, designed a mass spectrometer in which ions were dispersed by mass and focused by velocity--which improved MS resolving power by an order of magnitude over the resolution Thomson had been able to achieve. Aston received the 1922 Nobel Prize in Chemistry for isotope studies carried out with this type of instrument.
Around 1920, professor of physics A. J. Dempster of the University of Chicago developed a magnetic deflection instrument with direction focusing--a format later adopted commercially and still in use today. Dempster also developed the first electron impact source, which ionizes volatilized molecules with a beam of electrons from a hot wire filament. Electron impact ion sources are still very widely used in modern mass spectrometers.
In the 1940s the dominant commercial instrument in the U.S. was the Model 21-101 analytical mass spectrometer, manufactured by Consolidated Engineering Corporation (Pasadena, Calif.). The 21-101, which was based on Dempster’s single-focusing design, was used in the petroleum industry during World War II for quantitative analysis of organic gas mixtures.
“The magnetic sector type instrument was also very important in the early 1940s,” says McLafferty. “This instrument was developed by Professor Alfred O. C. Nier [of the department of physics at the University of Minnesota] during World War II to do isotopic analysis, with separation of uranium-235 from uranium-238 obviously of special importance. Nier isolated by MS the first sample of plutonium (10-9 g), for its first actual characterization. The Calutron, a three-story-high version of Nier's sector instrument, separated uranium-235 for the first atomic bomb. The gaseous diffusion plant at Oak Ridge, Tenn., supplied uranium-235 for the subsequent explosions.”
Mass spectrometers were manufactured during or just after World War II by several companies: Metropolitan Vickers in England (later Associated Electrical Industries, then VG and Micromass); Westinghouse and General Electric in the U.S. (in addition to Consolidated Engineering, mentioned above); and Atlas-Werke (later MAT [Mess und Analysentechnik]) in Germany.
High mass-resolution double-focusing instruments, in which ions are focused for both direction and velocity, were developed “for the purpose of accurately determining the exact atomic weights of the elements and their isotopes,” says chemistry professor Klaus Biemann of the Massachusetts Institute of Technology. One type of double-focusing instrument was developed in the 1930s by professor Josef Mattauch and his student Richard F. K. Herzog of the physics department of the University of Vienna, Austria, and another design was developed by Nier and physicist E. G. Johnson of the University of Minnesota.
“By the 1950s it became clear that the high resolving power of the Mattauch-Herzog and Nier-Johnson geometries would be very useful for the identification of organic compounds,” says Biemann. John Beynon of Imperial Chemical Industries (later chemistry professor at University College Swansea, Wales) made major contributions in this area.
Magnetic deflection instruments--both single-focusing (of the Dempster design) and double-focusing (of the Mattauch-Herzog design but especially of the Nier-Johnson design)--“dominated high performance mass spectrometry well into the 1990s,” says Biemann. “The cheaper time-of-flight (TOF), quadrupole, and ion trap mass spectrometers evolved in parallel to the preponderant and more expensive magnetic deflection instruments.”
The concept of TOF MS was proposed in 1946 by William E. Stephens of the University of Pennsylvania. In a TOF analyzer, ions are separated by differences in their velocities as they move in a straight path toward a collector in order of increasing mass-to-charge ratio. TOF MS is fast, it is applicable to chromatographic detection, and it is now used for the determination of large biomolecules, among other applications.
TOF instruments were first designed and constructed in the late 1940s and mid-1950s. Key advances were made by William C. Wiley and I. H. McLaren of Bendix Corp., Detroit, Mich.--the first company to commercialize TOF mass spectrometers. According to pharmacology professor Robert J. Cotter of Johns Hopkins University School of Medicine, Wiley and McLaren “devised a time-lag focusing scheme that improved mass resolution by simultaneously correcting for the initial spatial and kinetic energy distributions of the ions. Mass resolution was also greatly improved by the 1974 invention by Boris A. Mamyrin [of the Physical-Technical Institute, Leningrad, Soviet Union] of the reflectron, which corrects for the effects of the kinetic energy distribution of the ions.”
When commerical TOF instruments first came out “their performance in resolution was so poor that they never lived up to even single-focusing magnetic instruments,” says Biemann. However, he adds, “this analyzer has been greatly improved recently...to almost match the most sophisticated, and very expensive, double-focusing mass spectrometers.” Ion cyclotron resonance MS
Ion cyclotron resonance MS (ICR MS) is a technique in which ions are subjected to a simultaneous radiofrequency electric field and a uniform magnetic field, causing them to follow spiral paths in an analyzer chamber. By scanning the radiofrequency or magnetic field, the ions can be detected sequentially.
ICR MS was brought to the attention of chemists in the middle to late ‘60s through the work of electrical and computer engineering professor Darold C. Wobschall of the State University of New York at Buffalo, Peter M. Llewellyn of Varian Associates (Palo Alto, Calif.), and chemistry professor John D. Baldeschwieler at Stanford University (now at California Institute of Technology). The technique is particularly applicable to the characterization of ion-molecule reactions.
In 1974, Melvin B. Comisarow and Alan G. Marshall of the department of chemistry at the University of British Columbia, Vancouver, Canada (Marshall is now a chemistry professor at Florida State University, Tallahassee) revolutionized ICR by developing Fourier transform ICR mass spectrometry (FT-ICR MS). The major advantage of FT-ICR MS is that it allows many different ions to be determined at once, instead of one at a time. The technique is also known for its mass resolution, which is higher than that of any other type of mass spectrometer.
The direct coupling of gas chromatography (GC) and TOF MS was achieved in the mid-1950s by Roland S. Gohlke and McLafferty of Dow Chemical Co., Midland, Mich., in collaboration with Wiley, McLaren, and Dan Harrington at Bendix. At about the same time, GC was coupled to a magnetic sector instrument by Joseph C. Holmes and Frank A. Morrell of Phillip Morris, Richmond, Va., among others.
The great utility of modern GC-MS was made possible by the advent in the 1960s of carrier gas separators that removed the GC carrier gas prior to introduction of a sample into the high-vacuum mass spectrometer. Separators were developed independently by Einar Stenhagen (then at Uppsala University, later at Göteborg University, Sweden); Ragnar Ryhage of the Karolinska Institute, Stockholm; and Biemann.
The use of mass spectrometers as GC and liquid chromatography (LC) detectors is very widespread today. Applications of modern GC- and LC-MS include environmental analysis, forensics, drug testing, and pharmacological studies.
One type of instrument that proved to be ideal for coupling to a GC was the quadrupole mass filter, which was first reported in the mid-1950s by the group of physics professor Wolfgang Paul of the University of Bonn, who shared the 1989 Nobel Prize in Physics for his work on ion trapping. In a quadrupole device, a quadrupolar electrical field (comprising radiofrequency and direct-current components) is used to separate ions.
Although quadrupole mass spectrometers are not as accurate and precise as double-focusing instruments, they are fast, which is important for GC detection. But quadrupole instruments have also become very popular as standalone spectrometers. “Certainly, the number of quadrupoles sold and in use today far exceeds the total of all other types of mass spectrometers,” says McLafferty.
Another instrument that Paul originated was the quadrupole ion trap, which can trap and mass-analyze ions using a three-dimensional quadrupolar radiofrequency electric field. An ion trap system was first introduced commercially in 1983 by Finnigan MAT (San Jose, Calif.), originally as a GC detector. Its design was based on technology developed by Finnigan research scientist George C. Stafford and coworkers. In this instrument, ions of increasing mass-to-charge ratio successively become unstable as the radiofrequency voltage is scanned. “Stafford's ‘mass-selective instability’ scanning...converted quadrupole traps from a curiosity to a useful mass spectrometer,” says Comisarow. Today, ion trap instruments serve not only as GC detectors but also as LC-MS detectors and standalone mass spectrometers.
Novel ionization techniques have extended the capabilities of MS beyond those available with the electron impact source. Field ionization, in which a sample is ionized in a strong electric field gradient, was first observed in 1953 by Erwin W. Müller of the department of physics at Pennsylvania State University, University Park. A variation, field desorption--put into practice by Hans D. Beckey of the Institute of Physical Chemistry at the University of Bonn, Germany, in 1959--widened the range of MS by making it possible to study compounds that were involatile or thermally unstable. “Field desorption really opened the door for biological MS by demonstrating feasibility,” says chemistry professor Ronald D. Macfarlane of Texas A&M University, College Station.
Chemical ionization, a process in which ionization occurs as a result of ion-molecule reactions, was first observed in 1913 by Thomson in hydrogen gas, but he didn’t understand the phenomenon at that time and didn’t call it chemical ionization. Chemical ionization MS was first extensively described, characterized, and patented in the mid-1960s by Frank H. Field and Burnaby Munson, then at Esso Research Laboratories. Field is now professor emeritus at Rockefeller University, New York City, and Munson is a professor of chemistry at the University of Delaware. Chemical ionization MS is a “soft” ionization technique in which volatilized molecules are ionized by reaction with reagent gas ions. This process is gentler than electron impact ionization and generates fewer fragment ions.
In tandem MS (MS-MS), a precursor ion is mass-selected and typically fragmented by “collision-induced dissociation” (also called “collisionally activated dissociation”), followed by mass analysis of the resulting product ions. The technique requires two mass analyzers in series (or a single mass analyzer that can be used sequentially) to analyze the precursor and product ions. Tandem MS provides structural information by establishing relationships between precursor ions and their fragmentation products. The collision-induced dissociation procedure was introduced in 1968 by chemistry professors Keith R. Jennings of the University of Warwick, England, and McLafferty, who was then at Purdue University.
The combination of the newer soft ionization methods with collision-induced dissociation is what gives tandem MS its power in the analysis of mixtures--a feature first recognized by chemistry professor R. Graham Cooks of Purdue University, West Lafayette, Ind.
One of the most popular types of tandem MS instrument is the triple quadrupole mass spectrometer, invented at Michigan State University by Richard A. Yost (now a chemistry professor at the University of Florida, Gainesville) and chemistry professor Christie G. Enke (now at the University of New Mexico, Albuquerque). James D. Morrison of Latrobe University, Melbourne, Australia, helped Yost and Enke reduce the technique to practice. Tandem MS “was really popularized by triple-stage quadrupoles introduced first by Finnigan and Sciex (in 1980), followed by Extranuclear and Nermag, and some time later by VG,” says Michael S. Story of ThermoQuest Corp., San Jose, Calif.
A variety of desorption MS techniques have greatly advanced the capabilities of MS. The first of these was secondary ion MS (SIMS), a technique in which a beam of ions is used to ionize molecules on a surface. Richard E. Honig of RCA Laboratories, Princeton, N.J., “was the main driving force in the development of SIMS as an analytical method in the 1950s,” says Cooks.
Dempster first demonstrated the potential value of spark-source MS (SSMS), but that technique did not come of age until the 1950s, when N. B. Hannay described an SSMS instrument for semiconductor analysis. In SSMS, electrical discharges (sparks) are used to desorb ions from samples. The technique was widely used for trace analysis of a wide range of sample types.
In the 1960s, Georges Slodzian of the University of Paris developed the ion microscope, a SIMS instrument that combined spatial and depth resolution along with isotopic analysis, making it possible to obtain high-resolution chemical images. And physics professor Alfred Benninghoven and coworkers at the University of Cologne, Germany (now at the University of Münster, Germany), developed SIMS techniques for analyzing organic compounds.
Other desorption techniques include plasma desorption MS (PDMS) and laser desorption MS (LDMS). PDMS uses very high-energy ions to desorb and ionize molecules in solid-film samples. It was developed in the 1970s by Macfarlane and coworkers. According to Macfarlane, PDMS “was the first MS method to demonstrate feasibility for studying high molecular weight proteins and complex antibiotics.” In LDMS, a photon beam is used to desorb sample molecules. LDMS was developed in the late 1970s by Maarten A. Posthumus, Piet G. Kistemaker, and Henk L. C. Meuzelaar of the FOM Institute for Atomic and Molecular Physics, Amsterdam, the Netherlands.
In 1981 chemistry professor Michael Barber and coworkers at the University of Manchester Institute of Science & Technology, England, developed fast atom bombardment MS (FAB MS), or “liquid SIMS.” In FAB MS, beams of neutral atoms are used to ionize compounds gently from the surface of a liquid matrix, making it possible to obtain spectra of large, involatile organic molecules.
Two recently developed MS techniques have had a major impact on the ability to use MS for the study of large biomolecules: electrospray ionization MS (ESI MS) and matrix-assisted laser desorption/ionization MS (MALDI MS).
In ESI MS, highly charged droplets dispersed from a capillary in an electric field are evaporated, and the resulting ions are drawn into an MS inlet. The technique was first conceived in the 1960s by chemistry professor Malcolm Dole of Northwestern University, Evanston, but it was put into practice in the early 1980s by molecular beam researcher John B. Fenn of Yale University (now at the department of chemistry of Virginia Commonwealth University, Richmond).
MALDI MS, a form of laser desorption MS, was developed in 1985 at the University of Frankfurt, Germany, by professor of biophysics Franz Hillenkamp (now at the University of Münster, Germany) and Michael Karas (now professor of analytical instrumentation at J. W. Goethe University, Frankfurt), and independently by research scientist Koichi Tanaka and coworkers at Shimadzu Corp., Kyoto, Japan. In MALDI, sample molecules are laser-desorbed from a solid or liquid matrix containing a highly UV-absorbing substance.
ESI MS and MALDI MS have made MS increasingly useful for sophisticated biomedical analysis. Applications include: the sequencing and analysis of peptides and proteins (using techniques pioneered by Biemann); studies of noncovalent complexes and immunological molecules; DNA sequencing; and the analysis of intact viruses.
ESI and MALDI have made it possible for large biomolecules to be analyzed by low-cost instruments such as quadrupole, ion trap, and TOF mass spectrometers. This has “democratized” biomedical MS, making it available to hundreds of researchers who lack access to magnetic sector machines, which are much more expensive. “MALDI and ESI now promise a greatly expanded future with molecular characterization of proteins, DNA, and other large molecules, using instruments providing high sensitivity, specificity, and speed at lower cost,” says McLafferty.
A condensed version of this history appeared originally as part of a larger article, “Chemistry Crystallizes Into Modern Science” (Chemical & Engineering News, 12 January 1998, pp. 39-75).