Everyone loves a rainbow and most
people understand, at least roughly, how they work:
raindrops split a beam of white sunlight into rays of colored light,
bending the blueish ones more than the reddish ones to make the
well-known arc in the sky. Rain, then, is a brilliant method for
separating sunlight. Chemists and physicists use a similar method for
separating mixtures of substances into their components, turning them
into beams of particles and then bending them with electricity and
magnetism to make a kind of spectrum of different atoms that are easier to identify. This technique is called mass spectrometry
and it was pioneered by British physicist Francis Aston in 1919.
Let's take a closer look at how it works!
Photo: Rainbows bend short wavelength blue light more than long-wavelength red light. Mass spectrometers work in a very similar way, except they bend beams of ions (charged atoms) instead of beams of light.
Mass spectrometers are much simpler than they look—or sound. Suppose someone gives you
a bucketful of atoms of different chemical elements and asks you
what's inside. You need to separate out the atoms quickly and
efficiently, but how do you do it? Simple! Tip your bucket into a
mass spectrometer. It turns the atoms into ions (electrically charged
atoms with either too few or too many electrons). Then it separates
the ions by passing them first through an electric field, then
through a magnetic field, so they fan out into a spectrum. A computerized
detector tallies the ions in different parts of the spectrum and you can use this information to figure out what
kinds of atoms were originally in your bucket. That's the basic idea,
anyway. In reality, it's a bit more complex than this—there's no bucket, for a start!
Photo: A scientist uses a mass spectrometer in the Aeronomy Laboratory, Air Force Geophysics Laboratory (AFGL). Photo by William W. Magel courtesy of US Air Force.
How does a mass spectrometer work?
There are numerous different kinds of mass spectrometers, all working in
slightly different ways, but the basic process involves broadly the
You place the substance you want to study in a vacuum chamber inside the machine.
The substance is bombarded with a beam of electrons so the atoms or molecules it
contains are turned into ions. This process is called ionization.
The ions shoot out from the vacuum chamber into a powerful electric field (the
region that develops between two metal plates charged to high
voltages), which makes them accelerate. Ions of different atoms have
different amounts of electric charge, and the more highly charged
ones are accelerated most, so the ions separate out
according to the amount of charge they have. (This stage is a bit
like the way electrons are accelerated inside an old-style,
The ion beam shoots into a magnetic field (the invisible, magnetically active
region between the poles of a magnet). When moving particles with an
electric charge enter a magnetic field, they bend into an
arc, with lighter particles (and more positively charged ones) bending more than heavier ones
(and more negatively charged ones). The ions
split into a spectrum, with each different type of ion bent a
different amount according to its mass and its electrical charge.
A computerized, electrical detector records a spectrum pattern showing
how many ions arrive for each mass/charge. This can be used to
identify the atoms or molecules in the original sample. In early
spectrometers, photographic detectors were used instead, producing a
chart of peaked lines called a mass spectrograph. In
modern spectrometers, you slowly vary the magnetic field so each separate
ion beam hits the detector in turn.
How does it work in reality?
Artwork: Mass spectrometer designed by Robert Langmuir. Diagram courtesy of US Patent and Trademark Office.
Although that's a very simplified explanation, it's not too far from what really happens.
Take a look at this drawing of an early mass spectrometer designed by American
electrical and electronic engineer
Dr Robert V. Langmuir [PDF]. in the late 1930s and patented in 1945. I've colored
it to make it easier to follow and used the same numbering as I used up above to
emphasize the similarity. Here's how it works:
A sample of gas (blue) flows into the vacuum chamber (inner orange circle).
The sample is bombarded with electrons to make ions.
The ions are accelerated downward in an electric field (toward the curved electric plate labelled 3).
A magnetic field created by the electromagnet (outer red circle) bends the ions round in a semicircle (yellow).
The ions separate out and are picked up by the electronic detector apparatus (green).
Now why would you want to go separating a beam of atoms into a rainbow?
Like chromatography, with which it's often paired, mass spectrometry is an
important method for identifying the atoms or molecules in complex chemical substances. The inventor of the spectrometer, Francis Aston (1887–1945), used his machine to prove the existence of
many naturally occurring isotopes (atoms of the same element with different numbers of neutrons and different
mass). Apart from this kind of pure scientific research, there are all sorts of everyday fields in which mass spectrometers are indispensable tools, from crime scene investigation to archaeology and from environmental science to drug design.
Photo: A mass spectrometer being used to find the nitrogen content of plants
fed with fertilizer, photographed at the International Atomic Energy Agency (IAEA) in 1963. Photo by courtesy of International Atomic Energy Agency (IAEA), published on Flickr under a Creative Commons (CC BY-SA 2.0) licence.
Mass spectrometers are an obvious way of investigating situations where strange chemicals suddenly appear—and figuring out exactly what those things are. Materials scientists use them to identify the precise chemical composition of samples, which can help to explain why buildings or bridges collapse unexpectedly or why engineering components have suddenly failed.
Forensic science relies on being able to identify unusual substances found at a crime scene and match them precisely with similar substances found elsewhere. So, for example, if traces of explosive can be found at an airplane crash site, investigators can probably rule out mechanical failure. If detectives can go further, and trace the explosive's unique chemical signature to a particular place or person, with the help of mass spectrometry, that powerful evidence could lead to a successful criminal conviction. Security is a very closely related application, but works in a more pro-active way (aiming to prevent attacks altogether). Mass spectrometers are increasingly being used for things like baggage scanning and checking traces of chemicals found in suspicious mail packages, to identify what are termed CBRNE (chemical, biological, radiological, nuclear, and explosive) threats. And this sort of application is bound to become more important in future.
Photo: A fast atomic bombardment mass spectrometer. This one makes its ions not by bombarding atoms with electrons, but by smashing high-speed atoms into a surface to knock electrons out of them.
Photo by courtesy of US National Library of Medicine Digital Collections.
More optimistically, mass spectrometers can help us design new things that make the world a better place. In biotechnology, mass spectrometers are used for studying how proteins work and for identifying viruses and bacteria much more quickly than traditional culture-based methods allow. US chemist
John B. Fenn (1917–2010) won the 2002 Nobel Prize in Chemistry for jointly developing an improved form of mass
spectrometry called electrospray ionization (ESI), which is used for studying macromolecules (very large and complex molecules) such as proteins. Unlike normal mass spectrometry, this technique involves turning a sample into an aerosol burst of droplets, separating it into individual protein molecules, which can then be analyzed with a mass spectrometer in the usual way.
Widely used in the pharmaceutical industry, mass spectrometers can help chemists design new drugs much more quickly; among other things, they're useful for figuring out how to maintain the level of a chemical in the body for a certain period of time (which is the key to designing drugs that work effectively). Agricultural scientists who breed and genetically engineer plants use mass spectrometers to measure and maintain levels of desirable chemicals in new crops, or to study how quickly things like fertilizers (which have been radioactively labeled) are taken up from the soil.
Photo: A Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer at
Pacific Northwest National Laboratory. This highly accurate machine is used for molecular biology and genetics research. Photo by courtesy of US Department of Energy.
Mass spectrometers can also help us to understand our environment. We can use them to measure how quickly air and water pollution travels or to discover how fast things like pesticides break down when they're released into soil. Geologists use them to analyze rock samples for valuable minerals and to find out the precise chemical composition of oil and gas deposits. Archaeologists—human history detectives—also use mass spectrometers on soil samples, but with the aim of identifying the plant, animal, and mineral materials used in certain places hundreds or thousands of years ago.
So there's much more to a rainbow—at least a rainbow of atoms—than you might think!
Photo: A chemical ionization mass spectrometer being used by NASA scientists
to study air pollution and
climate change. Photo by Tom Tschida courtesy of NASA.
Mass Spectrometry by by James McCullagh and Neil Oldham. Oxford, 2019. Perhaps the best place to go next: a well-digested, comprehensive, illustrated introduction in about 200 pages.
Mass Spectrometry: A Textbook by Jürgen Gross. Springer, 2017. A fully illustrated reference covering the chemistry and theory of mass spectrometry more than the applications.
Mass Spectrometry Handbook by Mike Lee (editor). Wiley, 2012. This 1000-page academic reference has sections covering the use of mass spectrometry in biotechnology, pharmaceuticals, space research, forensics, security, archaeology, geology, and other common applications.
Mass Spectrometry: Principles and Applications (Third Edition) by Edmond de Hoffmann and Vincent Stroobant. Wiley, 2013. Aimed at a similar (undergraduate) audience to the one above, this book has exercises and covers applications.
This small selection covers recent advances in mass spectrometer technology and the sorts of things they can be used for in everyday life:
A Scale for Weighing Single Molecules by Douglas McCormick, IEEE Spectrum, September 7, 2012 California Institute of Technology researchers have developed a nanotechnology mass spectrometer for weighing individual molecules.
Can technology stop terror in the air? by Steven Ashley, Popular Science, November 1985. This old but excellent article from Pop Sci explores the various technologies that were being developed to stop terrorist attacks on airplanes back in the 1980s, including X ray machines and mass spectrometers.
If you're looking for a detailed technical description of how mass spectrometers work, patents are a really good place to start. Here are a few I've picked out from Google Patents:
US Patent #2,370, 673: Mass Spectrometry by Robert Langmuir, Consolidated Engineering Corporation, March 6, 1945. The earliest mass spectrometer patent I've found. Dr Langmuir went on to work for General Electric's Research Laboratories before joining Caltech's Electrical Engineering department.
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