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A rainbow.

Mass spectrometers

by Chris Woodford. Last updated: September 30, 2016.

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.

What is a mass spectrometer?

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: Positive ion/neutral quadruple mass spectrometer in the Aeronomy Laboratory, Air Force Geophysics Laboratory (AFGL).
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?

Diagram showing the five key processes at work in a typical mass spectrometer.

There are numerous different kinds of mass spectrometers, all working in slightly different ways, but the basic process involves broadly the same stages.

  1. You place the substance you want to study in a vacuum chamber inside the machine.
  2. 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.
  3. 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, cathode-ray television.)
  4. 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.
  5. 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?

Labelled diagram from Robert Langmuir's US mass spectrometer patent 2370673, March 4, 1945.

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:

  1. A sample of gas (blue) flows into the vacuum chamber (inner orange circle).
  2. The sample is bombarded with electrons to make ions.
  3. The ions are accelerated downward in an electric field (toward the curved electric plate labelled 3).
  4. A magnetic field created by the electromagnet (outer red circle) bends the ions round in a semicircle (yellow).
  5. The ions separate out and are picked up by the electronic detector apparatus (green).

You can read more about this in the full patent description, (also listed in the references at the end).

What is mass spectrometry used for?

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.

A mass spectrometer at the Seibersdorf laboratory of the International Atomic Energy Agency (IAEA) in 1963.

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: The 7-tesla fourier transform ion cyclotron resonance (FTICR) mass spectrometer.

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.

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, 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.

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!

Find out more

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This small selection covers the sorts of things mass spectrometers are used for in everyday life:


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:

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Woodford, Chris. (2009/2016) Mass spectrometers. Retrieved from [Accessed (Insert date here)]

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