Mass spectrometers

by Chris Woodford. Last updated: June 2, 2011.

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: Left: A mass spectrometer at Pacific Northwest National Laboratory. Photo by courtesy of US Department of Energy.

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 and Defense Imagery.

How does a mass spectrometer work?

Artwork showing the five key processes in a 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.

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, cathode-ray television.)
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.

Photo: gas chromatography sample being injected

What is mass spectrometry used for?

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). Mass spectrometry is also widely used in forensic science (to identify samples found at crime scenes), by materials scientists (for example, to study impurities in steel), and with radio-carbon dating to calculate the approximate age of important deposits unearthed by archaeologists.

Photo: Mass spectrometers are often used with gas chromatography machines like this. Photo by courtesy of NASA Kennedy Space Center (NASA-KSC).