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Weighing Atoms: The World’s Most Sensitive Scale

Learn why the Pliomax team uses strontium to date fossils


Current flows through the filament inside the mass spectrometer. Strontium daubed onto the filament is vaporized, releasing atoms that are accelerated through the device

Photo by Simon Werdmuller von Elgg


Watch a brief animation explaining how a mass spectrometer is used to measure the proportions of isotopes in a sample.

In order to date fossil beaches, the Pliomax team must determine the relative abundance of the two isotopes strontium-86 and strontium-87 contained in shells. Chemically, the two isotopes appear identical. Therefore, the researchers can’t separate the two isotopes using chemical reactions. Instead, they distinguish between the two kinds of strontium by the isotopes’ distinct weights. With its single additional neutron in each atom, strontium-87 weighs ever so slightly more than strontium-86.

Sorting atoms by weight is no easy matter. The Pliomax team uses a state-of-the-art mass spectrometer designed specifically to separate and count isotopes of strontium. The device is built around an airtight “L” shaped stainless steel chamber. The strontium sample, dabbed onto a fine coil of wire like the filament of an incandescent light bulb, is loaded in at the short end of the chamber. The apparatus is sealed shut and the air evacuated. An electrical current turns the filament white hot, vaporizing the strontium. Electrons fired at the vapor transform the strontium atoms into charged particles, or ions. The ions are focused into a narrow beam and accelerated into a magnetic field, where the chamber makes a 90-degree turn. The magnetic field bends the path of the ions, with a force dependent on each particle’s mass. The lighter ions, the ions of strontium-86, deflect more sharply. Therefore, the magnet converts the single stream of strontium-86 and strontium-87 ions into two distinct streams, one for each isotope. These separate ion streams smash into metal cups at the far end of the apparatus, known as the collectors. Each of the two streams lands in its distinct cup where the ions create an electrical current in proportion to their abundance. The ratio of the two measured currents is equal to the relative abundance of the two strontium isotopes.

The Pliomax team compares the relative abundance of the two isotopes they measure with a graph of the known history of strontium ratios in the ocean, published by other researchers. For the last 40 million years, the ratio of the two isotopes in a shell is sufficient to determine a unique age.