Mass spectrometers are one of the most essential tools in geology. It’s time for their story to be told.
It’s traveled to other planets, helped build atomic bombs, and made some of the most important discoveries in geology, but unless you’re a specialist, you probably have no idea what a mass spectrometer is. These machines balance electromagnetic and centrifugal forces, creating the unique ability to precisely separate and count different atomic isotopes. When paired with the science of radioactive decay, this empowers geologists to date rock samples with incredible precision.
It’s no understatement to say that modern geology relies on this power. It taught geologists that oceanic rocks are generally younger than continental rocks, which helped confirm the theory of plate tectonics. It’s also essential for our understanding of life on Earth; age-dating fossils and the rocks they’re found in provides a timeline for life’s evolution.
“We have a much higher resolution in the geologic record because of mass specs,” says Annie Bauer, a geochronologist and Assistant Professor of Geoscience at UW-Madison.
“It’s so crucial… [without mass spectrometry], you couldn’t do my entire field”
Annie Bauer
It all began in 1899, when the German physicist Wilhelm Wien built the first prototype of a mass spectrometer. After being refined by J. J. Thompson, mass spectrometers had swift and explosive successes throughout the 20th century. Perhaps most notably, a special kind of mass spec called a calutron was used by the Manhattan Project to purify the uranium that powered the atomic bomb at Hiroshima.
Take a famous geological discovery – say, the age of the Earth, or the death of the dinosaurs – and it’s a sure bet that mass spectrometers played a part. Like a telescope for astronomy or a test tube for chemistry, they’re so important to geology that they’ve become ubiquitous. Bauer has four mass spectrometers in her lab alone, and geologists from across the country flock there to use them.
Bauer herself uses mass spectrometers to research the eras of the Earth. Her research focuses on piecing together major geologic events, from the formation of the Earth’s crust to to the evolution of life. Age-dating can reveal far more than just wide geological events, however. Modern mass spectrometry fuels the research of an incredible range of geologic disciplines.
At the University of Minnesota, Robert Holzman, an earth sciences student researcher, works with mass spectrometers to study ancient climate change. His lab takes samples from cave formations and analyzes when they formed and the oxygen isotopes they’re composed of. This work reveals critical details of Earth’s ancient climates and how they change on scales of decades and centuries.

“In order to understand how human activity will influence climate, it’s absolutely critical to understand how climate acts of its own accord on these smaller timescales,” Holzman says. Without mass spectrometry, they’d be completely blind to these smaller changes.
Despite, or perhaps because of, their power, mass spectrometers can often be frustrating to use. “My grad student would tell you they’re the bane of her existence,” Bauer says. Holzman clarifies, “We’re measuring isotopes down to attogram precision… it often ends up being very tedious.”
The work can be slow, too. Holzman once had to spend seven hours simply starting up a mass spec. Another time, analyzing just four or five samples took fourteen hours. Oftentimes, this is because the spectrometer is broken or otherwise dysfunctional. After all, it’s hardly surprising that such a complex and precise instrument is liable to malfunction.
“It’s kind of unbelievable that we can do this at all,” Holzman says. Achieving an accuracy on the scale of decades among the four billion year history of the Earth is a triumph of human engineering and a testament to the power of mass spectrometers. Though they may not be widely known, mass spectrometers nonetheless rank as one of humanity’s greatest scientific tools.