A fair amount of Earth science research is based on incredibly subtle measurements of isotopes obtained from minerals or the remains of long-dead organisms. Surprising histories and insights can be recorded in minute shifts of these isotopes. But we never talk about how those measurements are made, since this particular sorcery is commonplace in modern science.

For example, a recent story about how quickly Yellowstone has coughed up eruptions in the past stated, “The researchers made spot measurements—tiny spots less than half a micron apart—of barium, strontium, and magnesium crossing the boundary of the [feldspar] crystal rims.” Hidden behind that sentence is some remarkable technology—and many hours spent using it. (That’s what happens when you reduce several years of 21st century research to 800 words.)

To make partial amends for this slight, Ars visited a lab in the University of Wisconsin-Madison’s geoscience department to check out one of the instruments behind this magic. That lab is managed by John Valley and Noriko Kita and is kept running through the hard work of several others. Like other geologists, Valley used to interrogate tiny crystals of the mineral zircon the only way he could—by tossing a pile of them into an instrument and making bulk measurements of their chemistry. If those crystals were uniform, with no difference between or within them, that would be great. But there was no way to know whether this was true.

New instruments changed that, allowing geologists to shrink down and inspect individual crystals. In 2005, Valley’s department got an instrument of its own: a room-filling, multimillion-dollar instrument called a Secondary Ion Mass Spectrometer (SIMS) built by the French manufacturer CAMECA. The instrument can make spot measurements of isotopes in areas as tiny as a quarter of a micron across—that’s less than one percent of the thickness of a sheet of paper.

There are only a handful of these instruments around the world, and most of them are designated for forensic investigations of nuclear material or debris from nuclear explosions. The academic labs that have one specialize in certain isotopes, so researchers will travel from all over the world to use the machine that performs the measurement they need. On the day I visited the Wisconsin lab, for example, Hafiz Ur Rehman from Kagoshima University in Japan was busy analyzing some zircons.

The instrument is called a Secondary Ion Mass Spectrometer because the atoms or molecules you measure are freed from the sample by a violent cannonball that is the primary ion. In other words, they accelerate particles and use the beam to blast material out of the sample for measurement.

Once the material is free of the sample, the machine operates using the same principles as other types of mass spectrometers. Different atoms or molecules, when ionized (so they have a positive or negative charge), have differing ratios of charge to mass. As a result, they take different paths when deflected by electric and magnetic fields. The electric or magnetic field supplies a force deflecting the atom around a bend, and the mass affects how much bending takes place. A heavy atom will take a wide berth around the bend; a light atom will turn sharply. By detecting the location where the atom splats against the wall that ends its flight, you can work out what its charge-to-mass ratio must have been and identify it.

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

Scott K. Johnson

To analyze zircons, these less-than-one-millimeter-wide crystals have to be separated from the rocks in which they’re found. The crystals are then locked into a thin layer of epoxy, and the whole mess is ground down to expose the interiors of the zircons. With that, they’re ready to go into the instrument.

From the computers than run the machine, you can see your samples through a high-powered microscope (rebuilt by the Wisconsin lab to use UV light for a sharper image that resolves tiny details). That pretty much makes this a point-and-click affair. You set the exact point you want your spot measurement, and the machine lines up the sample. The beam of cannonball ions—cesium, in this case—are fired at that spot, blasting out atoms and molecules and excavating a hole that's typically about ten microns across and one micron deep. “So that’s kind of like the break in pool,” Valley told Ars, “some stuff is so violently hit that it leaves the table.”

Some of atoms and molecules that spray out are ionized and accelerated down the length of the instrument, which is kept clear with thirteen powerful vacuum pumps (including some that use reactive titanium to gobble up the air molecules) and lined with magnetic “lenses” that focus the parade of particles.

The first 90° bend in the instrument houses a pair of electrically charged plates that push the ions around the corner. A slit on the downstream side allows you to filter out many of the things you’re not interested in while letting the ions you’re after continue on. This structure also compensates for the fact that the ions left your sample with differing amounts of kinetic energy.

The second 90° bend is a powerful magnetic field that does the main job of spreading the trajectories of the ions, sorting them by their charge-to-mass ratio.

The last section of the machine holds a number of detectors that are carefully placed in the right locations to catch the isotopes you’re measuring after they come careening around the magnetic curve. The detectors for oxygen-16 and oxygen-18, for example, are placed about 10cm apart on an axis that angles across the path of the ions.

There are a couple types of detectors available in this machine, depending on how many ions you’re going to be able to send flying into them. When ions are plentiful, a device called a Faraday cup is used, in which the colliding ions create a small current. An electron multiplier is more sensitive because each colliding ions results in a larger signal, but that also makes it susceptible to being overwhelmed and to background noise.

If that sounds like a lot of details to account for in order to fill your spreadsheet with data, that’s because it is. “After we’ve sorted for energy [at the first bend], then we can choose which part of the energy spectrum you want,” Valley said, “so that’s part of it, and then there’s another lens, there’s some deflectors—there’s like thirty different things that you have to tune on this machine. It’s really complicated. It takes like ten years to learn how to run this.”

And we’re not even getting into all the little biases that crop up because certain isotopes can make it through different parts of the machine more easily than others, or get blasted out of different sample materials more easily than others, or how the “beam” of ions gets blurry after passing through so many imperfect lenses… This is why laboratories specialize in certain isotopes. It takes work to pin all this down with precision.

Set output to “science”

So what kind of research gets done with this crazy contraption? Well, there are the zircons, of course, which turned out not to be uniform clones. Interesting stories can now be read within a single crystal.

And there’s plenty more, even for a lab that specializes in measuring just a few elements. When NASA’s Stardust mission brought back tiny specks of dust from the comet Wild 2, some went into this machine for measurements of oxygen isotopes. The results challenged our ideas about how comets were formed, as the specks appeared to have a vagabond history with chemical similarities to asteroids.

Samples from 2.4 billion-year-old glacial sediments told stories about the initial oxygenation of the atmosphere recorded in sulfur isotopes. Objects thought to be the oldest fossilized remains of early, single-celled life are being probed to see whether their carbon bears the isotopic fingerprint of living organisms or just mineral look-alikes.

Precipitated quartz and calcite, which glue together sandstone grains, are proving to contain very subtle layering that reveals complex geologic histories we could only dream about deciphering without an instrument like this one.

Similarly, cave stalagmites with microscopic seasonal growth rings have coughed up climate records of monsoon rainfall from thousands of years ago.

And sticking with climate, the tiny shells of ocean plankton called foraminifera form the backbone of climate records from ocean sediment cores. These can go back millions of years, and are now going through their own “zircon revolution.” Piles of these little shells are typically analyzed together (just as zircons were), but analysis with this instrument has discovered a surprising amount of variation within individual shells.

Better accounting for this may improve our climate records, and studying individual shells this closely may eventually lead to entirely new insights. “The records that people have built up for the last fifty years, of glacial and interglacial [periods], that’s real,” Valley said. “But that’s based on average numbers, and there’s a whole lot more information there that hasn’t been exploited yet.”

“We’ll spend a whole day on three or five foraminifera, and the people who are trying to make a record, and they have a hundred meters of core to do—they don’t want to hear this,” Valley said. “And so one of the tricks here is to figure out which are the critical samples that deserve this extra attention.”

A lot went into designing and building instruments like this one, and plenty more goes into maintaining and operating them to give samples that extra attention. But these instruments enable researchers to push scientific boundaries—even if we don’t often stop to give them credit for it.

Listing image by Scott K. Johnson