In a world filled with trillions upon trillions of atoms, spotting a handful that barely exist sounds almost impossible. Atoms of argon-42 (⁴²Ar), one of the rarest isotopes in Earth’s atmosphere, present at just one part in 10²¹, fall in the same category.

For years, this isotope showed up only indirectly, as a troubling background signal in dark matter experiments. Now, probably for the first time, researchers from the University of Science and Technology of China and the Chinese Academy of Sciences (CAS) have captured it directly, atom by atom. 

Until now, the main method for studying rare isotopes has been accelerator mass spectrometry (AMS), but even this technique fails when it comes to spotting something as rare as argon-42. 

The problem isn’t just sensitivity, but confusion: signals from similar atoms blurred the results, creating background interference that made precise detection unreliable. At the scale of ⁴²Ar, that interference completely overwhelms the signal, keeping it out of reach. 

However, the new approach promises to overcome this problem. Our study “demonstrates a powerful tool for detecting isotopes at previously inaccessible abundance levels, with implications for environmental dating and background characterization in next-generation liquid-argon detectors,” the study authors note.

Building a signal out of almost nothing

To get anywhere close to detecting ⁴²Ar, the researchers first had to deal with a basic problem: the isotope is buried under a massive excess of ordinary argon. So they began by reshaping their sample.

Using a high-flux mass spectrometer, they carried out a pre-enrichment step. This process selectively removed large amounts of the most common isotope, ⁴⁰Ar, from the gas. By trimming down the overwhelming majority, they effectively made the rare ⁴²Ar stand out more—boosting its relative presence by about 450 times. 

At the same time, they deliberately kept ³⁸Ar in the sample. This isotope acted like a built-in checkpoint, helping them confirm that what they were seeing later was real and not an artifact.

With the sample prepared, they turned to Atom Trap Trace Analysis (ATTA). Instead of measuring a bulk signal, ATTA works by targeting atoms individually. 

The team used lasers tuned so precisely that only ⁴²Ar atoms respond. When these atoms interact with the laser light, they slow down enough to be captured in a small trap, where each one can be detected separately.

This is what sets ATTA apart. It doesn’t just reduce background noise—it eliminates it. Since no other atom matches the exact laser frequency used, false signals simply don’t enter the measurement. However, even with such precision, progress was slow. Over 43 days, the system registered just 204 atoms, but this small count was enough. 

From it, the researchers calculated the isotope’s abundance in the atmosphere as (6.1 ± 0.5) × 10⁻²¹, extending the reach of atom-counting methods by four to five orders of magnitude compared to earlier limits.

“We demonstrated that isotopes as rare as 10⁻²¹ can now be analyzed by atom counting—pushing the detection limit four to five orders of magnitude beyond what was previously possible,” Zheng-Tian Lu, one of the study authors and a professor at the University of Science and Technology of China (USTC), said.

From background noise to new 

What makes the research even more interesting is that the study authors didn’t originally set out to chase ⁴²Ar. They had been improving ATTA for studying ³⁹Ar, which is far more abundant and used in dating ice and ocean water. 

However, as their system became more efficient, they realized they had quietly crossed a threshold—one that made tackling a much rarer isotope possible.

Pinning down the abundance of ⁴²Ar could immediately improve the reliability of dark matter experiments. In detectors filled with liquid argon, the decay of this isotope creates signals that can look deceptively similar to the ones scientists are searching for. 

With a clearer measure of how much ⁴²Ar is present, researchers can better separate genuine events from misleading ones.

However, the current approach is not yet fast enough for routine experiments. Detecting such tiny numbers of atoms takes time, and the setup requires careful calibration and advanced equipment. 

Making the method more practical will depend on increasing how quickly atoms can be captured and counted. Therefore, the researchers now plan to improve detection efficiency and push sensitivity even further, with the hope of identifying other ultra-rare isotopes that have so far remained invisible.

“These advances will not only benefit future applications of ⁴²Ar but also improve the analysis of isotope tracers already in use, including ³⁹Ar and ⁸¹Kr,” Lu said. Plus, “we would also be interested to learn whether there are other ultra-rare isotopes that scientists in different disciplines would like to detect,” he concluded.

The study is published in the journal Nature Physics.