One in a sextillion: Scientists directly detect Earth’s one of rarest argon isotopes

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, help…