Atoms can now be weighed to the nearest proton (Image: Don Farrall/Getty)

How do you weigh an atom down to the last proton? With scales accurate enough to measure the smallest unit of mass, aka the yoctogram, which is just one septillionth of a gram.

No ordinary scales will do ‐ the tiniest weights are measured using nanotubes, which vibrate at different frequencies depending on the mass of the particles or molecules on them. Until now, 100 yoctograms ‐ or a tenth of a zeptogram ‐ was the smallest mass the most sensitive sensor could detect.

To go even lower, Adrian Bachtold and his colleagues at the Catalan Institute of Nanotechnology in Barcelona, Spain, used short nanotubes. They give the best resolution and work at the low temperatures thought best for measuring frequency. Although the equipment was placed in a vacuum to minimise interference from other atoms, Bachtold removed any stray atoms by temporarily turning up the heat on the tubes to disrupt any bonds to atoms.


Then the sensor was able to weigh an atom of xenon to the nearest yoctogram, or 10-24 grams. This makes it the first scale capable of detecting a single proton, which weighs in at 1.7 yoctograms.

“The yoctogram mass sensitivity achieved by the Catalan team is certainly spectacular ‐ the challenge ahead will be to routinely manufacture nanotube sensors at low cost,” says Rachel McKendry, a nanoscientist at University College London.

Bachtold hopes the scales could be used to distinguish different elements in chemical samples, which might differ only by a few protons. They might also diagnose health conditions by identifying proton-scale differences in molecular mass that are markers of disease.

Journal reference: Nature Nanotechnology, DOI: 10.1038/NNANO.2012.42