Physicists in the US have built a device that can determine the mass of objects as light as a single gold atom.

The device works by attaching an object of interest to a "double-walled" nanotube, which consists of two concentric rolled-up carbon sheets, each just one atom thick. The object reduces the frequency at which the nanotube oscillates, with the change being proportional to the mass of the object.

The device, called a “nanomechanical mass spectrometer”, has been built by Kenny Jensen, Alex Zettl and colleagues at the University of California at Berkeley (Nature Nanotechnology doi:10.1038/nnano.2008.200).

One end of the double-walled nanotube, which is about 2 nm in diameter and some 200 nm long, is attached to a negatively-charged electrode, while the other end is free to vibrate like the tip of a diving board.

A positively-charged electrode is located near to the free end of the nanotube and electrons are able to flow from the tip of the nanotube to the positive electrode. The size of this current of electrons depends on the frequency at which the nanotube is vibrating. By measuring this current, the team is able to monitor changes in frequency and therefore tell if any tiny objects have become attached to the nanotube.

### Light-weight resonator

Although this is not the first such nanomechanical device for measuring mass, most other devices are made from silicon-based materials and have resonators that are much thicker and heavier than a carbon nanotube. This means that they are much less sensitive to very small changes in mass.

The team tested their device by introducing a small number of gold atoms into the vacuum chamber in which it was held, knowing that some of them would stick to the vibrating nanotube.

Exactly where and when each individual gold atom sticks to the nanotube is a random process, which means that the frequency changes in irregular jumps. The team used statistical analysis to study these changes, from which they could calculate the mass of a single gold atom.

The analysis revealed that the gold atoms have a mass of 0.29 +- 0.05 zg, where zg is a zeptogram (10-24 kg). This is consistent with the accepted mass of 0.327 zg as measured by a conventional mass spectrometer.

### 'Best-ever' sensitivity

The team measured the sensitivity of their device to be 0.13 zg Hz-1/2, which they claim is the best ever value for a nanomechanical mass spectrometer.

Although this is nowhere near as sensitive as a conventional mass spectrometer (in which an atom or molecule is ionized and then accelerated using electric and magnetic fields) there is no need to ionize the atom or molecule of interest. It could therefore be used to study large and fragile molecules that would be destroyed in a mass spectrometer.

The team are currently working on integrating the device into a chip.