The device — which is only a couple microns (millionths of a meter) in size — consists of a tiny, vibrating bridge-like structure. When a particle or molecule lands on the bridge, its mass changes the oscillating frequency in a way that reveals how much the particle weighs.
“As each particle comes in, we can measure its mass,” says Michael Roukes, the Robert M. Abbey Professor of Physics, Applied Physics, and Bioengineering at Caltech. “Nobody’s ever done this before.”
The new instrument is based on a technique Roukes and his colleagues developed over the last 12 years. In work published in 2009, they showed that a bridge-like device — called a nanoelectromechanical system (NEMS) resonator — could indeed measure the masses of individual particles, which were sprayed onto the apparatus. The difficulty, however, was that the measured shifts in frequencies depended not only on the particle’s actual mass, but also on where the particle landed. Without knowing the particle’s landing site, the researchers had to analyze measurements of about 500 identical particles in order to pinpoint its mass.
But with the new and improved technique, the scientists need only one particle to make a measurement. “The critical advance that we’ve made in this current work is that it now allows us to weigh molecules — one by one — as they come in,” Roukes says.