Light weight.

Question: What's the best way to weigh an atom or a molecule? Answer: With a very small scale.

The tiny device created by physicists at Caltech is called a "nanoelectromechanical resonator" and works like a tuning fork. It vibrates at a specific frequency so when scientists spray xenon atoms or nitrogen molecules on it, the frequency drops, just as added weight lowers the frequency of a guitar string. By measuring the frequency differentials, researchers can determine the weight of the added matter.

Michael Roukes, professor of physics, applied physics and bioengineering at Caltech, said the current generation of devices is sensitive to added mass at the level of a few zeptograms. (A zeptogram is a few billionths of a trillionth of a gram.)

Roukes said a practical application might be the isolation and identification of proteins secreted in the early stages of cancer.

Until now, molecules have been weighed by arrays of mass spectrometers that could fill an entire lab. The new technology could make the process both less costly and less cumbersome.