Science  07 Sep 2012:
Vol. 337, Issue 6099, pp. 1155

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  1. Weighing Individual Molecules

    Vibrating like a guitar string, a nanometer-sized beam of silicon can weigh individual molecules, opening the way to a new type of mass spectrometry.

    When a molecule sticks to the beam, the added mass lowers the frequency of vibration, in principle revealing the molecule's mass. However, the frequency shift also depends on where on the beam the molecule lands: A lighter molecule landing in the middle could produce the same frequency shift as a heavier one landing closer to one end. To avoid this ambiguity, Michael Roukes, a physicist at the California Institute of Technology in Pasadena, and colleagues shake a beam at two frequencies simultaneously. That's possible because a beam can vibrate in various patterns of motion, or modes, each with its own frequency. An alighting molecule lowers the frequencies of two modes by different amounts, and the two frequency shifts reveal both the molecule's position and its mass, the team reported on 26 August in Nature Nanotechnology.

    “How applicable this will be to generalized mass spectroscopy, time will tell,” says John Kasianowicz, a biophysicist at the National Institute of Standards and Technology in Gaithersburg, Maryland, who was not involved in the work. “But I think this is a major advance.”