Physicists at the University of California, Berkeley have developed a nanomechanical sensor — a cantilevered carbon nanotube — that can weigh an atom, replacing a large mass spectrometer.

The mass is determined by sending a radio-frequency signal to the nanotube and measuring its resonant frequency, which changes when different atoms are stuck to it.

Also see:

An atomic-resolution nanomechanical mass sensor, Nature Nanotechnology, July 20, 2008:

Unlike traditional mass spectrometers, nanomechanical mass spectrometers do not require the potentially destructive ionization of the test sample, are more sensitive to large molecules, and could eventually be incorporated on a chip.