A group of physicists headed by Dr. Michael Roukes have developed a nanoelectromechanical system (NEMS) based mass spectrometer, which can measure the mass of things as small as a single molecule. The professor and his group has been working on this for the past 10 years in Caltech's Kavli Nanoscience Institute.
In laymen terms, it is a NEMS resonator which keeps oscillating at a particular frequency. You drop a molecule on it, now because of the force excerted by this molecule, the standing wave frequency of the resonator changes. The change in the frequency is mapped to the mass of the molecule. Well, as you could imagine, the frequency could change depending upon where on the resonator the molecule has fallen. In order to avoid this, the molecule has to be dropped several times and the arithmetic mean of all measurement would give the mass. Here is the complete press release. [Image Credit: Caltech/Akshay Naik, Selim Hanay]
Now a curious question. Do they have a nanometric tuning fork to measure the change in standing wave frequency of this nanometric resonator?