  Mechanical resonators are widely used as inertial balances to detect small
quantities of adsorbed mass through shifts in oscillation frequency[1].
Advances in lithography and materials synthesis have enabled the fabrication of
nanoscale mechanical resonators[2, 3, 4, 5, 6], which have been operated as
precision force[7], position[8, 9] and mass sensors[10, 11, 12, 13, 14, 15].
Here we demonstrate a room-temperature, carbon-nanotube-based nanomechanical
resonator with atomic mass resolution. This device is essentially a mass
spectrometer with a mass sensitivity of 1.3 times 10^-25 kg Hz^-1/2 or,
equivalently, 0.40 gold atoms Hz^-1/2. Using this extreme mass sensitivity, we
observe atomic mass shot noise, which is analogous to the electronic shot
noise[16, 17] measured in many semiconductor experiments. 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.
