  We propose a mass sensor using optically trapped and cooled dielectric
microdisks with "measuring after cooling" scheme. The center-of-mass motion of
a trapped particle in vacuum can experience extremely low dissipation resulting
in robust decoupling from the heat bath. Thus the measurement can be
accomplished in a short time before a jump in the phonon number after the
optomechanical cooling is finished. This method can lead to 10^(-6) dalton mass
sensitivity which is a thousand times smaller than the mass of one electron.
