To do this, he and his colleagues use an apparatus called a quartz crystal microbalance.
From the economist.com
Other groups are working on electrochemical, microbalance and acoustic methods for detecting molecules.
From the sciencedaily.com
Currently, the team is at work integrating the microbalance disks with a heating element to enable the process to be simultaneous.
From the sciencedaily.com
The NIST group found one in the quartz crystal microbalance, essentially a small piezoelectric disk of quartz sandwiched between two electrodes.
From the sciencedaily.com
More examples
Balance for weighing very small objects
A microbalance is an instrument capable of making precise measurements of weight of objects of relatively small mass: of the order of a million parts of a gram. In comparison, a standard analytical balance is 100 times less sensitive; i.e. it is limited in precision to 0.1 milligrams. ...