Figure 7 Dispensing accuracy. A few microliters of 5 mM stock fluorescein was aspirated into each of 96 dispenser tips in a head and then 20 drops were delivered by each tip to a 96-well plate filled with 0.2 ml sodium borate buffer, pH 9.2. The height of each bar denotes the average volume of each drop. The average drop volume for the population of dispensers is 472±46 pl.

Above the valve, the entire hydraulic system is maintained at a fixed positive pressure of about 10 psi when the pathway is filled with liquid. The solenoid valve consists of a metal core wrapped by an insulated wire that is, in turn, surrounded over a portion by an annular conductor that is clamped at a fixed current (detent). When current flows through the wire wrapped around the core, the newly generated magnetic field forces the solenoid core into alignment with the field established around the annular conductor (active). The solenoid is mechanically fixed to obstruct the flow path in the detent field, so that when the solenoid is activated, the flow path toward the discharge orifice becomes contiguous with the pressurized hydraulic system and liquid moves out through the orifice. Because

of the time course of the valve movement, the activating and deactivating electrical currents can only be incremented on a millisecond time scale. This confines the linear range of volumes to 0.1-5 µl, which can be achieved by varying the duration over which the valve remains open. At short times volume precision is limited by the inertia of the volume that must be moved in order to obtain any liquid dispensing into the target well, and at long times the volume moved is sufficient to cause a droop in system