Electrostatic device for active transfer of submicroliter samples from syringe pipettors.

The electrostatic charger can be operated in automatic or manual mode. In automatic mode, the high voltage is controlled by means of an analog input ranging from 0–5 V, corresponding to 0–20 kV at the high voltage side of the device. In our case, a laptop running NI Labview 7.1® and a DAQCard 6062E® (National Instruments, Austin, TX, USA) was used to provide the analog control signal. The 0–5 V control signal is fed into a noninverting amplifier with a gain of 2.6, whose output is directly supplied to the SC-150® high voltage power supply (American High Voltage, Elko, NV, USA), and DPM 3AS-B® digital LCD display module (Martel Electronics, Londonderry, NH, USA). In manual mode, analog control of the high voltage is performed through a potentiometer. The potentiometer output is sent through a voltage follower to the SC-150 high voltage power supply and digital LCD display module. The SC-150 high voltage power supply is a voltage-controlled voltage source (VCVS) that provides an output voltage proportional to the input voltage (www.ahv.com). The maximum output is 20 kV, which is supplied to the microtiter plate charger via a 25kV high voltage cable (Belden Electronics Division, Richmond, IN, USA). An FSK15 Series® AC/DC converter (V-Infinity, Beaverton, OR, USA) provided the 15 V DC required for both automatic and manual operation. Figure S1 illustrates the circuit. The high voltage side contains the output pins of the SC-150 and the high voltage current limiting resistor (1 GΩ), which is inside a groove in an acrylic block. The groove is filled with a silicone rubber of high dielectric strength to prevent arcing. The high voltage side is supported on a 1/8′′ acrylic plate with the output pins of the SC-150 and the acrylic block epoxied to the acrylic plate. The separation of the high voltage side from the low voltage side in the illustrated manner ensures that the high voltage components are completely prevented from arcing to any other circuitry. The metal housing that encompasses the circuit is grounded.