US Patent 7117723 Fractionating apparatus

A proximity sensor for measuring a distance between a probe and a sample plate is provided a sideway of the probe. The sample plate is mounted on a stage and moved vertically and horizontally. The stage is connected to a control unit for controlling the movement of the stage. The control unit makes a feedback control for the stage so that the measured value of the proximity sensor may be set to a preset value. When dropping a liquid droplet from the probe, the sample plate is moved upwards to shorten the distance between the probe and the sample plate and contact the liquid droplet with the sample plate, whereby the solution is fractionated.