Automated pipetting systems and methods are disclosed for aspirating and dispensing fluids, particularly biological samples. In one aspect, a liquid dispenser includes a manifold and one or more pipette channels. The manifold includes a vacuum channel, a pressure channel, and a plurality of lanes. Each lane includes an electrical connector, a port to the pressure channel, and a port to the vacuum channel. The pipette channels can be modular. Each pipette channel includes a single dispense head and can be selectively and independently coupled to any one lane of the plurality of lanes. In some aspects, a valve in the pipette channel is in simultaneous fluid communication with a pressure port and a vacuum port of the manifold. The valve selectively diverts gas under pressure and gas under vacuum to the dispense head in response to control signals received through the electrical connector of the manifold.