An apparatus is provided for delivering precise volumes of a sample liquid. A piston is moved linearly into a first chamber containing an incompressible hydraulic liquid. The chamber includes one wall formed of a flexible diaphragm. The diaphragm separates the first chamber from a second chamber containing a sample liquid. The degree of expansion of the diaphragm and dispensing of the sample liquid from the second chamber are dependent upon the position of the piston within the first chamber.