The present invention relates to an apparatus for characterizing particles suspended in a liquid, especially a self-contained disposable cartridge for single-use analysis, such as for single-use analysis of a small quantity of whole blood. Furthermore, the present invention relates to a method for characterizing particles in liquid and a device for sampling a small and accurate volume of liquid. The apparatus comprises a housing having a mixing chamber and a collection chamber separated by a wall containing an opening, a first bore in the outer surface of the housing for entrance of a liquid sample, a first cavity for receiving and holding a first liquid sample, and a second cavity for receiving and holding a second liquid sample.