A method for characterizing particles suspended in a liquid, in a self-contained disposable cartridge for single-use analysis, such as for single-use analysis of a small quantity of whole blood. The method characterizes particles in liquid and samples a small and accurate volume of liquid. The cartridge includes 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.