A method of determining the charge of at least one test particle, comprising: applying one of an electric current or a voltage across an aperture connecting two chambers, whereby the chambers are at least partially filled with electrolyte and whereby the at least one test particle is suspended in the electrolyte of at least one of the chambers; measuring the other of the electric current or voltage across the aperture; varying a pressure differential between the two chambers; and determining the charge based on the measurements of the electric current or voltage.