The electrochemical detector responds to substances that are either oxidizable or reducible and the electrical output is an electron flow generated by a reaction that takes place at the surface of the electrodes. If the reaction proceeds to completion (exhausting all the reactant) the current becomes zero and the total charge generated will be proportional to the total mass of material that has been reacted. This process is called coulometric detection. If, however, the mobile phase is flowing past the electrodes, the reacting solute will be continuously replaced as the peak passes through the detector. All the time there is solute present between the electrodes, a current will be maintained, albeit varying in magnitude. Until relatively recently, this procedure was that most common employed in electrochemical detection and is called amperometric detection.