The previous section has been devoted to ensuring that areas in mobilograms map faithfully to areas in electropherograms. Without further treatment, this assumes that in turn electropherogram areas represent faithfully the amount of substance in the sample. This is highly dependent on the detection type. In the context of CE-MS, the critical element is the electrospray ionization (ESI) that nebulizes the output of the capillary to be fed into the MS. This ionization has two well known regimes of operation, the so-called mass and concentration modes . In mass mode, the ESI is able to ionize (nearly) all the substance coming from the capillary. If the capillary flow is increased, more substance will be ionized per unit time and the MS will simply register more counts. By contrast, when in concentration mode the ESI is saturated. It outputs an amount of ions proportional to the volumetric concentration of substance arriving from the capillary, independently of time. Crucially, increasing the flow in the capillary does not increase the number of counts.