Generally, it is advisable to set the intensity level of the PCIS as low as possible (to minimize costs and matrix effect induced by the PCIS), but without compromising the stability of the signal and without the signal going below the LLOQ during a chromatographic run. The intensity level can be optimized by varying the PCIS concentration and measuring the RSD and the intensity of the PCIS signal when combined with an isocratic LCflow, as shown in Fig. . The use of an isocratic flow prevents peaks or dips in the signal associated with the injected sample affecting the calculated RSD. As expected, the PCIS signal intensity linearly increased with the concentration. The RSD stabilized around 5% with concentrations at or above 50 ng/mL, but it is important to realize that the signal RSD is more related to its intensity than the concentration. It also means that if the PCIS intensity when injecting a biological sample is lower than during the isocratic infusion experiment (which could occur due to suppression), the PCIS concentration must be increased to reachieve the intensity level offering the desired stability.