I would like to ask this general question regarding solvent peak. Chromatographic peaks are related to a difference in absorption between the mobile phase and the compounds responsible of those peaks. But why when the mobile is injected, a peak can also be seen?

You are apparently talking about optical methods of detection. You can produce a peak due to refractive index changes, pressure changes (for instance, by moving the injection valve, this is also related to refr. index, or to equilibrium disturbances), picking up air somehow, dirt in the syringe, etc.

Thank u Mueller for ur comments