In the ESI source, ions are gently transmitted from the source region into the mass spectrometer. Changing the way ions are accelerated through the source optics can increase the number of collisions with background gas molecules, resulting in fragmentation of the ions. In-source fragmentation of volatile analyte ions is generally achieved by raising the Skimmer 1 potential. To fragment a more strongly bound species, ions have to be accelerated thought both funnels as shown in the voltage profile below by raising Skimmer 1Funnel 2Skimmer 2 voltage.

In-Source Collision Activated Dissociation (IS-CAD) 1In-Source Collision Activated Dissociation (IS-CAD) 2