We have implemented efficient and rapid collisionally activated dissociation (CAD) external to an ICR cell by use of a novel axial electric potential gradient mounted in an external ion accumulation octopole. The gradient is produced by eight tilted (in the axial direction) wires mounted between the rods of the octopole. Rapid switching of the wire potential between a positive and negative value drives the ion axial motion back and forth and, in the presence of nitrogen gas at suitable pressure, induces dissociation. A fragmentation period on the order of tens of milliseconds is typical. Precursor ion isolation is achieved in a quadrupole mass filter mounted between the electrospray source and the accumulation octopole. A scan rate of >1 Hz is possible with resolving power and mass accuracy equivalent to direct infusion experiments (for equivalent detection period for 1 scan). The method is thus sufficiently rapid for MS/MS with on-line LC sample introduction. Moreover, compared to CAD in the ICR cell, external CAD improves mass accuracy, producing thermal on-axis fragment ions for detection.