Injection of Mass-Selected Ions into a Quadrupole Ion Trap

A simple ion trap has been developed, which captures and confines ions using the fringing field of a linear ac quadrupole. The trap consists of a quadrupole electrode that has one end surrounded by a cage electrode. Ions generated outside the trap are injected through the internal field of the quadrupole and subsequently trapped in the external field extending to the cage. Theoretical calculations and experiments show that when Ar+ ions are injected into the trap at an ac frequency, f, of 1.2 MHz, the chirping of f to 0.6 MHz over ∼50 µ s improves the trap lifetime. This effect arises due to the adiabatic change in the trapping potential, which closes the injection pathway of the ions into the trap.