A gated electrostatic ion trap to repetitiously measure the charge and m/z of large electrospray ions

The design and operation of a new type of ion trap provides a way to measure mass, charge, and velocity of large individual electrospray ions (>1 MDa, >250 charges) repeatedly during the time the ion is trapped. The trap consists of an image charge detection tube mounted between two ion mirrors. The mirrors are sets of parallel electrodes drilled with holes aligned with the bore of the detector tube. The device relies on voltages applied to the electrodes to establish symmetrically opposing ion focusing mirrors for the purpose of cycling ions through the charge detection tube many times. This design does mot use magnetic or radio frequency fields to trap ions. Gating one of the mirrors to 0 V while maintaining voltages appropriate for reflecting ions on the opposite mirror allows an ion to pass through the holes il the mirror and enter the detector tube, A low-noise charge-sensitive amplifier, connected to the tube, reproduces the image charge of individual ions as they pass through the detector tube. When a highly charged electrospray ion enters the detector tube, its image charge triggers a circuit that enables the entrance electrodes, thus closing the electrostatic gate to the trap. Ion mass is calculated from simultaneous measurements of ion charge and velocity every time an ion passes through the detector. Individual ions have been trapped for as long as about 10 ms, during which time they cycled 450 times through the detector tube. At this level of trapping time, a theoretical precision for charge measurement as small as about two electrons rms can be achieved for 200 eV/charge ions carrying more than 250 charges. The operation of the system is demonstrated by trapping 2.88 MDa ions of DNA.