The mass spectrometer was triggered at 10 Hz by a function generator (DS345, Stanford Research Systems). Ions were stored in the first hexapole guide for 95 ms by placing a repelling voltage on its exit aperture. The pulse valve fired 3 ms after the ions exit the first guide to match the ions' arrival time to the trap. The gas burst had a duration of ~35 ms. An RF sweeping pulse on resonance with the secular frequency of the untagged parent ion was applied between 30 to 40 ms to remove untagged parents. Ions interacted with the laser pulses between 40 to 95 ms, at which time the exit electrode of the ion trap was triggered to extract ions into the TOF region.