METHOD OF OPERATING A LINEAR ION TRAP TO PROVIDE LOW PRESSURE SHORT TIME HIGH AMPLITUDE EXCITATION WITH PULSED PRESSURE

摘要

Methods for fragmenting ions in an ion trap are described. These methodsinvolve a) selecting parent ions forfragmentation; b) retaining the parent ions within the ion trap for aretention time interval, the ion trap having an operating pressureof less than about 1 x 10-4 Torr; c) providing a RF trapping voltage to theion trap to provide a Mathieu stability parameter q atan excitement level during an excitement time interval within the retentiontime interval; d) providing a resonant excitation voltageto the ion trap during the excitement time interval to excite and fragment theparent ions; e) providing a non- steady-state pressureincrease of at least 10% of the operating pressure within the ion trap bydelivering a neutral gas into the ion trap for at least a portionof the retention time interval to raise the pressure in the ion trap to avarying first elevated-pressure in the range between about 6 x10-5 Torr to about 5 x 10-4 Torr for a first elevated-pressure duration; andf) within the retention time interval and after the excitementtime interval, terminating the resonant excitation voltage and changing the RFtrapping voltage applied to the ion trap to reduce theMathieu stability parameter q to a hold level less than the excitement levelto retain fragments of the parent ions within the ion trap.The excitation time interval and the first elevated-pressure durationsubstantially overlap in time.