Mass spectometry method with applied signal having off-resonance

Abstract

A mass spectrometry method in which a combined field (comprising a trapping field and a supplemental field) is established and at least one parameter of the combined field is changed to excite ions trapped in the combined field sequentially (such as for detection). The supplemental field is a periodically varying field having an off-resonance frequency, in the sense that the supplemental field frequency nearly matches (but differs from) a frequency of motion of an ion stably trapped by the trapping field alone. Sequential ion excitation in accordance with the invention can rapidly eject a sequence of ions from a trap, or rapidly excite each ion to a degree sufficient for a desired purpose but insufficient for ejection from the trap, because the supplemental field will increase the trajectory of each ion in the sequence and because the supplemental field can have a sufficiently large peak-to-peak amplitude to increase each ion trajectory to a desired magnitude within a desired short time period. The trapped ions can be sequentially excited by holding the supplemental field fixed while changing at least one parameter of the trapping field, or by changing (such as scanning) at least one parameter of the supplemental field while holding the trapping field fixed. The amplitude of the supplemental field is kept sufficiently high to excite ions (via an off-resonance excitation mechanism) before they undergo resonant excitation.