Secular frequency modulation

This relatively straightforward resonant excitation process is complicated somewhat in a stretched ion trap (see above). Since ion axial secular frequencies increase as an ion approaches an end cap electrode, it is necessary to adopt one of several strategies to maintain resonance and thus ion excitation. There are three such methods of resonant excitation of ions of a selected miz value isolated within an ion trap, namely, single frequency irradiation (SFI), secular frequency modulation (SFM) and multifrequency irradiation (MFI). 

In 1994, Wells [41] proposed a new techniqne, RF modnlation dnring the CID period in ion trap tandem mass spectrometry. In this method, the precnrsor ion secnlar freqnency is bronght into resonance with the applied snpplemental AC dipole field, rather than matching the freqnency of the AC dipole with the ion secular frequency as was done in early methods. The frequency modulation is realized by modulation of the... 

FIGURE 15.24 Top, illustration of the modulation of an ion s -value and, thus, its secular frequency, by modulation of the RF trapping field. Bottom, the secular frequency of the ion oscillates about the fixed frequency of the AC field during the CID period. 

SFM involves modest variations in the amplitude Vg of the RF drive potential applied to the ring electrode, such that ions move into and out of resonance with the applied single frequency tickle waveform such slight modulation of Vg changes and so as to produce a sweep of the axial secular frequency g over a narrow range of some 1-2 kHz. Thus the resonance condition for ion activation is guaranteed to be met for some fraction of the time over which the resonant excitation voltage is applied. 

The presence of the matrix interferents can diminish fragmentation ratios. To solve this problem, the amplitude of the ion trapping radiofrequency can be slightly modulated during the activation phase by collision. This allows the secular frequency of the precursor ion to be modulated and for the activation to proceed in a range of frequencies instead of at one determined frequency. 

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