Recently, a method has been described for acquiring mass spectra using ion traps in which the frequency of a small supplementary alternating current (ac) signal is ramped through ion secular frequencies over time, a process which we term “secular frequency scanning”.[1] It has also been shown that secular frequency scanning can be used as a time-based method of mass-selective fragmentation. As a consequence, the combination of a low amplitude secular frequency scan and a high amplitude, constant frequency ejection waveform set at the secular frequency of a particular product ion can be used to record precursor ion spectra in a single ion trap.[2,3]
The attractiveness of recording mass spectra using the secular frequency scan compared with traditional sweeps of the radiofrequency (rf) amplitude is its simplicity since a linear ramp of the rf amplitude is not required. However, as in the rf frequency sweep method,[4] calibration remains difficult due to nonlinearity between the varied parameter (rf frequency or ac frequency) and m/z.[4]
Here, we introduce a simple algorithmic approach for the mass calibration of secular frequency scan mass spectra. The algorithm assumes a linear sweep of the ac frequency