A method for isolating ions in quadrupole ion traps using an excitation waveform generated by frequency modulation and mixing

Abstract We describe a new method for isolating ions in quadrupole ion traps using an excitation waveform generated by mixing a broadband waveform generated by frequency modulation (FM) with a sine-wave at the secular frequency of the ion to be isolated. In terms of resolution and efficiency, the mixed FM method exhibits performance nearly identical to isolation using the apex of the Mathieu stability diagram. A disadvantage of the mixed FM method is that isolations require additional time relative to apex-based methods. This disadvantage is shared by other methods that involve application of multi-frequency waveforms such as stored waveform inverse Fourier transform (SWIFT). An advantage of the mixed FM technique (also shared with other tailored waveform approaches), is applicability to a much larger m / z range than apex-based methods. Indeed, the mixed FM technique performs identically to SWIFT in many respects. While the mixed FM technique is not nearly as flexible as SWIFT in terms of the frequency content of the generated waveforms, the mixed FM technique is much simpler to implement as it requires only two function generators and a frequency mixer. Tuning important parameters of the waveform such as notch frequency, notch width, and excitation bandwidth is also facilitated with the mixed FM technique.