“Colored” Noise Waveforms and Quadrupole Excitation for the Dynamic Range Expansion of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry offers unparalleled analytical performance in most regards but has a dynamic range of typically no better than 10²−10³. This limitation reportedly arises from two opposing constraints, involving the maximum number of ions that can be effectively trapped (10⁶−10⁷) and the minimum number of ions required to produce a detectable signal (10²−10³). A potential solution to this dynamic range limitation is presented, based on the application of selected-ion accumulation using quadrupole excitation. We show that lower concentration species can be effectively accumulated in the FTICR trapped ion cell, while the more abundant species are continually removed by the application of quadrupolar excitation in the form of band-limited or “colored” noise waveforms. The result is that “room” is made in the cell for lower abundance species, even during extended accumulation periods. This approach was demonstrated with mixtures of the bovine proteins, insulin, ubiquitin, and cytochrome c. For normal accumulation, the dynamic range was ∼100. The application of selected-ion accumulation in the form of colored noise allowed the extension by 2 orders of magnitude and the detection of species of 1 × 10^-8 M concentration from a solution also containing another component at 9 × 10^-5 M. With this method, a putative new low abundance variant of bovine insulin was observed, and selected-ion accumulation and subsequent collisionally activated dissociation were used for its identification. Dipolar magnetron excitation was also explored to enhance selected-ion accumulation and was found to reduce the amount of buffer gas required for complete removal of the undesired species by a factor of 5. Further possible improvements are discussed, as are the complications due to the required balance between magnetron and cyclotron damping rates.