Coulombic shielding during ion cyclotron excitation in FT-ICR mass spectrometry

Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry relies upon linearity between the ion cyclotron excitation and the observed response. However, nonlinearities result from non-ideal applied electric and magnetic fields and Coulombic interactions. Here, we report nonlinear response at low excitation electric field magnitude due to Coulombic shielding. The measured ICR signal magnitude exhibits an excitation voltage threshold that increases monotonically with the number of shielding ions (i.e., unexcited ions). If shielding ions are not present, ICR signal magnitude versus excitation voltage is linear (e.g., for quadrupole-isolated ions of nearly a single m/z). Finally, we show that shielding results in a reduced cyclotron radius at low excitation voltage, resulting in an increased rate of transient decay; thereby exacerbating response nonlinearity and excitation threshold for long data acquisition period.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (145 K)Download as PowerPoint slideResearch highlights▶ A trapped ion packet can shield the ions so as to prevent rf resonant excitation in FT-ICR MS. ▶ In FT-ICR MS, higher resolution ion isolation requires a lower number of trapped ions. ▶ Coulombic shielding results in nonlinear FT-ICR response at low excitation electric field magnitude.