Ion kinetic energy modulation for improved ion trapping in electrospray ionization fourier transform ion cyclotron resonance mass spectrometry.

A new technique for manipulating the kinetic energy distribution of electrospray ions that arrive at a Fourier transform ion cyclotron resonance trapped-ion cell is presented. Narrow kinetic energy distributions can complicate the selection of appropriate trapping conditions for electrospray ions and introduce charge discrimination in resulting mass spectra. Modulation of the applied skimmer potential controllably broadens the kinetic energy distribution, which improves the reproducibility of acquired spectra and eliminates charge discrimination. Mass spectra of horse heart cytochrome c are presented to demonstrate the utility of the technique. For example, applied static skimmer potentials of 12 and 9 V yield charge state distributions ranging from [M+19H](+19) to [M+12H](+12) and [M+15H](+15) to [M+7H](+7), respectively. A 12 ± 2 V, 100-Hz modulation of the skimmer potential yields an electrospray spectrum with charge states that range from [M+19H](+19) to [M+7H](+7), which is more representative of the source distribution.