Fourier Transform Ion Cyclotron Resonance Mass Spectrometry at the Cyclotron Frequency

The phenomenon of ion cyclotron resonance allows for determining mass-to-charge ratio, m/z, of an ensemble of ions by means of measurements of their cyclotron frequency, ω_c. In Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), the ω_c quantity is usually unavailable for direct measurements: the resonant state is located close to the reduced cyclotron frequency (ω₊), whereas the ω_c and the corresponding m/z values may be calculated via theoretical derivation from an experimental estimate of the ω₊ quantity. Here, we describe an experimental observation of a new resonant state, which is located close to the ω_c frequency and is established because of azimuthally-dependent trapping electric fields of the recently developed ICR cells with narrow aperture detection electrodes. We show that in mass spectra, peaks close to ω₊ frequencies can be reduced to negligible levels relative to peaks close to ω_c frequencies. Due to reduced errors with which the ω_c quantity is obtained, the new resonance provides a means of cyclotron frequency measurements with precision greater than that achieved when ω₊ frequency peaks are employed. The described phenomenon may be considered for a development into an FT-ICR MS technology with increased mass accuracy for applications in basic research, life, and environmental sciences. .