Laser-induced fluorescence of trapped gas-phase molecular ions generated by internal source matrix-assisted laser desorption/ionization in a Fourier transform ion cyclotron resonance mass spectrometer.

The combination of laser-induced fluorescence with mass spectrometry opens up new possibilities both for detection purposes and for structural studies of trapped biomolecular ions in the gas phase. However, this approach is experimentally very challenging, and only a handful of studies have been reported so far. In this contribution, a novel scheme for laser-induced fluorescence measurements of ions trapped inside a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer will be introduced. It is based on an open FT-ICR cell design, continuous wave axial excitation of the fluorescence, orthogonal photon collection by fiber optics, and single photon counting detection. Rhodamine 6G ions generated by an internal matrix-assisted laser desorption/ionization source were used to develop and test the set-up. Due to photobleaching processes, the excitation laser power and the observation time window have to be carefully optimized. An ion tomography method was used to align the excitation laser. Potential applications for studying the gas-phase structure of fluorescent biomolecular ions and for investigating fluorescence resonance energy transfer of donor-acceptor pairs will be presented.