EXPERIMENTAL METHODS TO ALLEVIATE ION COUPLING EFFECTS IN MATRIX-ASSISTED LASER-DESORPTION IONIZATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

In Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometr y performed at high ion densities, coulombic interactions between ions of similar cyclotron frequencies may lead to coupled cyclotron motion s. For ions residing in these modes, a coalescence of closely spaced i on frequencies will result, and the observed spectral line shape of ot herwise distinct ion masses has the appearance of a single resonant cy clotron frequency. In this work, we use matrix-assisted laser desorpti on generated ions to provide examples of ion coupling and its effects on isotopic information. Two separate FT-ICR pulse sequences which alt er the applied trap potentials are demonstrated to reverse the effects of ion coupling by reducing the ion density inside the trapped-ion ce ll. The result is a mass spectrum with a resolved isotopic envelope fo r previously coupled ion frequencies. Analytical benefits obtained wit h these pulse sequences when applied to a gramicidin-Dubos sample incl ude a 300% increase in mass resolution and 100% improvement in mass me asurement accuracy in the MALDI/FT-ICR spectrum.