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 spectrometry performed at high ion densities, coulombic interactions between ions of similar cyclotron frequencies may lead to coupled cyclotron motions. For ions residing in these modes, a coalescence of closely spaced ion frequencies will result, and the observed spectral line shape of otherwise distinct ion masses has the appearance of a single resonant cyclotron frequency. In this work, we use matrix-assisted laser desorption generated ions to provide examples of ion coupling and its effects on isotopic information. Two separate FT-ICR pulse sequences which alter the applied trap potentials are demonstrated to reverse the effects of ion coupling by reducing the ion density inside the trapped-ion cell. The result is a mass spectrum with a resolved isotopic envelope for previously coupled ion frequencies. Analytical benefits obtained with these pulse sequences when applied to a gramicidin-Dubos sample include a 300% increase in mass resolution and 100% improvement in mass measurement accuracy in the MALDI/FT-ICR spectrum.