Selective fragmentation of single isotopic ions of proteins up to 17 kDa using 9.4 Tesla Fourier transform ion cyclotron resonance

Correlated sweep excitation in a 9.4 Tesla magnetic field Fourier transform ion cyclotron resonance instrument has been used to select individual isotope peaks of oxidised insulin chain-B (RMM = 3.5 kDa) and apomyoglobin (RMM = 17 kDa), In the case of apomyoglobin the experimentally achieved "front-end" mass resolution for ion selection was very close to the theoretically predicted attainable resolution. Subsequently, collisionally-activated decomposition of these mass-selected single-isotope precursor ions was accomplished using sustained off-resonance irradiation (SORI). Single isotopic peak selection prior to collisional activation brought a significant decrease in complexity of the fragment ion spectra. Single isotope selection made it possible to assign complementary fragment ions unambiguously, which in turn simplified the assignment of the isotope content of the precursor molecule. In SORI, different isotopic ions of an isotopic cluster are expected to be affected differently by a fixed frequency excitation field. Selecting a single isotopic ion out of the isotopic cluster prior to collisional activation avoided this problem. The methods introduced are general and can be used to measure collisionally-activated decomposition spectra of single isotopic ions of macromolecules up to approximately 20 kDa.