SELECTED-ION ACCUMULATION OF NONCOVALENT COMPLEXES IN A FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETER

An electrospray ionization-Fourier transform ion cyclotron resonance ( ESI-FTICR) mass spectrometer has been used in conjunction with the tec hnique of selected-ion accumulation (SIA) to investigate its utility f or the study of the gas phase behavior of noncovalent complexes from s olution. SIA is demonstrated to provide the selective accumulation of weakly bound species, providing a significant enhancement in the attai nable signal-to-noise ratio and dynamic range, The frequency response of quadrupole excitation was first determined by selectively accumulat ing single charge states of large, highly adducted, highly charged mol ecules such as bovine albumin and bovine albumin dimer. Under the cond itions employed, the response (selective accumulation) was found to be as narrow as 500 Hz. additionally the SIA was found to be sufficientl y gentle to allow accumulation of both specific and nonspecific weak n oncovalent complexes, such as the myoglobin-heme complex observed in b asic solutions and complexes of myoglobin and single amino acids, such as tryptophan. This result is probably due to the continual applicati on of quadrupole excitation during the high pressure accumulation even t, preventing any significant growth of magnetron motion, and subseque ntly, allowing interconversion to form only small cyclotron radii. We also demonstrate the simultaneous application of SIA with dipolar sust ained off-resonance irradiation (SORI) during the ion accumulation per iod so as to allow selective accumulation of dissociation products. Th e SIA/SORI combination promises to greatly extend (MS)(n) capabilities by allowing the selective accumulation of any dissociation product in any step along a given dissociation pathway.