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 technique of selected-ion accumulation (SIA) to investigate its utility for the study of the gas phase behavior of noncovalent complexes from solution. SIA is demonstrated to provide the selective accumulation of weakly bound species, providing a significant enhancement in the attainable signal-to-noise ratio and dynamic range, The frequency response of quadrupole excitation was first determined by selectively accumulating single charge states of large, highly adducted, highly charged molecules such as bovine albumin and bovine albumin dimer. Under the conditions employed, the response (selective accumulation) was found to be as narrow as 500 Hz. additionally the SIA was found to be sufficiently gentle to allow accumulation of both specific and nonspecific weak noncovalent complexes, such as the myoglobin-heme complex observed in basic solutions and complexes of myoglobin and single amino acids, such as tryptophan. This result is probably due to the continual application of quadrupole excitation during the high pressure accumulation event, preventing any significant growth of magnetron motion, and subsequently, allowing interconversion to form only small cyclotron radii. We also demonstrate the simultaneous application of SIA with dipolar sustained off-resonance irradiation (SORI) during the ion accumulation period so as to allow selective accumulation of dissociation products. The 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.