TARGET GAS EXCITATION IN COLLISION-INDUCED DISSOCIATION - A REINVESTIGATION OF ENERGY-LOSS IN COLLISIONAL ACTIVATION OF MOLECULAR-IONS OF CHLOROPHYLL-ALPHA

Losses of translational energy accompanying collisional activation (CA) of large organic ions at keV energies have been found to be unexpectedly large, as exemplified by work of Bricker and Russell (J. Am. Chem. Soc. 1986, 108, 6174) on molecular ions of chlorophyll-a. These energy shifts are of practical importance for tandem mass spectrometry applied to biochemistry and are also of theoretical interest with respect to the very large kinetic shifts thus implied for the dissociation. The original work attributed the chlorophyll-a shifts to ionization of collision target molecules as a mandatory accompaniment to CA, with negligible contributions from kinetic shift to the observed inelasticity. The present work has exploited improved experimental techniques to demonstrate that those trends observed by Bricker and Russell, which led to their target ionization hypothesis, were experimental artifacts and that target mass rather than ionization energy is the dominant parameter in determining the energy shift. Angular dependences of these energy shifts have been determined and shown to be consistent with predictions based on theoretical calculations of scattering effects based upon the limiting “elastic model” for inelastic collisions. These calculations required only one arbitrarily adjustable parameter (the magnitude of the average energy deposition leading to dissociation) and can account semiquantitatively for energy shifts as well as qualitatively for variations in peak shapes. The limitations of the present approach have also been assessed. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.