A novel method to determine collisional energy transfer efficiency by Fourier transform ion cyclotron resonance mass spectrometry

Abstract

A novel method is presented here to determine accurately the conversion efficiency in low energy collision processes. Using blackbody infrared radiation, the initial thermal energy of a selected molecular ion is both well defined and well known. Collisional activation is subsequently used to probe the additional energy needed to reach a particular final internal energy distribution, characterized by a given fragmentation rate (e.g. 50% of the molecular ion being decomposed). The method is discussed for collision induced dissociation under multiple collision conditions using resonant excitation in a Fourier transform ion cyclotron resonance ion trap. By variation of the thermal energy content the collisional energy necessary to obtain 50% fragmentation rate is also changed. Knowing this change, the collisional to internal energy transfer can accurately be determined. In the case of Leucine-Enkephaline using Ar collision gas it was shown that 4.4% of the laboratory frame collision energy is converted into internal energy in the resonant excitation collision cascade. In individual collisions 9.6% of the centre of mass collision energy is converted into internal energy. Note, that this value is accurately determined as an average for collisions in the 4‐6 eV centre of mass collision energy range, but is approximately the same in the 0‐4 eV range as well. # 1998 John Wiley & Sons, Ltd. One of the big challenges in bio-macromolecular research is to determine the relationship between molecular structure and functionality of large polyatomic molecules. Mass spectrometric studies are more and more frequently employed in the investigation of these macromolecules. Determination of molecular structure using tandem mass spectrometry relies heavily on gas-phase dissociative processes. Dissociation products of large polyatomic molecules provide information on the structure and composition of the molecule, for instance the primary and secondary structure of a peptide. The internal energy, the total energy of a particular species above its electronic, vibrational and rotational ground state, needs to be higher than the dissociation threshold to induce structurally relevant fragmentation. 1 The effect of collisional excitation strongly depends on the time scale of an experiment and the size of the molecule. In many cases reactions take place in a time window anywhere from picosecond to microsecond time scales. Typically these experiments employ an energetic mass selected beam which is transported through a collision cell filled with an inert collision gas, or through a photodissociation cell where the beam is allowed to interact briefly with a focused laser beam. In the interaction region the internal energy is increased such that fast (ps‐ms) dissociation is induced. The product ions in the beam are mass analysed after leaving the dissociation cell. These experiments become inadequate when applied to macromolecular systems, as the number of degrees of freedom increases dramatically. The internal energy required for fragmentation within a given time period increases with the number of degrees of freedom in the molecule. For large molecules, the fragmentation rates will be so low that no measurable fragmentation will occur on the time scale of these beam-type experiments. Consequently, there is a great need for new tools to explore the structure‐function relationship of macromolecules. The question arises as to what the role of gas-phase collisions can be in the examination of macromolecules. Different molecular conformations may