Modeling collisional kinetic energy damping, heating, and cooling of ions in mass spectrometers: a tutorial perspective

Abstract

Many powerful methods in mass spectrometry rely on activation of ions by high-energy collisions with gas particles. For example, multiple Collision Induced Dissociation (CID) has been used for many years to determine structural information for ions ranging from small organics to large, native-like protein complexes. More recently, Collision Induced Unfolding (CIU) has proved to be a very powerful method for understanding high-order protein structure and detecting differences between similar proteins. Quantifying the thermochemistry underlying dissociation/unfolding in these experiments can be quite challenging without reliable models of ion heating and cooling. Established physical models of CID are valuable in predicting ion heating but do not explicitly include mechanisms for cooling, which may play a large part in CID/CIU in modern instruments. Ab initio and Molecular Dynamics methods are extremely computationally expensive for modeling CID/CIU of large analytes such as biomolecular ions. In this tutorial perspective, limiting behaviors of ion kinetic energy damping, heating, and cooling set by "extreme" cases are explored, and an Improved Impulsive Collision Theory and associated software ("Ion Simulations of the Physics of Activation", IonSPA) are introduced that can model all of these for partially inelastic collisions. Finally, examples of modeled collisional activation of native-like protein ions under realistic experimental conditions are discussed, with an outlook toward the use of IonSPA in accessing the thermochemical information hidden in CID breakdown curves and CIU fingerprints.

Keywords: collisional activation; ion thermochemistry; kinetics; simulations.

Figure 1.

Schematic illustration of collision geometries and temperature changes in the five models described in the text. Color range from blue to red indicates relatively cold to warm temperatures, respectively. Collisions resulting in ion heating are shown for the totally inelastic and original ICT models, whereas an example of a collision resulting in ion cooling is shown for the IICT model.

Figure 2.

Kinetic energy damping for native-like cytochrome c⁷⁺ with 20 V injection potential into N₂ buffer gas (21 μbar) predicted with IonSPA for collision models described in the section 2. See section 3 for simulation details.

Figure 3.

Ion heating and cooling predicted with IonSPA for the totally inelastic in IICT models for native-like (a) cytochrome c⁷⁺ and (b) GroEL⁷⁰⁺ at injection potentials of 20 and 50 V. For GroEL⁷⁰⁺, solid lines indicate trajectories through the 18-cm Collision Cell, and dotted lines indicate continuation of these trajectories for another 18 cm under the same constant elution field and gas pressure.

Figure 4.

Heating and cooling of native-like cytochrome c⁷⁺ predicted by IonSPAavg for the IICT model in three different buffer gases. (a) illustrates vibrational energy vs. distance through the Collision Cell, and (b) illustrates vibrational energy vs. time.

Figure 5.

Heating and cooling of native-like cytochrome c⁷⁺ at an E/N of 1 Td and a gas pressure of 2.1 mbar in He, N₂, and Ar buffer gas with an injection potential of 50 V. Inset shows zoomed-in region where heating and cooling are most rapid.