An Energetic-Control Framework for Electrospray Ionization Using Thermometer Ions

Abstract Electrospray ionization mass spectrometry (ESI-MS) is increasingly used to investigate chemical reactivity. During these experiments, source conditions are frequently adjusted, either deliberately or as part of routine method optimization. However, the energetic consequences of these changes are rarely known, meaning that experiments may unknowingly be performed under different energetic conditions. Consequently, differences observed between experiments may reflect changes in ion activation rather than changes in solution or droplet chemistry, particularly when reactive intermediates or labile species are investigated. Here, we establish a practical, source-specific energetic-control framework using thermometer ions to provide an independent relative energetic reference for a defined electrospray source. A multivariate Design of Experiments revealed that the energetic landscape is governed by strong interactions between source parameters, making the energetic consequences of changing source settings difficult to anticipate and potentially variable between nominally similar experiments. The framework also identified operating regimes in which differential ion transmission distorted the apparent energetic response. After identifying and excluding these transmission-distorted regions, the remaining energetic landscape was accurately described by a predictive multivariate model. The resulting framework provides an independent energetic reference that can be established alongside chemical measurements to distinguish source-induced energetic effects from genuine chemical transformations. We therefore propose that, analogous to routine mass calibration, the energetic response of an electrospray source should be experimentally characterized whenever ion internal energy may influence the observed chemistry.