Role of Ion Charge and Size in Susceptibility to Acoustic Ion Manipulation (AIM)

The recent coupling of electrospray ionization (ESI) with acoustic ion manipulation (AIM) has expanded the range of ionic species that can be successfully controlled with an acoustic field. The use of a drying tube was critical to this process to ensure sufficient desolvation and enable the strongest AIM interaction. Here, we explore how desolvation of small-molecule and protein ions impacts the AIM process in more detail. Specifically, acoustic gating of ions from ubiquitin, cytochrome c, and a standard mass calibration mixture was studied for drying tube temperatures from 30 to 210 °C. Transmission of an ion beam trajectory through an acoustic antinode has been previously established as a reliable and simple measure for the acoustic-ion interaction cross section. The transmission of different charge states of protein ions was found to be heavily temperature dependent. This unique observation enabled exploration of the potential impact of different ESI mechanisms on the availability of ions for AIM interactions. The behaviors of singly charged and multiply charged ions traversing a standing acoustic wave were compared and suggest that the electrostatic properties, as well as the resulting higher-order structure, of ions at least partially explain AIM behaviors. Additionally, the increased desolvation temperature led to ion transmission values consistent with dry, plasma-produced ions (i.e., less than 5%) across all species tested. The use of a higher drying temperature ultimately further optimized ESI-AIM, as well as highlighted the role of ion properties during the AIM phenomenon.