Manipulation of Electrospray-Produced Protein and Peptide Ions with Acoustic Fields at Atmospheric Pressure

The coupling of electrospray ionization (ESI) with the newly discovered acoustic ion manipulation (AIM) phenomenon is demonstrated for biopolymer analytes, including peptides, proteins, and DNA. It is shown that large (i.e., up to ca. 12 kDa) and multiply charged ions are influenced by the presence of the acoustic fields. Specifically, we demonstrate the ability to gate, separate, redirect, and focus these biomolecular ions through the use of an acoustic field. To ensure effective acoustic-ion interactions, it was necessary to perform desolvation of the electrospray droplets and ions prior to the AIM field, through optimization of a drying-tube temperature and spray-gas pressure. In general, small and more highly charged ions were more readily deflected from the unstable antinode region towards the acoustic nodes. This trend extended to a charge-state dependent behavior for protein ions, where higher charged ions of the same molecule were redirected more than their lower charge counterparts. This work lays a foundation for the implementation of AIM in the study of biomolecules, as well as other analytes introduced by ESI to ion-based spectroscopic techniques, such as mass spectrometry and ion mobility spectrometry.