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Introduction The effects of the introduction of ambient gases into the region of the electrospray ionization (ESI) source have been investigated only in a very limited way. Dopant gases added to the ambient environment of the ESI Taylor cone can influence a variety of parameters associated with the ESI process. Even in the absence of covalent attachment to analytes, ambient gases can influence ESI characteristics such as the onset potential for electrospray, as well as mass spectral features such as the charge state distribution. In this study, a new model has been developed to account for modified onset potentials observed in response to analyte protein characteristics, and as a consequence of the presence of dopant gas in the ESI source region. Methods All experiments were performed on a SolariX ® Fourier Transform-Ion Cyclotron Resonance (FT-ICR) mass spectrometer (Bruker, Bremen, Germany) fitted with a CaptiveSpray ® ion source equipped with a nanoBooster ® (Bruker). This combined accessory is an enclosed chamber that allows the low-pressure addition of volatile dopant gas into the region surrounding the ESI Taylor cone. A manually operated toggle valve switches the arrival of pressurized N 2 gas (default mode) to that of the gas contained in the headspace of an attached solvent-containing glass bottle (the nanoBooster ®). Thus, the opening of the toggle valve introduces solvent vapor into the immediate vicinity of the ESI emitter. This solvent vapor will interact with the liquid stream undergoing ESI, thus acting as a dopant. Preliminary Data A new model has been developed to explain adsorption behavior at the electrospray emitter based upon data obtained using a series of dopant ambient gases in the nanoBooster ® headspace chamber. Four volatile dopants (acetone, acetonitrile, ethyl acetate, methanol) were added sequentially into the CaptiveSpray chamber prompting interactions with aqueous solutions containing three separate proteins (cytochrome c, lysozyme, myoglobin). The isoelectric point of the protein was found to exert a significant influence on the observed onset potential for ESI. Lysozyme, the most basic of the three tested proteins, consistently afforded the lowest onset potential, regardless of which dopant was present in the spray chamber. Myoglobin, the least basic of the three proteins, always yielded a higher onset potential than lysozyme, whereas the intermediate basicity cytochrome c gave variable rankings. These results build upon complementary findings that considered the effects of physical parameters of the employed dopants (dielectric constant, dipole moment, proton affinity, surface tension) on ESI behavior. Of these investigated parameters, the dopant proton affinity was found to exert the clearest influence on ESI onset potential, with lower onset potentials observed for higher proton affinity dopants. These converging trends related to the basicity of the dissolved protein undergoing ESI, and the proton affinity of the added dopant in the spray chamber, are presented here for the first time. This data indicates the importance of surface protons in attracting dopant gases whose arrival thereby modifies the surface of the solution undergoing ESI. A new model to explain the complementary nature of these converging trends is proposed and presented in a visual depiction. These results have implications that can enable a means to increase spray stability when performing ESI using purely aqueous solutions. Novel Aspect A new model has been formulated to explain gaseous dopant-dissolved analyte behavior in the ESI chamber.