Alleviation of ion suppression effect in sonic spray ionization with induced alternating current voltage.

In this study, alleviation of ion suppression effect in sonic spray ionization mass spectrometry (SSI-MS) was investigated. Ion suppression effect was firstly compared between electrospray ionization (ESI) and conventional SSI, and more severe ion suppression effect was observed with SSI. Ion suppression effect of SSI was also found difficult to be alleviated by simply optimizing major parameters. Alternatively, we found that with the assistance of an alternating current (AC) voltage with low amplitude, the ion suppression effect was greatly alleviated (comparable with conventional ESI). That AC voltage was applied outside the SSI spray tip, and no direct contact between the electrode and spray solution was necessary. Besides the alleviation of the ion suppression effect, this newly-developed method, termed as induced electrosonic spray ionization (IESSI), appeared to preserve similar charge state distribution with SSI for protonated cytochrome c, hemoglobin, and bradykinin. IESSI could also obtain significantly improved ion intensities (~1000-fold over conventional SSI). In addition, tolerance of concentrated salts for IESSI-MS was investigated through the analysis of cytochrome c in the presence of concentrated sodium chloride (NaCl) or ammonium acetate (NH4 OAc).