Lighting Up at High Potential: Effects of Voltage and Emitter Size in Nanoelectrospray Ionization

Effects of electrospray voltage on cluster size and abundanceformedfrom aqueous CsI were investigated with emitter tip diameters between260 +/- 7 nm and 2.45 +/- 0.30 mu m. Cluster size increaseswith increasing voltage, increasing solution concentration and increasingemitter diameter consistent with formation of larger initial dropletsizes. For emitters with tip diameters above similar to 1 mu m, varyingthe voltage either up or down leads to reproducible voltage-dependentextents of cluster formation. In contrast, higher voltages with submicrondiameter emitters can lead to only Cs+ and Cs-(H2O)(+) and no clusters. This change in ion formation reproduciblyoccurs at spray potentials >1.3 kV for 260 nm emitters and appearsto be induced by a corona discharge and material build-up at the emittertip. Under conditions where abundant Cs+ is observed andno clusters are formed, ions such as K+ and Cu1+ are also observed but ions with more negative solvation energies,such as Ba2+, are not. Similarly, ions from bradykininand ubiquitin are observed predischarge but not post discharge. Ionswith more positive solvation energies can desorb directly from theair-water interface that is created at the tip of these emitters,whereas ions with more negative solvation energies as well as peptideand protein ions do not. These results indicate that ion desorptiondirectly from solution can occur, and similar experiments with evensmaller emitters may lead to new insights into ion formation in electrosprayionization.

Automated summary

The effects of electrospray voltage on the size and abundance of aqueous CsI clusters were studied. It was found that increasing voltage, solution concentration, and emitter diameter led to larger cluster sizes. Reproducible voltage-dependent extents of cluster formation were observed for emitters with tip diameters above 1 μm, while for submicron diameter emitters, higher voltages only resulted in Cs+ and Cs-(H2O)(+) ions without cluster formation. This ion formation change was induced by corona discharge and material build-up at the emitter tip.