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

Abstract

Effects of electrospray voltage on cluster size and abundance formed from aqueous CsI were investigated with emitter tip diameters between 260 ± 7 nm and 2.45 ± 0.30 μm. Cluster size increases with increasing voltage, increasing solution concentration and increasing emitter diameter consistent with formation of larger initial droplet sizes. For emitters with tip diameters above ∼1 μm, varying the voltage either up or down leads to reproducible voltage-dependent extents of cluster formation. In contrast, higher voltages with submicron diameter emitters can lead to only Cs+ and Cs­(H2O)+ and no clusters. This change in ion formation reproducibly occurs at spray potentials >1.3 kV for 260 nm emitters and appears to be induced by a corona discharge and material build-up at the emitter tip. Under conditions where abundant Cs+ is observed and no 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 bradykinin and ubiquitin are observed predischarge but not post discharge. Ions with more positive solvation energies can desorb directly from the air–water interface that is created at the tip of these emitters, whereas ions with more negative solvation energies as well as peptide and protein ions do not. These results indicate that ion desorption directly from solution can occur, and similar experiments with even smaller emitters may lead to new insights into ion formation in electrospray ionization.

Overcoming aggregation with laser heated nanoelectrospray mass spectrometry: thermal stability and pathways for loss of bicarbonate from carbonic anhydrase II

“…Emitter flow rates were determined by measuring the mass of the emitter loaded with ∼5-10 µL of solution using an Ohaus Analytical Plus balance (Parsippany, NJ) before and after electrospray at 1.3 kV for 5 minutes. 63,64 To determine the mass loss due to evaporation, the same emitter was placed in front of the mass spectrometer with no spray voltage applied and the mass was measured after 5 minutes. Mass loss due to solution flow during electrospray and evaporation were obtained using a density of 0.99821 g mL −1 for water at 20 °C.…”

“…In order to obtain an estimate of the time that a protein spends in the solution that is directly heated by the laser, the solution flow rate and volume in the heated portion of the emitter were measured. The flow rate was measured gravimetrically, 63,64 and the volume of the tip was approximated by a truncated cone. The inner diameter of the emitter tip and the inner diameter 300 μm away from the tip are ∼2.4 μm and ∼45 μm determined from scanning electron and optical light microscope images, respectively.…”

Modern Electrospray Ionization Mass Spectrometry Techniques for the Characterization of Supramolecules and Coordination Compounds

“…Parameters for common nano-ESI tips can be found elsewhere. 45,47 For successful ionization using nESI, a voltage needs to be applied to the solution inside the tip. Two common options exist: the insertion of an inert metal wire (often Pt) that is connected to the source, or the coating of the nanotip with a conductive material (often Au, Pd or Pt).…”

“…The design of these “nano-tips” needs some consideration, and depending on usage, either the purchase of premade nanotips or the in-house design with capillary pullers is possible. Parameters for common nano-ESI tips can be found elsewhere. , …”

Ion emission from 1–10 MDa salt clusters: individual charge state resolution with charge detection mass spectrometry

“…34 CDMS has the advantage that the m / z , charge, and mass of ions are measured on an individual basis, thereby circumventing issues associated with the spectral congestion of heterogeneous samples analyzed using conventional mass spectrometers. 18,34–48 This capability has extended the size range of ionic clusters that can be investigated by more than 100-fold. The extent of charging on aqueous nanodrops relative to the Rayleigh limit charge for an aqueous nanodrop depends on the identity of ions contained in the droplet.…”