The Taylor Cone

If the applied electrical field is high enough, the formation of charged droplets from the cone apex is observed which, due to their charge, further migrate through the atmosphere to the counterelectrode. Experimental data have shown that the droplet formation is strongly influenced by 

In the case of positive-ion analysis, the capillary is usually placed at a positive voltage while the counterelectrode is placed at a negative voltage (this is the case shown in Fig. 1.1). The reverse is used in the case of negative-ion analysis. In both cases, a high number of positive (or negative) charges are present on the droplet surface. 

The formation of the Taylor cone and the subsequent charged droplet generation can be enhanced by the use of a coaxial nitrogen gas stream. This instrumental setup is usually employed in the commercially available electrospray sources Then the formation of charged droplets is due to either electrical and pneumatic forces. 

ESI can be considered as an electrolysis cell and the ion transport takes place in the liquid, not the gas phase. The oxidation reaction yield depends on the electrical potential applied to the capillary, as well as on the electrochemical oxidation potentials from the different possible reactions. Kinetic factors can exhibit only minor effects, considering the low current involved. 

The effect of oxidation reactions at the capillary tip will be the production of an excess of positive ions, together with the production of an electron current flowing through the metal (see Fig. 1.1). An excess of positive ions could be the result of two different phenomena that is, the production of positive ions themselves or the removal of negative ions from the solution. 

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Liquid Metal Sources. The source feed is a metal of low melting point - Ga and In are commonly employed. It is introduced as a liquid film flowing over a needle towards the tip whose radius is relatively blunt (10 pm). The electrostatic and surface tension forces form the liquid into a sharp point known as the Taylor cone. Here the high electric field is sufficient to allow an electron to tunnel from the atom to the surface, leaving the atom ionized. 

Figure 11.2 A schematic of the electrospray process, showing the release of charged droplets from the Taylor cone and the Z-spray arrangement with respect to the sample inlet, sample cone, and the subsequent path of the ions into the analyzer.

Fig. 11.12. Electrospray from a nanoESI capillary. The jet emitted from the Taylor cone is clearly visible and separate from the region of rapid expansion into a plume of microdroplets. By courtesy of New Objective, Woburn, MA.

An alternative model for ESI was developed by the group of Siu [24-25]. They correlated the ion envelope of a protein with the predicted distribution of charge states of the protein in solution. The latter depends on pH and pK values of the acidic and basic amino acids in the protein. From the good correlation obtained, they conclude that the ion envelope nicely reflects the abundances of the preformed ions of the protein in solution. Based on further experiments, they postulate that the droplet evaporation is not that important and that the ESI-MS spectrum results from ions directly emitted from the Taylor cone [25-26]. 

Figure 8.8 Liquid metal ion source. The primary ion beam is extracted from the Taylor cone of liquid metal. (Reproduced with permission from J.C. Vickerman and D. Briggs, ToF-SIMS Surface Analysis by Mass Spectrometry, IM Publications and SurfaceSpectra, Chichester and Manchester. 2001 IM Publications.)...

Fig. 5.3. A photograph of the spray generated in an electrospray ionization source (top) and a schematic view of the processes involved (bottom). From the Taylor cone, a jet is emitted and expands to a plume of micrometer-sized charged droplets from which finally desolvated ions are produced.

Figure 1.1. Schematic of an electrospray source showing the production of charged droplets from the Taylor cone.

Figure 3.3 SIMION electric field models for 28 pm diameter sprayers located 4.7 mm from the ion orifice counter electrode and a spray (solution) voltage of 1.6 kV. Equipotential lines are shown every 50 V. Electric field calculated at the tip of the Taylor cone for each model is shown. (B, C) A comparison of the models shows a 50-fold increase in the electric field generated at the tip of the Taylor cone when the silicon underlying a dielectric film is held at ground potential rather than at the spray potential. Electric field shown in (B) is equivalent to that of a 2 pm diameter pulled capillary with a 1.0 kV spray voltage at a distance of 3 mm from a counter electrode.

Figure 3.4 Graph plotting the changes in the electric field calculated at the tip of the Taylor cone as the distance between the sprayer and counter electrode varies using Equation (3.1) and the SIMION models shown in Figures 3.3A and B. The ESI Chip model (Figure 3.3B) shows that the electric field is independent of the sprayer distance from the counter electrode.

Optical image of a triangular tip before (a) and after (b) the Taylor cone formation (from ref. 12). 

The first attempts to generate ESI from the chip were performed by electrospraying the sample solution directly from the channel terminus at the chip edge (Figure 53.1a). Two major, somewhat interrelated, problems were associated with this approach, that is, the elecfrospray Taylor cone-related dead volumes and the spray instability. Ramsey and Ramsey have estimated that the volume of the Taylor cone at the chip edge is 12 nL, that is, much broader than are the typical CE peak volumes. The flat edge was not ideal for the establishment of a strong electrical field even if... 

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