Charged droplet explosion

Fig. 11.5. Diagram illustrating the components of an ESI source. A solution from a pump or the eluent from an HPLC is introduced through a narrow gage needle (approximately 150 pm i.d.). The voltage differential (4-5 kV) between the needle and the counter electrode causes the solution to form a fine spray of small charged droplets. At elevated flow rates (greater than a few pl/min up to 1 ml/min), the formation of droplets is assisted by a high velocity flow of N2 (pneumatically assisted ESI). Once formed, the droplets diminish in size due to evaporative processes and droplet fission resulting from coulombic repulsion (the so-called coulombic explosions ). The preformed ions in the droplets remain after complete evaporation of the solvent or are ejected from the droplet surface (ion evaporation) by the same forces of coulombic repulsion that cause droplet fission. The ions are transformed into the vacuum envelope of the instrument and to the mass analyzer(s) through the heated transfer tube, one or more skimmers and a series of lenses.

Figure 14.2 Principle of electrospray ionization, (a) The analyte is dissolved in an appropriate solvent and sprayed via a capillary into an electric field. Here, the liquid filament finally forms charged droplets, (b) The solvent of the charged droplets evaporates, resulting in an increase of the surface charge up to a critical boundary, at which a Coulomb explosion occurs. The newly formed droplets undergo the same process. The final products are the desolvated, naked ions, which are then entering the mass spectrometer.

Figure 6.18 HPLC-MS interface for ESI. 1 —from HPLC column 2 = nebulizing gas 3 high voltage 4 = charged droplet (drawn too large) 5 = evaporation 6 = coulomb explosion 7 = drying gas 8 — skimmer 9 = quadrupole.

The charges, with the signs opposite to the sign of the electric potential applied to the MS inlet, accumulate on the liquid meniscus at the capillary outlet. This process is followed by the formation of a Taylor cone and Coulomb explosions which lead to the formation of fine droplets. The mechanism responsible for generation of charged droplets and ions in Pl-ESI appears to be similar to that found in the conventional ESI process (Section 2.4). 

Figure 1.4 Schematic representation of the explosion of charged droplets.

Schematic of an electrospray system. In order to prevent droplet explosion during evaporation of aerosol droplets, the highly charged aerosol is passed through a radioactive neutralizer before evaporation occurs. Reproduced with permission from Chen, X. Hu, X. Feng, J. Nanostruct. Mater. 1995, 6, 309. Copyright 1995 Elsevier B.V.

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