Droplet fission

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.

API-electrospray ionization involves three stages. First, there is the formation of charged droplets. Once the droplets are formed, solvent evaporation and droplet fission occur. Droplet fission is due to an increase in charge repulsion at the surface of the droplet as the solvent evaporates. Once the droplets become small enough (<10 nm), it is believed that charge repulsion produces ion evaporation from the surface of the droplet. Thus, ions are transferred from the solution to the gas phase. Factors affecting the production of the desired ions include analyte concentration, flow rate, matrix content, and analyte surface activity. In... 

Richardson et al (1989) performed similar measurements for droplets of sulfuric acid and dioctyl phthalate (DOP) in a quadrupole. Sulfuric acid droplets exploded prior to the Rayleigh limit (at 84 20% of the Rayleigh limit), and the DOP droplets fissioned approximately at the Rayleigh limit... 

As a consequence, the droplet breaks up into a stream of smaller droplets, each one continuing to shrink by evaporation until the Rayleigh stability limit is reached again. The process of droplet fission is repeated several times and it is called uneven fission or droplet jet fission [5,6],... 

The formation of a miniemulsion requires high mechanical agitation to reach a steady state given by a rate equilibrium of droplet fission and fusion. 

Tallin, D.C., T.L. Ward, and E. James Davis. Electrified droplet fission and the Rayleigh limit. Langmuir 5(2) (1989) 376-384. 

It is clear that the process of repeated droplet fissions of both parent and progeny droplets ultimately will lead to very small charged droplets that are the precursors of the gas-phase ions. The mechanisms by which the gas-phase ions are produced from... 

The assumptions with which the scheme (Figure 1.6) for water as solvent was obtained are described in detail in the section entitled Calculations and Experimental in Peschke et al. The stability limits of droplet fission at droplet charge Z=0.9Z/ (just before the droplet fission) and Z = 0.1 Zr (just after the fission due to Beauchamp and co-workers for water) were used (see Figure 1.4 in the present work). These are for droplets of radii in the 13- to 3-pm range, while the nano-droplet evolution scheme (Figure 1.6) involves close to a hundred times smaller radii. It is not known to what extent the droplet fissions of such small droplets follow the same stability limits. Unfortunately, no measurements for such small droplets exist because these droplets evaporate and fission very fast, within several microseconds, while the large droplets fission within intervals of some 40 ms (see Figure 1.4). 

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