Sprays of fine droplets

Electrospray uses an electric field to produce a spray of fine droplets. 

Under suitable conditions, a stream of solvent containing a substrate (solute) dissolved in it can be broken up into a spray of fine droplets at atmospheric pressure (nebulized). 

As the name implies, thermospray uses heat to produce a spray of fine droplets. Plasmaspray does not produce the spray by using a plasma but, rather, the droplets are produced in a thermospray source and a plasma or corona is used afterward to increase the number of ions produced. 

Sprays of fine droplets can be generated by first mixing a liquid with liquefied gas under pressure and then expanding the mixture through a nozzle. This technique, referred to ssliquefied gas atomization, has been used in many applications such as commercial aerosol cans. The mean droplet size generated with this technique is very small. In very few systematic studies, the measured droplet size distribution was found rather widely spread.[881 It is not clear, however, how the liquid amount, pressure, and nozzle design affect the mean droplet size and size distribution. 

Charged liquid exiting the capillary forms a cone and then a fine filament and finally breaks into a spray of fine droplets (see Figure 22-17c and the opening of this chapter). A droplet shrinks to —1 p,m by solvent evaporation until the repulsive force of the excess charge equals the cohesive force of surface tension. At that point, the droplet breaks up by... 

The rapid rate of stirring desirable for maximum reaction rate often causes spraying of fine droplets of mercury from the seal. This can be prevented by a layer of paraffin oil over the mercury. It is important for the gas-inlet tube to extend below the surface of the stirred liquid, for absorption of hydrogen occurs chiefly at the rapidly agitated surface. 

In general, a pneumatic nozzle can produce sprays of fine droplets to provide a large interface area for heat and mass transfer but the power consumption for atomization is very high. In some cases, e.g., when it is used in technical equipment for environmental protection to remove harmful gases, its high power consumption may become a significant economic problem. 

Electrospray is viewed as the most versatile ionization technique for neutral compounds and ions in solution, and at the same time a general-purpose interface for LC-MS [14-24]. In electrospray ions are formed in solution and then transferred to the gas phase. This differs from APCI where neutral molecules are first transferred to the gas phase and then ionized by gas-phase ion-molecule reactions. The heart of the electrospray source is a metal capillary through which the sample solution flows. A potential of 3-6 kV is applied to the capillary forming a spray of fine droplets directed towards a counter electrode with a sampling orifice located about 1-3 cm from the capillary tip. A positive potential is applied to the capillary to generate positive ions, and a negative potential for negative ions. To accommodate different liquid flow rates droplet formation is assisted by optimization of the orifice diameter of the capillary sprayer, the use of a coaxial gas flow, and heat to increase the rate of solvent evaporation. 

Instrument detection limits (IDLs) for most metals by FIAA are in the low-ppm realm in contrast to graphite furnace AA (GFAA). The conventional premixed chamber-type nebulizer burner is common. The sample is drawn up through the capillary by the decreased pressure created by the expanding oxidant gas at the end of the capillary, and a spray of fine droplets is formed. The droplets are turbulently mixed with additional oxidant and fuel and pass into the burner head and the flame. Large droplets deposit and pass down the drain 85-90% of the sample is discarded in this way. Figures 10-15 in Ref. 2 (pp. 216-218) provides a good schematic of the laminar flow burner. 

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