B–4
Model 3480 Electrospray Aerosol Generator
C a l c u l a t i n g P r i m a r y D r o p l e t D i a m e t e r
As the liquid reaches the end of the capillary, the electric field
induces a surface charge on the liquid. The electrostatic charge of
the liquid produces forces that cause the liquid to disperse into a
fine spray of charged droplets. The diameter of the charged droplet
as it first leaves the tip of the capillary is referred to as the primary
droplet diameter. After the primary droplet is generated, the liquid
from the primary droplet evaporates in the ionization chamber and
an aerosol particle remains. If a sucrose solution is used to
generate an aerosol, the sucrose concentration can be varied to
generate different particle diameters. For any sucrose particle
diameter D
p
, the primary droplet diameter D
d
can be calculated if
the sucrose concentration C (expressed as a decimal) is known by
using the following expression:
p
d
D
C
D
⋅
=
3
/
1
1
Equation
B-2
Empirical formulas expressing the particle diameter (from which
the primary droplet diameter can be calculated) of a 0.1% V/V
sucrose solution as a function of (Q/K)
1/3
, where Q is the liquid flow
rate and K is the electrical conductivity of the liquid, have been
reported in publications such as:
Chen, Da-Ren, David Y.H. Pui, and Stanley L. Kaufman [1995]
“Electrospraying of Conducting Liquids for Monodisperse Aerosol
Generation in the 4 nm to 1.8 µm Diameter Range.”
J. Aerosol Sci.,
26:963-977.
E l e c t r o s p r a y C h a m b e r
Immediately after the highly charged primary droplets are
generated at the capillary tip, they are transported by an air/ CO
2
sheath flow mixture through an orifice plate to the ionization
chamber. This must be done quickly because after the primary
droplet is generated, the liquid begins to evaporate, which
decreases the surface area of the droplet, thereby increasing the
surface charge density. If the surface charge density becomes too
large, the Coulombic repulsive forces on the droplet will cause
Rayleigh disintegration, resulting in a less monodisperse aerosol.
To reduce the charge on each primary droplet, the Electrospray
chamber houses a Polonium-210 radioactive alpha-emitter ionizer.
The nature of the flow through the orifice plate into the ionization
chamber and the placement of the ionizer cause the droplets to
encounter the ions nearly immediately upon their formation.
