Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 31

Neutralizer

Pre-separator

D
E
M
C

HV supply

CPC

FIGURE 2.4 Typical setup of a scanning mobility particle sizer (SMPS).

neutralizer by soft X-ray sources, which provide comparable charge distributions
(Shimada et al. 2002; Lee et al. 2005), but have lower regulatory and administrative
requirements. Downstream of the neutralizer, the charged particles are introduced
into a DEMC (Liu and Pui 1974; Winklmayr et al. 1991). A DEMC is essentially a
coaxial arrangement with inner and outer electrodes as shown in Figure 2.5.
Particles are introduced near the outer electrode, whereas the space near the inner
electrode is flushed with a particle-free sheath air flow. Depending on manufacturer
and setting, the aerosol flow rate is commonly between 0.1 and 1.0 L/min and the
ratio of sheath air to aerosol flow rate is usually 10:1. Toward the exit, the inner elec-
trode contains a small slit, where the monodisperse aerosol flow leaves the classifier
at the same flow rate as the aerosol inlet flow. If no voltage is applied to the DEMC,
all particles leave the DEMC with the sheath air outlet and the monodisperse aerosol
outlet is particle free. If a voltage is applied between inner and outer electrodes, par-
ticles of one polarity migrate toward the inner rod at a velocity that is defined by the
classifier voltage and the electrical mobility Z p of the particle (Hinds 1999):

⋅⋅
⋅⋅⋅

ned
Cd

m
cm

Z =

(2.2)

p

3 πη ( )

In Equation 2.2, n is the number of particle-borne elementary charges e , d m is
the electrical mobility diameter, h is the gas viscosity, and C c is the Cunningham
slip correction factor (Cunningham 1910; Kim et al. 2005) that corrects for the fact
that molecular reflection from particle surfaces in the submicron size range can no