Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 33

near the inner electrode and are deflected toward the outer wall. The outer electrode
consists of an array of electrometer rings. Since the location where the particles
hit the outer electrode depends on the electrical mobility of the particles, each one
of the currents from the electrometer electrodes along the outer electrode represents
the concentration of particles in a certain electrical mobility range. The measured
currents are then deconvoluted into a number size distribution by making use of
the known particle charge distribution. Since concentrations are measured continu-
ously over the entire size range covered, the FMPS can provide a much higher time
resolution and thus also allows for the study of dynamically changing aerosols.
The high measurement speed, however, comes at the price of a decreased accuracy,
especially for agglomerated particles and for particles larger than 100 nm (Asbach
et al. 2009b; Jeong and Evans 2009; Kaminski et al. 2013). In these cases, large
deviations were found for both sizing and concentration measurements, whereas
much higher accuracies were obtained in the same studies among SMPS systems of
various manufacturers.

2.3.2 i nstruments B ased on i nertial C lassifiCation
Particles can be classified based on their inertia, which is defined by the particles'
masses and velocities. Inertial classification is usually done by using impactors. In an
impactor, the incoming flow is accelerated in a nozzle before facing a perpendicular
impaction plate to form a stagnation point flow (see Figure 2.6). While small par-
ticles with low inertia follow the low streamlines, particles with higher inertia divert
from the streamlines and are deposited on the impaction plate. The aerodynamic
particle size at which 50% of the particles are deposited, the cut-off diameter, can be
easily predicted by the nozzle diameter, gas flow rate, and the particle density.

Nozzle

Particle trajectories

Flow streamlines

Impaction plate

FIGURE 2.6 Principle of an impactor.