Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 30

on the model the Partector comes with an in-built sampler for sampling the particles
onto transmission electron microscopy (TEM) grids.
2.3 SIZE-RESOLVED AND TIME-RESOLVED MEASUREMENTS
Size-resolved and time-resolved measurements provide information on the particle
size distribution over time. This type of instrument not only provides the highest
amount of physical information but also requires the highest efforts for data evalu-
ation. Due to the nature of the particles, instruments for the submicron and micron
size range require different measurement techniques. While micron particles are
large enough to scatter sufficient light to be sized optically or provide a sufficient
mass to be classified based on their inertia, particles below 300-500 nm can only be
classified by their electrical mobility and, with some limitations, by inertial impac-
tion. Since the measured particles are usually not spherical, the instruments can
only provide particle size information based on equivalent diameters. In the case of
electrical mobility analysis, the instruments' output is based on the electrical mobil-
ity equivalent diameter, which describes that the respective particle behaves in the
electrostatic classifier like a spherical particle with this particular diameter. If the
particles are classified according to their inertia, besides the particle size, the particle
density also affects the classification. Results are hence reported as aerodynamic
equivalent diameters. Particle sizing by light scattering is not only affected by the
particle size but also by the surface properties of the particles as well as the refractive
index. Particle sizes are therefore commonly reported as PSL-equivalent diameter,
that is, the particle under consideration scatters the same amount of light as a spheri-
cal PSL particle of this diameter.
2.3.1 i nstruments B ased on e leCtriCal m oBility a nalysis
Instruments for measuring particle size distributions based on electrical mobility
analysis commonly comprise of four main components, (a) a preseparator to remove
too large particles, (b) a particle charger to establish a known particle charge distri-
bution, (c) a differential electrical mobility classifier (DEMC, terminology according
to ISO 15900), in the literature often found as DMA, and (d) an instrument for the
quantification of the mobility-classified particle concentrations. Electrical mobility
analysis is useful for measuring the number size distribution of submicron particles
with time resolutions between ≤1 s and several minutes. Electrical mobility analysis
is described in detail in ISO 15900:2009.
The most commonly used type of instrument is the scanning mobility particle
sizer (SMPS, Wang, and Flagan 1990), shown in Figure 2.4. Incoming particles that
are too large are removed in an inertial preseparator, usually an impactor. Particles
are then brought to charge equilibrium (Fuchs 1963), that is, the particles acquire a
bipolar charge distribution, which can be easily predicted (Wiedensohler 1988). Since
the effective global charge level of the particles is nearly zero, the charger is usu-
ally referred to as neutralizer. In the past, the neutralizers all contained radioactive
sources, mostly 241 Am or 85 Kr, to produce the required bipolar ion atmosphere. More
recently, most manufacturers also offer the possibility of replacing the radioactive