Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 39

surface filters, whereas fiber filters are used for depth filtration. In a membrane filter,
the air is drawn through small well defined holes. Particles deposit on the surface by
impaction, interception, or diffusion. A fiber filter contains many zigzagging fibers.
Particles are deposited on the fibers inside the media based on the same deposition
mechanisms as previously described for the membrane filter. The pressure drop of
fiber filters is for a given efficiency lower than in membrane filters. The choice of
surface or depth filtration media depends on the purpose of the sampling. Particles
collected on a filter surface can much more easily be retained and then, for example,
be used in toxicological in vitro or in vivo experiments or dispersed in a liquid to
analyze their potential to generate reactive oxygen species (Hellack 2012). Depth
filters are more commonly used for determining the particle mass of the sampled
particles or for digesting wet chemical analyses.
Single stage impactors with subsequent particle filtration are often used to sam-
ple particles in certain size fraction, such as the thoracic (<10 µm) or the alveolar
(<4 µm) fraction. The obtained filters can be weighed to determine the particle mass
concentration and used for wet chemical analyses. These systems are available as
large and bulky versions with high flow rates for sampling large amounts of par-
ticles, which may become necessary, depending on the objective of the sampling.
Other such systems are small and usable as personal samplers (e.g., Tsai et al. 2012)
2.5.2 e leCtrostatiC P artiCle s amPlers
Electrostatic precipitators (ESPs) are used to deposit electrically charged particles
by exposing them to a Coulomb force in an electric field. ESPs are used in a wide
range of applications, including sampling of particles for consecutive analysis. In
commercial ESPs, suitable for workplace aerosol analysis, the incoming aerosol flow
faces a perpendicular substrate, which is maintained at a high electric potential,
whereas the housing of the ESP is kept at ground potential (Dixkens and Fissan
1999). Consequently, particles of opposite polarity as the collecting electrode are
collected on the substrate. These ESPs can be used to sample airborne particles by
their naturally occuring charge, without additional charging. Aged particles in the
atmosphere carry a bipolar equilibrium charge distribution and hence a nonnegli-
gible fraction of the particles is charged and will be collected. To enhance the sam-
pling efficiency, particles may be charged unipolarly upstream of the ESP. However,
currently there are no commercially available particle chargers that could be used
for this purpose and hence all studies that report on the use of a particle charger
upstream of a commercial ESP used home-built unipolar chargers (e.g., Van Landuyt
et al. 2014). Another option is to use an ESP downstream of a DEMC to sample
mobility-classified particles. Particles leaving a DEMC are always charged, however,
most of them bear only a single elementary charge, resulting in a rather low collec-
tion efficiency especially for larger particles.
More recently, a novel, commercial handheld ESP was introduced (ESPnano
model 100, Miller et al. 2010). The sampler has a small size and is battery oper-
ated and can hence easily be taken to sample particles at a certain location, where
an increased particle concentration or a particle release is expected. A schematic of
the ESP is shown in Figure 2.11. This ESP includes a unipolar charger to generate