Biomedical Engineering Reference
In-Depth Information
generated in the corona discharging chamber, after the particles and vapor contaminants
are removed by high-efficiency particulate air (HEPA) and active carbon filters. The two
split flows are mixed in the aerosol charging chamber. Particles exited from the charging
chamber pass through an ion trap, with the voltage set at 20 V, before the electrical charges
carried by particles are measured in an aerosol electrometer of a Faraday cage-type down-
stream of the ion trap.
A modified EAD (MEAD), which consists of a unipolar charger to electrically charge
sampled particles, an ion trap to remove excess ions, and an aerosol electrometer to meter
the resultant current, is used in this study with its ion trap controlled by an external,
high-voltage power supply (Stanford Research System Inc., Model PS325/2500 V-25 W,
Sunnyvale, CA, USA). The ion trap in the MEAD now serves as a size selector, by chang-
ing its voltage and allowing different fractions of charged aerosols to pass through. The
total current of passed aerosols is then measured as the output of the MEAD. Owing to the
charging method used in EADs, a difference in the output of the EAD signal is expected
when test particles are in the same size distributions with different materials.
Li et al. (2009a) evaluated the performance of EADs with the same ion trap voltage
settings as those in NSAMs. Poly- and monodisperse nanoparticles of Ag, NaCl, and oleic
acid were generated and used as test aerosols. For the EAD TB curves, the variation of
EAD TB line slopes is about 15% when varying the dielectric constant of particle materi-
als from infinite to 2.5, and the dependence of size on TB correlation curves is negligible
(less than 5%). The variation of the slope on EAD A correlation lines is generally within
10%. Much of the variation is a result of the size, not from particle materials. In spite of
detectable particle material and size effects on the EAD TB and A correlation curves, it
was concluded that the effects were generally minor at a relatively low concentration (i.e.,
EAD readout less than 1 pA). However, the particle material effect is more pronounced for
polydispersed particle testing. The slope variation due to the particle size in the correlation
line is estimated to be about 5% (Li et al., 2009b). The charging efficiencies and the charge
distribution of particles passing through the EAD charger were further characterized using
monodispersed Ag and PSL particles. It was found that the effect of the particle material
on NSAM readouts is generally minor at low aerosol concentrations. The material effect
is, however, more pronounced at high aerosol concentrations. To reproduce the measured,
intrinsic charging efficiency and charge distribution of particles in the EAD charger, the
birth-and-death particle charging model with two Nit (ion concentration times aerosol
residence time) values were proposed. Modeling results using the proposed model are in
very good agreement with the experimental charge distribution of particles with a diam-
eter less than 1 μm. Based on the information above, it was concluded that MEAD could
be modified as a sizer to indicate particle concentrations and size distributions (Li et al.,
2009a). However, the above instrument has never been validated in the field, especially for
nanoparticles with different material characteristics.
2.4 OCCUPATIONAL NANOPARTICLE EXPOSURES IN VARIOUS INDUSTRIAL
AND DEVELOPMENTAL LABORATORIES
Table 2.1 provides a summary of nanoparticle exposure concentrations from various industry
workplaces or development laboratories. The workplace levels, outdoor ambient levels, or the same
activity levels were measured and listed for comparison. Many monitored activities were related
to the end phase in a commercial-scale production (i.e., bagging and packaging of the product)
(Wake, 2001; Kuhlbusch et al., 2004; Fujitani et al., 2008; Demou et al., 2008; Peters et al., 2009;
Wang et al., 2010). Others were monitored for research-scale activities only, focusing on welding
and grinding (Wake, 2001; Zimmer and Biswas, 2001; Maynard, 2003; Stephenson et al., 2003;
Möhlmann, 2005; Cheng et al., 2008; Dash and D'Arcy, 2008; Evans et al., 2008; Elihn and Berg,
