Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 28

instrument, but in a smaller packaging and means for operation independent of mains
power and an external computer. miniDiSC/DiSCmini and nanoTracer are signifi-
cantly smaller, handheld, and independently operating instruments that are particu-
larly suited for tier 2 screening measurements (see Chapter 11), and the Partector is
the first true personal monitor to measure the nanoparticle exposure in the breath-
ing zone of a worker. All aforementioned diffusion charger-based instruments use
a very similar set up as shown in Figure 2.3. The particles pass through a unipolar
charger, where they acquire a known and predictable number of elementary charges
(Kaminski et al. 2012). These unipolar chargers use corona discharge, caused by a
highly inhomogeneous electric field near a corona electrode (usually a thin wire or a
sharp tip), to ionize the air. Brownian motion causes these produced ions to collide
with the particles and transfer their charge. In the next stage, the charged aerosol
enters an ion trap, which is an electrostatic precipitator operated at a rather low
voltage, sufficient to trap ions, but ideally leaving charged particles unaffected. In
an NSAM and Aerotrak 9000, the ion trap can be set to two different voltages (100
or 200 V), which are both higher than required for the removal of ions only (20 V).
As a consequence, a certain fraction of very small (<20 nm) charged particles are
also captured in the ion trap. This is done to adjust the response function of the
instrument to the functions required to mimic particle deposition in the alveolar or
tracheobronchial region of the human lung (Fissan et al. 2007; Shin et al. 2007). It
was, however, found that the measured differences are minute and that a single ion
trap voltage can be used to mimic both deposition rates as well as the deposition in
the total respiratory tract (Asbach et al. 2009a). The ion trap and electrical manipula-
tion step, shown in Figure 2.3, are hence combined in an NSAM. The particles are
eventually captured on a filter in a Faraday cup electrometer to measure the particle
induced current. The LDSA is then derived from the current by the application of a
simple calibration factor.

Time

I 1 , I 2

I sensor = I 1
I sensor = I 2
I sensor = 0

- nanoTracer
- nanoCheck

EI. Manip.
PSD

Filter

Diffus.
charger

Ion
trap

Mech.
Man. PSD

- miniDISC

Filter

I 1 I 2

FIGURE 2.3 Schematic setup of diffusion charger-based monitors.