Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 29

The nanoTracer (Marra et al. 2010) uses a slightly different principle, originally
invented by Burtscher and Schmidt-Ott (2009). The charger is followed by an electri-
cal manipulation stage, which consists of an electrostatic precipitator, with a square
wave voltage applied to it. During times when the electrostatic precipitator (ESP)
voltage is low, only ions are removed, whereas when the voltage is high, a certain
fraction of small particles is also collected in the ESP. The current stemming from
the particle-borne charges is continuously measured downstream of the ESP, result-
ing in two independent current levels being measured during high and low voltage
periods. When a low voltage is applied to the ESP the measured current I 1 is again
proportional to the LDSA concentration. Due to the size dependence of the par-
ticle charging and hence the particle collection in the ESP, the ratio of the currents
I 1 / I 2 measured with the different ESP voltages is a function of particle size and is
therefore used to determine the mean particle diameter. The mean particle diameter
and the LDSA concentration are then used to determine the total particle number
concentration. Results are delivered with a time resolution of 16 s. According to the
manufacturer's specifications, the modal diameters of the measured aerosols need to
be in the range from 20 to 120 nm for the nanoTracer to measure accurately.
The miniDiSC (Fierz et al. 2011) and its commercial counterpart DiSCmini
(Matter Aerosol, Switzerland) use a similar approach to the nanoTracer, but the
manipulation of the particle size distribution is done by mechanical instead of elec-
trostatic means, that is, it follows the lower branch in Figure 2.3. The instrument uses
a dual stage particle deposition system. Small particles are preferentially deposited
on a stack of diffusion screens in the first stage and all remaining particles are col-
lected on a high efficiency filter in the second stage. Both stages are connected to
separate electrometers such that the resulting two currents are measured simultane-
ously, resulting in a high time resolution of 1 s. The overall size range of the mini-
DiSC is limited to 10-300 nm.
The aforementioned diffusion charger-based instruments have been subject to
several round robin and comparability tests (Asbach et al. 2012; Meier, Clark, and
Riediker 2013; Mills, Park, and Peters 2013; Kaminski et al. 2013). The main out-
come is that, although some limitations apply, this type of instrument can be reliably
used for the determination of airborne particle concentrations. However, the accu-
racy of the results should be expected to be only around ±30%.
The Partector (naneos, Switzerland) is the latest and smallest diffusion charger-
based instrument on the market. It is approximately the size of a cigarette box and
can hence be used as a personal monitor. It follows the same overall principle as
shown in Figure 2.3, but lacks a manipulation step. The particles are charged in a
unipolar corona diffusion charger and the ions are removed in an ion trap. The par-
ticle charge is detected without particle deposition through induced currents in an
induction stage. Since induction only occurs in the presence of a charge gradient, the
particle charger is operated in a pulsed mode (Fierz et al. 2014). As previously dis-
cussed, the charge level acquired by particles in a unipolar diffusion charger is pro-
portional to the alveolar LDSA concentration, which is hence the metric measured
by the Partector. The manufacturer specifies the particle size range to be from 10 nm
to 10 µm. Since the particles are not deposited for the measurement of the particle
charge, they are available for electrostatic collection for further analyses. Depending