Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 32

Aerosol flow

Slit

Sheath air
(closed circuit)

Inner electrode

Outer electrode Inlet

FIGURE 2.5 Schematic of a differential electrical mobility classifier (DEMC). (Courtesy of
Palas GmbH, Karlsruhe, Germany.)

longer be assumed to be specular. The classified particles are then counted in a CPC.
By changing the voltage in the classifier, the full range of electrical mobility can
be covered. In a SMPS, the voltage is continuously or sequentially ramped, which
requires approximately between 2 and 6 minutes. Only very recently much faster
scanning SMPS systems have been introduced (Farnsworth et al. 2013) that accom-
plish a full scan in down to 10 s. As previously described, the primary output of an
SMPS is the distribution of the particle number concentration downstream of the
classifier as a function of the particle electrical mobility. A complex data decon-
volution scheme is then used to obtain the particle number size distribution from
these data (Hoppel 1978; Fissan, Helsper, and Thielen 1983). This data deconvolu-
tion requires the appropriate use of a preseparator. Otherwise, if particles larger than
the ones to be measured in the largest particle size bin enter the DEMC, the multiple
charge correction will overcompensate for multiply charged particles in a lower size
bin, resulting in more or less strong dips in the determined particle size distribution,
which require additional correction (He and Dhaniyala 2013). SMPS systems can
cover particle sizes ranging from 2.5 nm up to around 1000 nm (Winklmayr et al.
1991; Chen et al. 1998), depending on the DEMC and CPC used and the SMPS set-
tings and model. The sizes of particles and molecular clusters smaller than 2.5 nm
can also be classified using a DEMC. The downstream quantification of the particle
number concentration is only possible with an aerosol electrometer, because all com-
mercially available CPCs are only usable down to 2.5 nm. In workplace aerosols,
however, such small particles are usually irrelevant.
A possibility of measuring airborne particle number size distributions with a
higher time resolution of up to 1 s was introduced by Tammet, Mirme, and Tamm
(2002). Their instrument was commercialized as Fast Mobility Particle Sizer
(FMPS model 3091, TSI) and covers a particle size ranging from 5.6 to 560 nm. It
essentially follows a very similar principle set up to an SMPS. Initially, too large
incoming particles are removed in a cyclone. The remaining particles pass a uni-
polar particle charger before being classified based on their electrical mobility. The
classifier has a similar design to the DEMC shown in Figure 2.5, but particles enter