Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 36

Particle induced
voltage signals

Relative
frequency distribution

V

2

V N

1

Beam trap

10 20 ms 123 µm

Time Particle size

Signal
processing

Amplifier

Light source

Photo detector

Vacuum pump

FIGURE 2.9 Principle of an optical particle spectrometer. (Borggräfe, P. (2000): Erweiterung
des Meßbereichs von optischen Partikelzählern durch gezielte Reduzierung der Störquellen
und mit digitalen Signalverarbeitungsmethoden, Ph.D. Thesis at Gerhard Mercator University
Duisburg, available online: http://duepublico.uni-duisburg-essen.de/servlets/DerivateServlet/
Derivate-5033/bodiss.pdf [accessed November 23rd, 2013].)

is used and the particle sizes delivered as “PSL-equivalent” sizes. By counting the
number of particles of different sizes, the particle number size distribution is deter-
mined. Some of the commercially available optical particle spectrometers calculate
the mass concentration fractions in specific size ranges (e.g., PM 10 or PM 2.5 ), based
on assumed particle size-dependent mean refractive indices and particle densities.
These instruments are usually calibrated using empirically derived mean values of
atmospheric particles. Hence, care needs to be taken when such spectrometers are
used for the determination of particle mass concentrations in workplaces, where
refractive indices and particle densities may differ significantly from those in the
atmosphere.
Several manufacturers couple an optical spectrometer for the characteriza-
tion of mainly micron size particles with SMPS measurements for submicron par-
ticles (e.g., Wide Range Aerosol Spectrometer WRAS model EDM 665, Grimm
Aerosol, Germany, or Wide Range Particle Spectrometer model M1000XP, MSP,
USA). Grimm Aerosol additionally introduced two different instruments that couple
a spectrometer coupled with diffusion charger-based monitors. In the case of the
nanoCheck, the diffusion charger instruments follow the identical principle to the
nanoTracer (see the aforementioned) to provide the total number concentration and
mean size of particles below 300 nm. The diffusion charger-based instrument in
the miniWRAS (model 1.371) is more sophisticated and uses a simplified electri-
cal mobility classifier to provide particle size distributions below 300 nm with a
10 channel resolution. Similar to the combination of APS with SMPS, merging of
the data from the two measuring techniques requires the conversion from electrical
mobility equivalent diameter to the optical equivalent diameter or vice versa.