Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 35

Time-of-right
measurement

Light
scattering
measurement

Light scatter
to electrical
pulse

FIGURE 2.8 Schematic of the time of flight particle sizing in an aerodynamic particle sizer
(APS). (Courtesy of TSI Inc., Shoreview, MN.)

and ELPI+ are rather bulky instruments and require the use of a large pump, which
make them suitable only as stationary measurement instruments and the size infor-
mation is limited to a resolution of 14 size channels.
Another instrument that determines the particle size distribution based on the
aerodynamic particle size is the Aerodynamic Particle Sizer (APS, TSI Inc., USA;
Baron 1986). An APS is a mains-operated desktop instrument, in which the aerosol
is accelerated in a nozzle. The velocity of particles leaving the nozzle is a function
of particle size. The particles pass through two laser light sheets and the particle
size is determined by the time of flight between the two sheets (see Figure 2.8). The
number of particles is then stored into the corresponding particle size bin to eventu-
ally determine the particle number size distribution. An APS nominally covers the
size range from 0.5 to 20 µm and provides size distributions with a resolution of
up to 32 size channels per size decade with a time resolution of up to 1 s. An APS
is often used in combination with an SMPS to cover the full size range from a few
nanometers up to 20 µm. The joint data evaluation from the two instruments requires
conversion of either the aerodynamic diameters from the APS to electrical mobility
diameters or vice versa by using Equation 2.1, which sometimes can be problematic
if the (effective) density of the measured particles is unknown (Khlystov, Stanier,
and Pandis 2004).

2.3.3 i nstruments B ased on o PtiCal C lassifiCation
Optical particle spectrometers measure the light scattered by particles in an aerosol
to measure the particle number size distribution. As a rule of thumb, the minimum
detectable particle size is approximately half the wavelength of the light source used,
that is, commonly around 250 nm. The upper size limit is usually defined by particle
losses in the system and is typically between 10 and 40 μm, approximately. A sche-
matic of an optical particle spectrometer is shown in Figure 2.9.
The aerosol is focused into a narrow particle stream, which passes the light beam
from a light source. Each particle scatters light, which is detected in a photodetector,
where it produces a voltage spike. The height of the spike depends on the amount of
scattered light, which depends on the particle size and refractive index. If the refrac-
tive index is (assumed to be) known, the particle size can be determined. Since in
many cases the particle material is unknown, the refractive index of PSL particles