Biomedical Engineering Reference
In-Depth Information
equivalent size, that is, the particle under consideration scatters the same amount of
light as a PSL particle of this size.
2.2 SIZE-INTEGRATED AND TIME-RESOLVED MEASUREMENTS
Size-integrating and time-resolving instruments deliver information on the total
particle concentration (e.g., number, surface area, or mass) within a given particle
size range with a high time resolution. The size range over which the concentra-
tion is integrated is commonly defined by the measurement technique involved.
Additionally, the upper end of the size range is often, deliberately, limited by the use
of an inertial preseparator, that is, a cyclone or an impactor.
2.2.1 C
ondensation
P
artiCle
C
ounters
The most commonly used instrument for measuring nanoparticle number concentra-
tions is the condensation particle counter (CPC, sometimes also referred to as con-
densation nucleus counter). In a CPC, the incoming particles are first exposed to a
supersaturated atmosphere of a working fluid, commonly butanol, isopropyl alcohol,
or water. The principle of the CPC dates back to the late 19th century, when Aitken
(1888) designed the apparatus to quantify dust in the air. A schematic of a modern
CPC is shown in Figure 2.1.
The particles, which otherwise would be too small to scatter light, grow to
optically detectable sizes due to the condensation of the working fluid vapor onto
their surfaces. The resulting droplets are counted in a measurement cell by light scat-
tering to determine the number concentration with high time resolution of commonly
Optical sensor
with LED-light technology
LED
Photo multiplier
Condenser
working fluid condenses
onto the nanoparticles
forming droplets
Saturator
with constantly heated
helical U-shaped channel
Reservoir with
working fluid, e.g.
butanol, water
Aerosol flow
FIGURE 2.1
Schematic of a condensation particle counter (CPC). (Courtesy of Palas
GmbH, Karlsruhe, Germany.)
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