Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 191

Probably the most frequently used method of estimating particle surface area is
indirectly from the number size distribution. If it is assumed that all particles have
simple spherical geometry, then given the number of particles in each size bin, it is
a simple matter to calculate their surface area, which can then be summed across
any specifi ed range of particle sizes. Typically, measurements from the SMPS are
treated in this way and it is perfectly feasibly to estimate a geometric surface area
from SMPS data. If the geometry of the particle structures differs substantial from
spherical, and particularly if the particles are comprised of fractal clusters, then
simple calculations of surface area based upon assumed spherical geometry could
be very considerably in error.
The diffusion charger functions by transmitting the air sample through a corona
charger containing a high concentration of unipolar air ions. The charged ions
diffuse to the particle surface and attach at a rate determined for smaller particles
by gas kinetic considerations, and for larger particles by the rate of diffusion of the
ions through the laminar boundary layer at the particle surface. The resultant
charged particles are collected on a fi lter, where the collected charge is quantifi ed.
In a similar device called an epiphaniometer (Baltensperger et al. , 1988 ), the active
(or Fuchs) surface area of the particles is measured by attaching radioactive lead
atoms to the particle surface. The particles are subsequently collected on a fi lter
which is assayed continuously for its alpha radioactivity. Lead atoms which do not
attach the particle surface are not counted. Both the diffusion charge and epipha-
niometer have the disadvantage of measuring active rather than geometric surface
area except for very small particles, whilst for ultrafi ne particles this diffi culty does
not apply. It is, however, necessary to pre-separate larger particles from the gas
stream before they enter the charging zone of the instrument. Making a size cut in
the nanoparticle range can be achieved by impaction, but this becomes instrumen-
tally quite complex because of the large pressure drops required. Measurements
made with the epiphaniometer can relate well to those calculated from particle size
distribution data (Shi et al. , 2001b ).

5.6.3 Mass Concentration

There are a number of cascade impactors which provide separation of particles
within the nanoparticle range. Most notably amongst these are the Micro-Orifi ce
Uniform Deposit Impactor (MOUDI) and the Berner-type low-pressure impactor.
In addition to the standard MOUDI, there is nano-MOUDI with cut-points down
to 10 nanometres. These devices separate particles on the basis of their inertial
properties by accelerating particles through a fi ne jet below which is a fl at plate on
which particles of higher inertia deposit, whilst those of lower inertia are able to
follow the gas streamlines and avoid impaction. By using progressively higher
velocities on sequential stages, progressively smaller particles deposit on the impac-
tion plates, from which they can be removed or weighed for chemical analysis. Low
pressure impactors, as well as depending on very high velocities for particle impac-
tion, use the fact that the Cunningham slip correction of the particle is reduced at
lower pressure, causing particles to impact more easily than at atmospheric pressure
(Hinds, 1999). The disadvantage of impactors for work with NPs is that for work in