Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 331

it may be that specifi c surface area is independent of agglomeration/aggregation.
One early example of such a device is the Epiphaniometer (Gaggeler et al. ; 1989 ).
In this device the aerosol is passed through a charging chamber where lead isotopes
created from a decaying actinium source are attached to the particle surfaces. Once
collected on a fi lter, the amount of radioactivity measured is proportional to the
particle specifi c surface area. This is a complex and diffi cult instrument which has
not found widespread acceptance and use.
New specifi c surface area measurement systems are becoming available
however. Wilson et al. (2007) described the use of an Electrical Aerosol Detector
(EAD) as an indicator for the total particle specifi c surface area deposited in
the lung. In this device, sampled aerosol is passed through a diffusion charger in
which the attachment of ions to particles relates to the total specifi c surface area
of an aerosol. By capturing and measuring this charge in an electrometer, an
estimate of the specifi c surface area can be derived. Shin et al. (2007) showed that
the response function of this instrument can be further modifi ed by altering the
voltage of an ion trap within the instrument; this selectively removes a portion of
the aerosol such that the output provides a measure of the deposited specifi c
surface area in the lung. A commercial device, based on this approach is available
from TSI.
Measurements of specifi c surface area are more commonly carried out using bulk
methods such as BET (Brauner- Emmett -Teller) analysis. This widely used process
depends on gas adsorption using nitrogen, krypton, argon or carbon dioxide gas.
The sample sizes used are typically greater than that which might be expected based
on occupational hygiene sampling. Nevertheless, this is an important measurement
process by which bulk materials may be characterised and, as such, could be used
as part of an overall strategy for assessment of exposure.
In addition to these processes, there are a number of imaging processes which
may be used along with scanning (SEM) or transmission (TEM) electron micro-
scopes to obtain size, shape, structure and, with appropriate detectors and imaging
software, quantitative chemical, compositional and morphological information
from single aerosol particles or their agglomerates. Conventional SEMs typically
have a spatial resolution of 5-10 nm whereas TEMs can resolve sizes below 1 nm.
Samples can be collected directly onto fi lters, fi lter substrates or impaction sub-
strates. Filters can collect particles smaller than their pore size; however, it is prefer-
able to use fi lters with a pore size comparable to the smallest particles of interest.
Samples may also be collected directly on to SEM supports using electrostatic
precipitation or diffusion. Prior to imaging, samples are generally coated with gold
or carbon and a commercial sputter coating device that deposits a layer of atoms
a few nanometres thick under vacuum over the sample. Various commercial imaging
packages are available to facilitate the analysis process. Although it has higher
defi nition, TEM often requires more complex arrangements for sample collection
than the SEM. For both SEM and TEM there is an important need to calibrate the
loading of the particles onto the sample fi lter correctly. It is necessary to provide
suffi cient coverage to allow analysis, but to ensure the particles do not touch or
overlap. This can be particularly diffi cult in cases where samples comprise a wide
range of particle sizes.