Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 189

grids are available (Maynard 1995), but size-specifi c knowledge of collection effi -
ciencies is essential if absolute number concentrations are to be estimated.
The electrical low pressure impactor (ELPI) provides another rather indirect
means of estimating particle number concentrations (Figure 5.12). Air entering the
ELPI fi rst encounters a unipolar corona charger which imparts electrical charges
upon the particles. The particles then enter a cascade impactor in which they are
separated according to their aerodynamic size (see below). The smaller size frac-
tions lie within the ultrafi ne particle size range. The impacted particles are collected
on electrometers and the rate of charge deposition on those electrometers is related
to the particle number concentration within that size range. Careful calibration of
charging effi ciencies as a function of particle size together with correction for dif-
fusional deposition of ultrafi ne particles on the early (coarse particle) stages of the
impactor allows an estimation of particle number size distribution.
There are also instruments which are a kind of hybrid between the SMPS and
ELPI. Within such instruments, particles are initially charged in a unipolar corona
charger as with the ELPI. They then move into a differential mobility spectrometer
within which they migrate in an electric fi eld orthogonal to the fl ow rate of gas.
However, instead of this instrument being tuned to sample different sizes of par-
ticles sequentially by ramping the voltage, the potential gradient remains constant
but the walls of the differential mobility analyser are lined with electrometers,
Figure 5.12 Diagram of the operating principle of an Electrical Low Pressure Impactor
(ELPI).