Environmental Engineering Reference
In-Depth Information
Figure 5.11 Schematic of a Scanning Mobility Particle Sizer (SMPS).
be applied for multiply charged particles and is incorporated in the instrument
software.
In a recent variant on the Scanning Mobility Particle Sizer referred to as the
SMPS+E, the condensation particle counter is replaced with a Faraday cup elec-
trometer that measures the accumulated electric charge upon particles which enter
it. Such a device is not subject to the lower size limitation imposed by the conden-
sation particle counter and, therefore, has potential attractions in relation to mea-
surement of nanometre size particles.
The precise measurement range of a Scanning Mobility Particle Sizer is deter-
mined by the fl ow rates used within it but typically such instruments contain an
impactor designed to remove larger particles and are limited to sizing particles of
no greater than 1
m diameter, although often they are operated in a size range of
only up to around 500 nm diameter.
Although rarely used in practice, off-line electron microscopy of sampled par-
ticles is capable of generating size-fractionated number concentrations. A major
diffi culty, however, lies with collecting suitable samples for microscopy. Filtration
is unlikely to be very suitable as most aerosol fi lters are depth fi lters and NPs can
get lost between the fi bres. Nuclepore fi lters have a fl at surface but are of limited
effi ciency for very small particles. Sampling devices based upon electrical charging
and deposition by thermophoretic precipitation directly onto electron microscopy
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