Biomedical Engineering Reference
In-Depth Information
Impactors can be used as a cascade with cut-off diameters decreasing from stage
to stage to obtain size-resolved samples. An Electrical Low Pressure Impactor (ELPI
or ELPI+; Dekati, Finland; Keskinen, Pietarinen, and Lehtimäki 1992) uses a total
of 14 stages (ELPI+) to sample particles in the size range from 6 nm to 10 µm. Since
the particle inertia scales with the third power of the particle size, nanoscale particles
can only be sampled by inertia if the gas pressure is significantly reduced. At reduced
pressure, the drag force, which would otherwise cause the particle to be transported
with the flow, decreases and therefore also particles of lower inertia deviate from
the streamlines. ELPI uses a pressure of below 100 mbar in the last stage to collect
such small particles. Before being introduced into the cascade impactor, particles are
getting electrically charged to a known charge level in a unipolar diffusion charger.
The current induced by the deposition of charged particles is measured from each
impaction stage by the use of electrometers to obtain the particle concentration in the
corresponding size bin. As a result, the particle size distribution is measured with a
time resolution of up to 0.1 s. A schematic of ELPI is shown in Figure 2.7. ELPI is
the only instrument on the market that covers the size range from a few nanometers
up to 10 µm based on the same measuring principle and therefore using the same
equivalent particle size, that is, the aerodynamic diameter. Furthermore, it is the only
instrument that not only provides online information on the particle size distribution
but also provides the particle samples from each impaction stage for consecutive
analyses, for example, by electron microscopy or chemical analysis. However, ELPI
Flush pump
and filter
Charger
HV source
Connection to
PC (Optional)
Electrometers
and A/D
Internal PC
Impactor
Display and
controls
Ext. In/Out
Pressure sensor
Vacuum pump
FIGURE 2.7
Schematic of the electrical low pressure impactor (ELPI). (Courtesy of Dekati,
Kangasala, Finland.)
Search WWH ::
Custom Search