Biomedical Engineering Reference
In-Depth Information
Table 9.3 Changes in APSD of aerosols from OIPs, their detection by the CI method, and
potential for EDA to fail to detect such shifts
Nature of change
CI observation
Condition required for EDA failure
Increasing MMAD
Shift in mass from higher- to
lower-numbered stages in CI
Entire APSD contained within
either LPF or SPF
Decreasing MMAD
Shift in mass from lower- to
higher- numbered stages
Entire APSD contained within
either LPF or SPF
Broadening APSD
(constant MMAD )
Decreased mass on middle
stages and increased mass
on peripheral stages
Either: LPF/SPF boundary
coincident with MMAD
Or: entire APSD contained within
either LPM or SPM
Narrowing APSD
(constant MMAD )
Increased mass on middle stages
and decreased mass on
peripheral stages
Either: LPF/SPF boundary
coincident with MMAD
Or: entire APSD contained within
either LPF or SPF
Change in overall
shape
Change in mass distribution
across all stages accompanied
by change in MMAD
Entire APSD contained within
either LPF or SPF
Change in modality
(unimodal to
bimodal)
Emergence of mass at new
mode, balanced by decrease
mass at original mode
Entire APSD contained within
either LPF or SPF
9.3
Potential Failure Modes: Theoretical Considerations
This topic is best approached by fi rst deriving conceptual changes to the APSD in
which both the sum, ISM , and the ratio, LPM/SPM , terms would remain unaltered
during transport from the inhaler through to the CI-based measurement system.
This strategy is similar to that which would have to be taken in the wider consider-
ation of APSD changes to the OIP-produced aerosol, in the context of product sta-
bility evaluations, forming part of that specifi c OIP development process.
Mitchell et al . envisaged a series of hypothetical situations that have the potential
for EDA to fail [ 3 ]. However, at the outset, it was recognized that the probability is
small that such changes would occur precisely so that the portion of the APSD
beyond the boundary between either LPF or SPF , depending on the scenario under
consideration, is unaffected.
Figure 9.2 summarizes in simplifi ed form the aerosol transport conditions appli-
cable in a typical CI system.
Apart from the intentional deposition of particles to the various stages following
size-fractionation on the basis of their differing inertia, nonideal deposition to the
interior walls of the system may occur to a greater or lesser extent, depending on the
CI design. The backup fi lter is assumed 100% effi cient as a collector of particles
that penetrate beyond the last impaction stage, but this assumption may not be true
for the MOC used with the NGI. Besides mechanisms responsible for particle depo-
sition, there is the possibility of APSD change being brought about as the result of
particle-particle interactions in the aerosol phase.
 
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