Good Cascade Impactor Practices - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 107

of data points straddling the 50th mass percentile, all yielded MMADs that were in
good agreement with each other [ 79 ]. More importantly, these estimates of MMAD
more accurately fitted the actual data than did values derived using the USP method.
Usefully, the alternative approaches neither require log-transformation of the raw data
nor rely on underlying assumptions about the form of the APSD, other than requiring
that it be unimodal. True lognormal distributions are also adequately addressed by any
of the proposed generalized approaches. The linear two-point interpolation method,
based on the closest data points straddling the MMAD value, is the simplest of these
options to implement and therefore the preferred choice.

4.4.2.3 Other Derived Metrics

There are many other metrics beyond the moments that can be derived from
CI-measured APSDs. These additional measures are conveniently classified into two
subsets: metrics related to EDA and those associated with the more conventional
approach to data reduction that focuses on providing measures that may be predictive
of size-related deposition in the HRT. Table 4.4 summarizes the options for size-
related metrics that are commonly chosen, together with those that can be determined
by AIM-based methods in which an APSD is not obtainable as part of the process.
Each mass subfraction ( LPF , SPF , EPF , FPF , and CPF ) can be determined
directly from a cumulative mass-weighted APSD as the mass fraction correspond-
ing to the chosen size limits for the subfraction of interest. Likewise the correspond-
ing metrics based on absolute mass ( LPM , SPM , EPM , FPM , and CPM ) are obtained
from the original raw data expressed as mass per stage. Stage groupings may be
used to establish these absolute mass values, since more than one stage is usually
involved in the collection of each mass subfraction. However, EDA is more efficient,
in that only LPM and SPM are needed to undertake the assessment of the CI size-
fractionated portion of the OIP aerosol.
It is self-evident from Table 4.4 that the AIM-based approach is capable of pro-
viding the same degree of flexibility with regard to all of these derived metrics,
regardless of the eventual application.

4.4.3 CITDAS and Other Software for Assessing APSD Data

from CI Measurements

A laboratory may develop its own computerized techniques for assessing CI APSDs.
However, commercially available software has been available for some time in
order to assist those in the quite complex process of deriving the most appropriate
measures to report. CITDAS ® (Copley Scientific Ltd, Nottingham, UK) is the most
tailored product for OIP-related applications. Lewis has recently reported that this
software is both versatile and easy to use with an intuitive user interface [ 82 ].
CITDAS allows standardized data processing for the operating conditions of four
alternative impactors: ACI, MSLI, MMI, and NGI. In the current release (CITDAS