Biomedical Engineering Reference
In-Depth Information
Fig. 10.2
“Glaxo” twin
impinger (TI) (
Courtesy of
Copley Scientific Ltd.,
Nottingham, UK
)
and all remaining particles pass through to a final collection media. The upper stage
of the Copley-Fisons impactor has a cut-off diameter of 9.8
m
d
ae
at 60 L/min, so
the boundary between fine-coarse particle size separation would again likely be
viewed as higher than acceptable for present-day OIP applications.
The introduction of the TI was supported at the time by details of its experimen-
tal use, in that the value of the device was perceived as its ability for distinguishing
between “good” and “poor” aerosols, in particular its application in the detection of
agglomerating pMDI formulations, during product development [
5
]. The lack of
sensitivity relative to fuller APSD measurement by the 4-stage multistage liquid
impinger was, however, identified in the mid-1990s as a potential drawback for
more discriminating investigation, especially for the comparison of generic with
innovator OIPs [
9
]. Further studies confirmed initial claims about the insensitivity
of Twin Impinger measurements to variability in operational parameters such as
collecting fluid composition and volume and test flow rate [
7
]. The use of the TI at
low rates in excess of the design value of 60 L/min for low resistance DPI testing
may, however, be a practical proposition [
10
]. Calibration with solid monodisperse
aerosols has confirmed, that like other types of inertial impactor, its cut-off size for
the upper stage decreases at sampling flow rates in excess of the routinely used
60 L/min [
8
], as predicted by the relationship:
μ
12
/
Q
Q
1
d
=
d
ref
(10.1)
ae,50,1
ae,50,ref
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