Current Approaches to APSD Measurements of OIPs Based on Inertial Impaction - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 14

Chapter 2
Current Approaches to APSD Measurements
of OIPs Based on Inertial Impaction

Jolyon P. Mitchell and Daryl L. Roberts

Abstract The AIM and EDA concepts are founded on the principles of inertial
impaction of aerosol particles under laminar flow conditions. This chapter examines
the current CI systems that are recognized by the pharmaceutical compendia,
providing a summary of the key parameters that affect the size-resolving capability
of each system. The potential for bias introduced through the assumption that
individual stage collection efficiency curves are step functions at the calibration size
is explored, with attention given to the effect of removing stages in order to achieve
an AIM-based configuration. Non-sizing accessories, such as the induction port (IP)
entry and preseparator (PS), are discussed and the chapter concludes with consider-
ation of how add-on devices, such as spacers and VHCs that are commonly used in
conjunction with MDIs, should be evaluated.

2.1 Introduction

Aerosols produced by all types of OIP comprise either solid particles or liquid drop-
lets suspended in air that the patient receiving therapy is intended to inhale. If sus-
pended particles are present, each droplet itself will strictly be inhomogeneous, if
one or more particles are incorporated within each droplet. For simplicity, the term
“particles” will be used from now onwards to include droplets as well as solid
particles. Aerosols are by definition semistable phenomena, in that the size distribu-
tion of particles continually changes with time due to several physical processes
happening simultaneously [ 1 ]. The most important of these processes affecting the