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Cascade Impactor Calibration, Mensuration and In-Use Qualification Methods: A Practical Guide

Mitchell JP, Roberts DL, Chambers F, Ali A, Shelton C, Nichols SC.

Respiratory Drug Delivery 2010. Volume 1, 2010: 173-184.

Abstract:

A practical guide to the qualification of new and in-use cascade impactors (CIs) is described. The scientifically sound method of CI calibration using aerosolized reference particles of known sizes requires considerable specialized equipment and expertise to achieve accurate and reproducible results. It is time-consuming and excessively expensive to qualify routinely a multi-stage CI in this way. In the future, such calibrations are likely to be confined to determining performance of so-called ‘archival’ CIs, in which care has been taken to fabricate each impaction stage as closely as possible to the nominal specification. Stage mensuration by optical inspection equipment is currently proposed for routine qualification because such methods are easy to automate. However, there is a variety of optical inspection equipment available with different measurement algorithms. A recent multi-center study quantifying precision and accuracy for these systems concluded that despite these differences, they are fit-for-purpose to qualify CIs to their pharmacopeial specifications. The optically certified circular opaque image spots and transparent apertures chosen for accuracy assessment were 2-dimensional, with ring gauges added to mimic more realistically 3-dimensional nozzle profiles. More recently, an alternative calibration object comprising six, 3-dimensional orifices shaped like those of stages 2 to 7 of the Next Generation Pharmaceutical Impactor (NGI) with the nominal NGI stage nozzle diameters has been certified by the National Institute for Standards and Technology (NIST) to the S.I. length standard. The routine measurement of pressure drop across CI stages is a simple-to-do test to monitor impactor performance as part of good laboratory practice (GLP).