Andersen Cascade Impactor (ACI)
Cascade impactors operate on the principle of inertial impaction. Each stage of the impactor comprises a series of nozzles or jets through which the sample laden air is drawn, directing any airborne particles towards the surface of the collection plate for that particular stage.
Whether a particular particle impacts on that stage is dependent on its aerodynamic diameter. Particles having sufficient inertia will impact on that particular stage collection plate, whilst smaller particles will remain entrained in the air stream and pass to the next stage where the process is repeated.
The stages are normally assembled in a stack or row in order of decreasing particle size. As the jets get smaller, the air velocity increases such that smaller particles are collected. At the end of the test, the particle mass relating to each stage is recovered using a suitable solvent and then analysed usually using HPLC to determine the amount of drug actually present.
To see an animation of how a cascade impactor works, please click on the following link: How Does a Cascade Impactor Work?
The Andersen Cascade Impactor (ACI) is arguably the most commonly used impactor within the pharmaceutical industry for the testing of inhaled products. The ACI is an 8-stage cascade impactor suitable for measuring the aerodynamic particle size distribution (APSD) of both MDIs and DPIs.
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