Calibration at Different Flow Rates of a Multistage Liquid Impinger

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Abstract
The multistage liquid impinger (MLI) is an impactor suitable for size-selective characterization and testing of pharmaceutical inhalation products, especially dry powder inhalers (DPIs). The four-stage version of the MLI, which was adopted by the European Pharmacopoeia for testing at 60 L/min, has been calibrated by two different laboratories (Astra Draco AB, Sweden, and AEA Technology, UK), reaching consistent results. The MLI has been extended with an additional impaction stage and all impaction stages have been calibrated at flow rates of 30, 40, 50, 60, 80, and 100 L/min. It is shown that in this flow range, the cutoff diameter depends on the flow rate according to a simplified impaction equation, i.e., the cutoff diameter is proportional to the inverse of the square root of the flow rate. Based on the calibration data and the simplified equation, a cutoff value for any flow rate in the range 30–100 L/min can be assigned. This is an important possibility as a DPI should be tested at a flow rate derived from the air flow resistance of the DPI. The cutoff values at 60 L/min for the four impaction stages are 13, 6.8, 3.1, and 1.7 μm, the fifth stage being a filter stage of the five-stage MLI. The apparatus has also been shown to be robust and to have a high degree of uniformity between replicas.