CALIBRATION AT DIFFERENT FLOW-RATES OF A MULTISTAGE LIQUID IMPINGER

The multistage liquid impinger (MLI) is an impactor suitable for size- selective characterization and testing of pharmaceutical inhalation pr oducts, especially dry powder inhalers (DPIs). The four-stage version of the MLI, which was adopted by the European Pharmacopoeia for testin g at 60 L/min, has been calibrated by two different laboratories (Astr a Draco AB, Sweden, and AEA Technology, UK), reaching consistent resul ts. The MLI has been extended with an additional impaction stage and a ll 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 how 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 th e simplified equation, a cutoff value for any flow rate in the range 3 0-100 L/min can be assigned. This is an important possibility as a DPI should be tested at a how rate derived from the air flow resistance o f the DPI. The cutoff values at 60 L/min for the four impaction stages are 13, 6.8, 3.1, and 1.7 mu m, 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. (C) 1997 Ame rican Association for Aerosol Research.