Pressure Drop Characteristics of the Reduced NGI Configuration with Several Common Glass Fiber Filters

Measurement of aerodynamic particle size distribution, a clinically relevant in vitro attribute of inhalable drug products, involves multistage cascade impactors and is tedious and expensive. A leading candidate for a quicker method is the reduced NGI™ (rNGI). This method involves placing glass fiber filters on top of the nozzles of a chosen NGI stage, selected often to collect all particles with an aerodynamic diameter smaller than approximately five microns. These filters contribute additional flow resistance that can alter the flow rate start-up curve, potentially affecting the size distribution and mass of the drug product dispensed by passive dry powder inhalers (DPIs). The magnitude of these additional flow resistance measurements is currently unreported in the literature. We placed glass fiber filters on top of the stage 3 nozzles of an NGI, along with the necessary support screen and hold-down ring. We measured the pressure drop across NGI stage 3 with the assistance of a delta P lid and a high-precision pressure transducer. With each filter material type and multiple individual filters, we gathered eight replicates at flow rates of 30, 45, and 60 L/min. The filters typically doubled the total pressure drop through the NGI. For example, at a flow rate of 60 L/min, the Whatman 934-AH filters introduced a pressure drop of about 9800 Pa at stage 3, reducing the absolute pressure exiting the NGI to about 23 kPa below ambient, compared with a typical value of 10 kPa for the NGI alone at this flow rate. The pressure drop across typical filters is approximately equal to that through the NGI alone and therefore will affect the flow start-up rate intrinsic to compendial testing of passive DPIs. This change in start-up rate could cause differences between results of the rNGI configuration and those of the full NGI and will increase the required vacuum pump capacity.