Specified as Apparatus 4 in the United States Pharmacopeia and as Apparatus C in the European Pharmacopoeia, the 5-stage Liquid Impinger (MSLI) originally designed by Astra Draco, Lund, Sweden is a versatile cascade device that can be used for testing both MDIs and DPIs for the determination of Particle Size Distribution.

The Multi-Stage Liquid Impinger consists of 4 impaction stages – each containing a small volume of liquid during operation – and a final filter stage. The liquid in the impinger can be expected to reduce particle bounce and re-entrainment but should be validated if this is and important aspect of the test performance.

The Multi stage Liquid Impinger (MSLI) needs an Induction Port to connect it to the inhaler but does not require a preseparator to run with DPIs. However when operating with a DPI the unit should follow a similar systematic approach as when using the standard configuration of the 8-Stage impactor specified in the Pharmacopoeia.

At 60 lpm the cutpoints of the stages are as follows:

• Stage 1 – 13.0 microns
• Stage 2 – 6.8 microns
• Stage 3 – 3.1 microns
• Stage 4 – 1.7 microns
• Final Filter – <1.7 microns