To avoid re-entrainment, the collection surfaces need to be coated with a suitale material.
I choose three coating matrials(glycerol coating, 1%Span in hexane ,1% Tween in EtOH) and uncoated cups to test, and having results between the four groups. But how to evaluate which one is better to my DPIs products?
There is no standard method of coating plates on an impactor. The coating by reducing re-entrainment of particles aims to reduce variability. But the type of coating largely depend on the type of formulation or in other word on the characteristics of the DPIs. MIcronized lactose or more classical coarse lactose based formulations do not require particular coating. At variance ultrafine powders not requiring deaggregation mechanisms may be "resistant" to any type of coating and the NGI is not the ideal impactor. In these cases Andersen of MSLI are much reliable.
To come to you question you may compare variability without coating and after different coating systems.
At the links below you can some information I hope you will find useful. One of the link is on a paper on this topic I found here on Research Gate.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2976949/
Article A Methodology to Study Impactor Particle Reentrainment and a...
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