Air permeability

Specific surface area determination by air permeability

The test is intended for the determination of the specific surface area of dry powders expressed in square metres per gram in the sub-sieve region (<30 µm particle size). 

Similarly to the specific surface area determined by nitrogen adsorption method these results can be used to predict bioavailability, for evaluation of product performance and manufacturing consistency. In this method the air flow rate through a compressed powder bed is measured, which is correlated to the surface area of the substance. The air permeability method will give you different information compared to the nitrogen adsorption since the air is not permeating inside the pores and in the result you will get the outer specific surface area (see the illustration below).

Specific Surface area measured by the air permeability will

  • Give you additional value to correlate with particle size distribution when evaluating batch-to-batch variation of raw materials. The degree of correlation can be higher than for the BET method
  • When compared to the BET results can give an insight on the size of the internal surface area 
  • Will be the best value to correlate with the dissolution results since this area is available for water.
  • Help you to understand the surface properties of your particles
  • Be the utmost critical quality attribute for the development of the dry powder inhalation products
  • Becomes your first aid in the building of your QbD or DoE for micronisation and crystallisation process 

For Material Experts: 

Instrument specifications and measuring principle  

The air permeability instrument applied by Particle Analytical build based on the principals described in the Ph. Eu.2.9.14 an it determines the specific surface area (m²/g) of pharmaceutical powders. 

InstrumentDescribed in Ph. Eur 2.9.14 
USP/Ph. Eur.Ph. Eu.2.9.14
ResultSpecific surface area in m²/g or m²/cm³.

Consult our Experts

Dr. Wenbo Wang
QC Chemist

Dr. Anna Shevchenko
Principal Scientist

Literature

Mathews DH (1957) Measurement of the specific surface area of fine powders. A comparison of the ‘gas‐adsorption’ and ‘air‐permeability’ methods. J App Chem 7(11):610-613. 

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