By my experience there is virtually no difference in the results you will get - as long as the sample is more or less uniformly porous. But you do have to check this once - if there are significant differences between the samples of the various sieving fractions, it might still be the case that your measurment protocoll doesn´t reach equilibria...

Dear Yonas,

The BET surface area should slightly larger for the smallest particles as the outer surface area depends on the size of the particle.e.

Best Regards

Dear Abdul Salam M. Al-Jumialy ,

Thanks.

Do you mean that I should use the smallest particles (from the example given above: using sieve 50 micro meter ) for measuring BET surface area and get higher?

Dear Yonas,

Yes,

BUT, If the particles are porous materials, the internal surface area should be similar for all particles at the different sizes.

Best Regards