I'm trying to measure surface area of ibuprofen powder, melting point 70 degC. I cannot heat it above ~50 degC. Is there anyone who tested this compound before?
Dear Olga,
I have not tested ibuprofen. However, the following file gives some hints how to activate a sample for BET. I have copied important text from the publication for quick view:
Sample Activation
Sample preparation (or activation) prior to sorption analysis is a key aspect of material characterization. To perform a meaningful analysis the material should be in a state that is representative of how it will be used and pretreated accordingly. It is necessary to remove gas and vapors which may have adsorbed onto the surface from the ambient air. If this is not done, the surface area result can be low and non-reproducible since an indeterminate amount of the surface will be covered with these materials. This step must be done whilst making every effort not to change the original surface of the sample. Activation consists of heating a sample under
conditions of dynamic vacuum or purging with an inert gas to remove adsorbed, volatile compounds from the surface. Isotherms and, consequently, BET and pore size distribution results can be greatly influenced by sample preparation since the pore structure can be collapsed upon too stringent activation or insufficiently cleaned (leaving the internal pore structure partially blocked) upon too mild a conditioning. High temperature activation, therefore, can either increase or decrease the porosity. Caution must be used when heating some common pharmaceutical excipients because melting, dehydration, sintering, and decomposition are processes that can drastically alter the surface properties of the
sample.
Hoping this will be helpful,
rafik
Thanks Rafik! This is great. If you know someone who did it with pharmaceuticals, it would be extremely helpful.
Olga
The question is obvious?BTW, ibuprofen is not a volatile compound at all? So what is the relationship between volatility and ibuprofen?
As far I know BET measurements done under using an inert gas.
Olga,
Pharmaceutical ingredients (actives and excipients) ARE routinely measured for surface area by gas adsorption, including low melting point (and low Tg) materials. Degassing the sample must of course be done below the melting point, and low temperatures can in fact still be employed, although longer duration might be required to fully remove adventitiously adsorbed moisture. In fact, the USP monograph for magnesium stearate indicates just 40degC for 2 hours. Repeated measurement of the surface area can be a practical solution to the problem too. Desorption of the nitrogen at the end of the measurement (as the sample warms to room temperature) ablates the surface and does a pretty good job of cleaning it up. Re-analyze until the measured surface area results stabilize.
Good luck.
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