Many microporous materials, such as zeolites and activated carbons, trap and hold helium in their complex pore structures for many hours after being exposed to helium. Helium trapped in micropores can interfere with the analysis at low pressures, causing an “S-shaped” curve at the lower end of the isotherm*. For this reason, it is recommended that you enter the warm and cold free-space volumes when performing micropore analyses, thereby avoiding exposure of the sample to helium. Two techniques can be used for determining warm and cold free-space values.
The first method is to perform a short analysis on the sample after partial degassing (one pressure point with no incremental dosing), but prior to final sample preparation. Measure the free space during this analysis. The measured free-space values will be printed on the report and may then be entered into the sample file after more thorough sample preparation.
Many microporous materials, such as zeolites and activated carbons, trap and hold helium in their complex pore structures for many hours after being exposed to helium. Helium trapped in micropores can interfere with the analysis at low pressures, causing an “S-shaped” curve at the lower end of the isotherm*. For this reason, it is recommended that you enter the warm and cold free-space volumes when performing micropore analyses, thereby avoiding exposure of the sample to helium. Two techniques can be used for determining warm and cold free-space values.
The first method is to perform a short analysis on the sample after partial degassing (one pressure point with no incremental dosing), but prior to final sample preparation. Measure the free space during this analysis. The measured free-space values will be printed on the report and may then be entered into the sample file after more thorough sample preparation.
Please download to read more.