Adsorbents are usually characterized using parameters such as specific surface area, pore volume and the pore size distribution. These quantities can be derived by analysis of gas sorption isotherms by applying an appropriate theory used to treat the adsorption and/or desorption data.

However, the results obtained for the surface area, pore size etc. are dependent on the applied theoretical method for data analysis and, to some extent, on the chosen experimental method. In order to overcome these problems, the use of certified reference materials and standardized measurement procedures allow one to check and calibrate the performance of sorption analyzers and to compare results from different laboratories. More than twenty certified reference materials for surface area and pore size analysis are now available from the four internationally recognized standard authorities including BAM (Germany), IRRM (European Community), LGC (UK), and NIST (USA). These reference materials generally consist of powders of inert materials such as alumina, titania, silica/quartz, carbons and silicon nitrides.

Much work was done recently in standardizing measuring methods for the surface area and pore size characterization of porous solids. A comprehensive survey of standards on surface structure characterization can be found in the review of Robens et al. Such standards are available from the national standardization organizations, but as a result of the globalization of research and industries, standardization is shifted more to the international organizations, e.g., ISO (International Organization for Standardization, ASTM International (American Society for Testing and Materials).

Often, for the purposes of laboratory analysis, it is necessary to obtain a small quantity of powder from a larger batch. For maximum accuracy and reproducibility, it is necessary that the sample chosen be representative of the larger initial quantity. Here, the term representative means that the sample must possess the same particle and pore size distributions and specific surface area as the larger quantity from which it was obtained. 

To some extent, under even slight agitation, particles tend to segregate with the finer ones settling toward the bottom of the container. When poured from a container into a conical pile, the smaller particles will collect towards the center. This behavior is caused by large particles rolling over the smaller ones and the small particles settling through the voids between the larger ones.

It is generally impossible to make a segregated sample completely homogeneous by shaking, tumbling or any other technique. Often these attempts only further enhance the segregation process. Devices such as the spinning or rotary riffler can be used to obtain representative samples. Rifflers operate on the principle that a sample need not be homogeneous in order to be representative.