Bet Surface Area Analysis

BET Specific Surface Area

The BET (Brunauer, Emmett and Teller) theory is commonly used to evaluate the gas adsorption data and generate a specific surface area result expressed in units of area per mass of sample (m2/g).  The technique is referenced by several standard organizations such as ISO, USP and ASTM. 

This method is widely used for most materials, but is most reliable for materials with a Type II or Type IV isotherm with sufficient level of interaction between the adsorbate gas and the surface.  For materials with other types of isotherms, the BET theory may not apply for various reasons and therefore should be treated with caution. 

Prior to analysis, the sample must be preconditioned to remove physically bonded impurities from the surface of the powder in a process called degassing or outgassing. This is typically accomplished by applying elevated temperature to the sample in conjunction with vacuum or continuously flowing inert gas. This process must be carefully controlled and monitored in order to generate the most accurate and repeatable results.

The specific surface area of a material is then determined by the physical adsorption of a gas (typically nitrogen, krypton, or argon) onto the surface of the sample at cryogenic temperatures (typically liquid nitrogen or liquid argon temperatures). The choice of gas to be used is dependent on the expected surface area and the properties of the sample. Once the amount of adsorbate gas has been measured (either by a volumetric or continuous flow technique), calculations which assume a monomolecular layer of the known gas are applied.  BET surface area analysis must be done in the linear region of the BET plot, which could be systematically evaluated using the Rouquerol transform. 

Specific surface area can also be expressed in units of area per volume of sample (m2/cm3).  This value is termed volume-specific surface area (VSSA) and is obtained by multiplying the BET surface area with the density of the material.  Per the 2011 European Commission Recommendation on the definition of nanomaterials, the VSSA can be used as a complementary qualifier to identify a potential nanomaterial in addition to the particle size distribution of the material.  PTL also has the capability to perform density analyses as well as particle size analyses of your materials. 

PTL can advise whether Static Pressure (Volumetric) or Dynamic Flow (Continuous Flow) will be most suitable for you, and/or whether nitrogen, krypton, or argon gas is best in any particular case. Our lab is well equipped with instruments capable of either adsorption technique, and we possess the technical expertise to provide meticulously executed surface area analysis using the most appropriate gas and procedure. The volumetric analyses can be extended in-house to provide information on the porosity of the material as well.

Single and multi-point BET analyses are available; and PTL utilizes the following BET specific surface area instruments:

Quantachrome Autosorb 3B (Static Pressure / Volumetric Technique) - Click here for a typical sample report.

Micromeritics Tristar II 3020 (Static Pressure / Volumetric Technique) - Click here for a typical sample report.

Micromeritics Flowsorb II 2300 (Dynamic /Continuous Flow Technique) - Click here for a typical sample report.

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