Have you found yourself confused about how best to determine the surface area of your sample? How does the analysis work? What does “BET” mean? How many points do I need to collect? What is the appropriate analysis gas? Here, we offer answers to some of the most frequently asked questions about gas adsorption surface area measurements.
How is surface area determined using gas adsorption?
To determine specific surface area, we are measuring the quantity of gas required to form a layer of molecules around the surface of a provided sample. The gas used to cover this sample is referred to as the adsorbate and has a known molecular cross-sectional area. If we can determine the quantity of gas molecules required to encapsulate the surface of the sample, we can use the cross-sectional area of the adsorbate to determine the approximate surface area of the sample.
This technique requires the surface of the sample to be free of contaminants. Left unprepared, the sample’s surface can be masked by these contaminants, resulting in an inaccurate measurement. Preparation of the samples is done using a combination of heat and either vacuum or inert gas flow. Once prepared, the sample is introduced to vacuum at cryogenic temperatures. The cryogenic temperatures allow the intermolecular forces between the probe gas and the sample to exceed the forces between the adsorbate gas molecules. This results in the gas molecules being drawn close to the surface of, and physically bonding to, the analyte. When using the volumetric gas adsorption technique, the quantity of gas adsorbed to the surface of the sample is determined by monitoring the pressure in the sample cell before and after dosing a known quantity of adsorbate into the sample cell. The difference between the quantity of gas dosed and the quantity of gas that remains in the free space of the sample cell is the quantity of gas that was adsorbed by the sample.
What is a “BET” surface area measurement?
In 1938 Stephen Brunauer, Paul Hugh Emmett and Edward Teller presented a theory that multilayer gas adsorption systems can be used to quantify specific surface area. The BET (Brunauer, Emmet and Teller) theory, was an extension of the theory presented by Irving Langmuir that described monolayer gas adsorption (Langmuir surface area). The Langmuir model assumes only a monolayer of gas will adsorb to the surface of the sample, while the BET model assumes multilayer adsorption on the sample is possible before monolayer adsorption is complete. Because the Langmuir model does not account for multilayer gas adsorption, which frequently occurs, the specific surface area calculated from this model is often overestimated. Particle Technology Labs uses the BET model as our default model for determining specific surface area. However, we recognize there are times when comparative data may require the use of the Langmuir model. We are happy to calculate your data using either of these models upon request.
How many points do I need to collect? 1, 3, or 5 points?
Particle Technology Labs offers three different analysis options for determining BET specific surface area – a 1, 3, and 5-point analysis option. A single point BET analysis is recommended for analyzing samples with a well-established test method and a specified surface area. This analysis sacrifices precision for speed and is typically reserved for quality control testing in which minimal sample variability from lot to lot is anticipated. If your material has an unknown surface area or you anticipate variability between lots or chemical manufacturers, a 3 or 5-point analysis would be the most suitable. The difference between these testing options is the definition of data collected. The same pressure range will be measured for either a 3 or 5-point analysis. By increasing the number of data points within the measured pressure range, the precision of the measurement is increased.
Which analysis gas should be used?
At Particle Technology Labs, nitrogen and krypton gas are the two most commonly used adsorbates to determine BET specific surface area. Nitrogen is commonly used for materials with a surface area of 1 m2/g or greater while krypton is used for surface area measurements of <1m2/g. This is not a hard and fast rule as the total surface area in the sample cell is just as important as the normalized surface area. For a nitrogen analysis, Particle Technology Labs typically recommends a total of 1m2 of measured surface area in the sample cell. If your sample has a surface area of less than 1m2/g, the total surface area in the cell could be increased to over 1m2 be measuring more than 1g of sample. However, for low surface area samples, increasing the sample mass measured to meet a total surface area measured of 1m2 isn’t always feasible. In this event, krypton gas can be used as an alternative.
Nitrogen has a slightly smaller molecular cross-sectional area when compared to krypton, so why is krypton used for samples with smaller surface area? As noted, the volumetric gas adsorption technique quantifies the volume of gas adsorbed to the sample by taking pressure measurements before and after dosing a known quantity of adsorbate into the sample cell. If too little surface area is available, the difference between the quantity of gas molecules dosed and the quantity of gas molecules left un-adsorbed in the cell will not be large enough to determine an accurate measurement. Krypton when submersed in liquid nitrogen has a vapor pressure significantly lower than nitrogen. This results in approximately 1/300th the number of adsorbate molecules available in the free space of the sample cell. By decreasing the number of gas molecules available in the cell, we effectively increase the difference between the dosed volume of gas and the un-adsorbed quantity of gas after equilibration. This results in a more accurate measurement of low surface area samples.
What is P/Po and why is the measured P/Po range different for some materials?
When determining surface area by volumetric gas adsorption, the quantity of gas adsorbed is measured at various pressure points. These pressure points are relative to the saturation pressure of the adsorbate where P is the pressure in the sample cell and Po is the saturation pressure of the adsorbate. The conventional P/Po measurement range for a BET specific surface area analysis is 0.05-0.3. When you submit a sample to Particle Technology Labs, this is the initial measurement range that will be used to collect data for your sample. Once the data has been collected, we process it using a Rouquerol transform plot. This plot allows us to determine the appropriate maximum P/Po measurement to report. While a P/Po of 0.3 is commonly recognized as the conventional maximum pressure for surface area, this convention does not hold for all samples. Monolayer adsorption is often completed before the P/Po pressure of 0.3. As such, any adsorption measured after monolayer adsorption is completed will result in an inaccurate measurement. Once we have determined the appropriate measurement range, we select the number of points requested for the analysis within the optimum range and determine if further system suitability criteria are met. The data must have a linear fit to the BET transform of 0.9975 or greater. The BET “C” constant must be greater than 0 and the y-intercept of the BET plot must be greater than 0. If all these conditions are met, the BET specific surface area is reported. It should be noted that microporous materials will typically have a shifted BET range lower than P/Po 0.05-0.3.
Still confused? Have no fears – you can count on Particle Technology Labs’ extensive knowledge to select the ideal analysis conditions for your sample.
