Multi-point BET specific surface area how to choose points - Database & Sql Blog Articles

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The multi-point BET equation is a widely used method for determining the specific surface area of porous materials. However, it's important to note that the pressure range (0.05–0.35) provided by most instruments is typically suitable for mesoporous samples but may not always be appropriate for your particular sample. Therefore, it’s essential to carefully evaluate the data and ensure that the selected pressure points and the C constant are reasonable.

It's advisable not to use pressure points that are too low, as they may fall close to the origin (0,0). These points often do not form a proper monolayer and cannot be used effectively in the BET calculation. Similarly, using pressure points that are too high can lead to inaccuracies, such as poor linear regression coefficients, negative C constants, or incorrect assumptions about the maximum adsorption capacity.

In some cases, the BET equation may not apply at all. For example, if the BET curve continues to rise without reaching a maximum within the P/P0 < 0.15 range, this could indicate that the material has an unusual structure, such as micropores or irregular surfaces, where traditional BET analysis is not suitable. In such situations, alternative methods may be required for accurate surface area determination.

When selecting pressure points, it's recommended to avoid low-end non-equilibrium data, as shown in Figure 1 on the right. It’s also best to use at least three data points, ideally five, and choose them as evenly spaced as possible. Unless statistical tests show significant deviations, intermediate data points should not be discarded. Additionally, it’s crucial to observe the sample’s behavior at higher pressures during the BET test. If the curve rises above the expected range, it might suggest that the sample has a high adsorption capacity, commonly seen in microporous materials. On the other hand, if the curve bends within the normal range, it could indicate stronger multilayer adsorption or capillary condensation in narrow mesopores (e.g., 2–4 nm), which may affect the accuracy of the BET results.

Some advanced applications include improvements in systems like Autosorb-1MP/C, MCM-48, certain MCM-41 structures, and materials such as stearic acid and its derivatives. These developments help refine the measurement process and improve the reliability of surface area calculations for complex materials.