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

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The pressure point selection for the multi-point BET equation is a critical step in determining the specific surface area of a material. While the standard measurement range (0.05–0.35) provided by the instrument works well for most mesoporous samples, it may not always be suitable for your particular sample. Always review the BET results to ensure that the selected pressure points and the C constant are reasonable.

It’s important not to use data points at too low relative pressures. Some of these points may approach the origin (0,0), which means they don’t form a complete monolayer and cannot be used to calculate the specific surface area accurately.

Similarly, avoid using pressure points that are too high. High-pressure points can lead to poor linear regression, negative C constants (which indicate an invalid intercept), or incorrect maximum values when calculating the BET surface area. For some samples, especially microporous ones, the BET curve might rise sharply within the "normal" range, indicating that the adsorption capacity exceeds theoretical expectations. In such cases, it's better to collect data at lower pressures.

If the BET curve bends within the normal range, it could mean that the adsorption behavior deviates from the expected model, possibly due to capillary condensation in narrow mesopores (between 2–4 nm). This is common in materials like MCM-48, certain MCM-41 structures, or even in materials like stearic acid and its derivatives.

When selecting pressure points, it's recommended to use at least three points, ideally five, and try to make them as equidistant as possible. Don't discard intermediate data unless a statistical test confirms a significant deviation. Also, pay attention to the sample's behavior at higher pressures—this can help you determine whether the BET model applies or if alternative methods are needed.

Finally, always check the BET curve to ensure that the selected points fall within the appropriate range where the adsorption isotherm behaves predictably. If the curve doesn't show a clear maximum, or if the surface area continues to increase with pressure, the BET equation may not be suitable for that sample. In such cases, consider using other techniques to evaluate the surface properties.

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2, MCM-48, and some MCM-41, stearic acid and stearate