
Practical BET measurements
This is a very brief list of some considerations before running BET measurements. If you would like more detail about using your BELSORP for BET analysis, contact us.
Understand your instrument.
- How the AFSM function works
- Know the sensitivity and accuracy
- What is the recommended sample size
- …
Know your sample
- Can it be heated, exposed to vacuum ?
- How long must it be degassed ?
- Does it contain any solvent or water ? Does it contain evaporable components ?
- Zinc, Magnesium can evaporate when heated in vacuum.
- Is there a risk of damaging it ?
- Heat can cause damage to polymers
- Heating zeolites which contain water can cause hydrothermal decomposition
- Residual solvents can decompose MOF-type materials
- Natural biomaterials can crack and deform when dried
- Could there be any pores ? What sizes ?
- What form is it ?
- If it is granular or monolithic :
- Does it fit inside the cells ?
- Is it necessary to grind or cut it ?
- If it is a powder :
- Is there a problem of elutriation - movement of the powder when vacuum is applied ?
- Is there a problem of static adhesion inside the cell?
Choose analysis conditions
- Sorbate and temperature:
- Nitrogen, Argon, Krypton,
- appropriate amount
- cell and accessories
- degassing and sample preparation method
- measurement method
- use AFSM
- Use GDO ?
Verify your equipment
- Clean cell, frits, cell caps/ autoseal valves
- Caution : In case of static problems, the cleaning method may be contributing, if the detergent leaves silicone or other residues.
- New seals or o-rings ?
- Recent verification of instruments ?
- date of last periodic BELSORP system-check.
- last measurement of a reference sample
- last service of pumps - especially rotary pumps where a turbomolecular pump is not used
- Verify the laboratory microbalance
Loading sample
- Prepare the empty cell - clean it; evacuate and backfill it?
- Weigh the empty cell
- Load the sample
- avoid sample in the stem of the cell
- Evacuate or purge
- Verify no leaks
- Degas
- External degasser? in-situ? both?
- Set correct Temperature
- Record Duration
- Verification of apparent leak rate
- Cooling
- Backfill
- Reweigh
configure the measurement
- Measurement \(P_0\) ( saturated vapour pressure )
- Measurement range
- include desorption
- include full isotherm ?
- Dosing criteria
- Equilibration criteria
After measurement :
- verify reversibility within the BET range (or understand why the isotherm is not reversible)
- Verify that the \(C\) constant is positive : \(C \gt 0\)
- Choose the region where the BET model is valid
- Specifically, \(n_{ads}\,\left(1 - P/P_0\right)\) is increasing continously with \(P/P_0\)
- This is recommended by ISO9277 / IUPAC
- It is particularly important in the presence of micropores
- Specifically, \(n_{ads}\,\left(1 - P/P_0\right)\) is increasing continously with \(P/P_0\)
- Verify that the relative pressure \(P/P_0\) corresponding to the monolayer coverage, \(n_m\), lies inside the range of \(P/P_0\) used for the fit.
- Choose A_m
Reporting
Explicit mentions :
- Gas
- Temperature
- Outgassing conditions and duration
- Range of P/Po used for fitting
- Quality of fit, usually as an R^2 value