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
• 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