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A BET surface area analyser: the BELSORP-mini-X is a volumetric sorption analyser, which can measure isotherm data and perform the calculation of BET surface area.
A BET surface area analyser: the BELSORP-mini-X is a volumetric sorption analyser, which can measure isotherm data and perform the calculation of BET surface area.

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


Explicit mentions :

  • Gas
  • Temperature
  • Outgassing conditions and duration
  • Range of P/Po used for fitting
  • Quality of fit, usually as an R^2 value

Mercer Instruments specialises in the supply and support of scientific instruments for measurements of sorption, and characterisation of pore size and surface area.
Mercer Instruments
for gas and vapour sorption.
In France, Italy, Switzerland, and Benelux.
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