advancing sorption science
 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

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

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