advancing sorption science
A BET surface area analyser: the BELSORP-mini-X.
A BET surface area analyser: the BELSORP-mini-X.

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
    • or maybe your instrument uses some other method of free space control
  • Know the sensitivity and accuracy
  • What is the recommended sample size
  • Has the instrument been serviced ? Was a reference sample run recently ? Or other controls ?

Know your sample

  • Can it be heated ? Can it be exposed to vacuum ?
  • Does it contain any solvent or water ?
    • The effect of heating may be far greater when solvents are present.
    • It may be necessary to degas in several stages, starting at low temperature, and progressively increasing.
  • How long must it be degassed ?
  • Does it contain evaporable components ?
    • Even metals, such as Zinc and Magnesium can evaporate when heated in vacuum.
  • Is there a risk of damaging it ?
    • Heat can cause damage to polymers and other organic compounds
    • Heating zeolites which contain water can cause hydrothermal decomposition
    • Cements, clays, silicates, and many related materials
    • Residual solvents can decompose MOF-type materials
    • Natural biomaterials can crack and deform when dried
  • Heat can also cause structural changes:
    • Recrystallisation after heating above the Glass transition temperature
    • Strain relief and annealing of disorder
  • Could there be any pores ? What sizes ?
    • If you want to get knowledge about the pores, it is necessary to measure at suitable pressures
    • The presence of micropores normally increases the difficulty of outgassing.
  • What form is it ?
    • If it is granular or monolithic :
      • Does it fit inside the cells ?
      • Is it necessary to grind or cut it ?
      • Is it necessary to use it whole ?
    • 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 on the sides of the cell?
  • Do you already know what BET surface area you expect ?

Choose the analysis conditions

  • Sorptive and temperature:
    • Nitrogen 77 K
    • Argon 77 K
    • Argon 87 K
    • Krypton 77 K
    • Water at 298 K
    • Other vapors
  • Select an appropriate amount of sample
    • Typically, your manufacturer will indicate a minimum amount of surface area in the cell.
    • Remember there are other considerations in choosing sample size…
  • 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 - it must be clean and dry
    • It may be necessary to evacuate and backfill it with inert gas
    • Some samples absorb significant amounts of nitrogen, even at room temperature, so it may be necessary to use helium at every weighing stage
  • Weigh the empty cell
  • Load the sample
    • avoid sample in the stem of the cell
  • Weigh the sample + cell, before any outgassing
  • Evacuate or purge
  • Verify there are no leaks
  • Degas
    • External degasser? in-situ? both?
    • Set the correct Temperature
      • or temperature profile
    • Record the duration of degassing
    • Verify the apparent leak rate
    • Cooling
    • Backfill the cell with the same gas used for the empty weighing
  • Reweigh the sample when the moisture has been removed

Configure the measurement

  • Measurement of the value of \(P_0\) ( saturated vapour pressure )
  • Measurement range of \(P\) or \(P/P_0\)
    • Must at least cover the BET range
    • It is very useful to include desorption points
      • If the adsorption interaction is truly physisorption, then the desorption curve should superpose the adsorption curve - it should be reversible. Otherwise, there may be a problem with the measurement.
    • Do you want to include a full isotherm to see details of the mesopores ?
  • Dosing criteria
  • Equilibration criteria
    • These are especially important for measurements of microporous materials at low pressures, where the equilibration can be very slow.

After the measurement :

  • Verify the 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, \(n_{ads}\,\left(1 - P/P_0\right)\) should be increasing continously with \(P/P_0\)
      • This is recommended by ISO9277 / IUPAC - it is commonly called the Rouquerol criterion
      • 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 the result

Explicit mentions to accompany the BET area:

  • Gas species
  • Temperature
  • Outgassing conditions, duration, verification method
  • Range of \(P/P_0\) used for fitting
    • and why that range was chosen
  • Quality of fit, usually as an R^2 value
  • The C 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.
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