advancing sorption science

# 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.
In France, Italy, Switzerland, and Benelux.
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