Last reviewed: October 4, 2026

Adsorbate Selection Guide

Nitrogen vs Argon Adsorption

Compare N₂ at 77K versus Ar at 87K for BET surface area and micropore analysis. Understanding quadrupole effects and IUPAC recommendations for optimal adsorbate selection.

View Comparison IUPAC Guidelines

N₂ Nitrogen at 77K

Temperature 77.35 K (-195.8°C)
Cross-sectional area 0.162 nm²
Quadrupole moment -1.52 × 10⁻²⁶ esu·cm²
P/P₀ range (mono) 0.05 - 0.35
Kinetic diameter 0.364 nm
Molecular shape Linear diatomic

Ar Argon at 87K

Temperature 87.27 K (-185.9°C)
Cross-sectional area 0.138 nm²
Quadrupole moment 0 (spherical)
P/P₀ range (mono) 0.02 - 0.20
Kinetic diameter 0.340 nm
Molecular shape Spherical monatomic

Comprehensive Comparison

Technical comparison of nitrogen and argon for gas physisorption analysis

Parameter Nitrogen (77K) Argon (87K)
Physical Properties
Measurement temperature 77.35 K (liquid N₂) 87.27 K (liquid Ar)
Saturation pressure (P₀) ~760 torr ~760 torr (87.3 K is argon’s normal boiling point)
Quadrupole moment -1.52 × 10⁻²⁶ esu·cm² 0 (no quadrupole)
Polarizability 1.74 × 10⁻²⁴ cm³ 1.64 × 10⁻²⁴ cm³
Cross-sectional area 0.162 nm² 0.138 nm²
Measurement Characteristics
BET range Typically 0.05–0.30 P/P₀; lower for microporous solids (Rouquerol criteria) Same approach: choose the range with the Rouquerol criteria
Micropore filling Narrowest micropores fill at very low P/P₀ (~10⁻⁷–10⁻⁵) The same micropores fill at noticeably higher P/P₀
Equilibration time Slow in narrow micropores (diffusion at 77 K) Faster: ~10 K higher temperature and higher filling pressures speed up diffusion
Surface interactions Specific (quadrupole) Non-specific
Advantages
Main advantages • Most common, extensive data
• Inexpensive coolant
• Long reference/kernel history
• Good for mesopores 		• No quadrupole interactions
• Better for micropores
• More accurate for heterogeneous surfaces
• Recommended by IUPAC (2015) for micropore analysis
• Micropores fill at higher P/P₀, with faster equilibration
Limitations
Main limitations • Quadrupole interactions
• Effective cross-section varies on polar surfaces (~0.135 nm² on hydroxylated silica vs. the conventional 0.162 nm²)
• Orientation effects
• Issues with polar surfaces 		• Liquid argon is costlier and less available than liquid N₂
• Requires liquid Ar or a cryostat
• Fewer historical datasets for comparison
Recommended Applications
Best for • Non-polar materials
• Mesoporous materials
• Routine QC testing
• Comparative studies
• Initial screening 		• Microporous materials
• Zeolites and MOFs
• Polar/ionic surfaces
• Reference materials
• IUPAC compliance

Understanding the Quadrupole Effect

What is the Quadrupole Moment?

The quadrupole moment arises from the non-spherical charge distribution in the N₂ molecule. This creates specific interactions with surface functional groups and exposed ions that can:

  • Cause preferential orientation of N₂ molecules on surfaces
  • Make the effective area per molecule depend on the surface (the conventional 0.162 nm² can be too large on hydroxylated or cationic surfaces)
  • Create uncertainty in cross-sectional area calculations
  • Shift micropore-filling pressures, which distorts pore size distributions from kernels
Key Impact: For materials with surface heterogeneity, polar groups, or exposed cations (zeolites, MOFs, functionalized carbons), argon provides more reliable results.

Molecular Interaction Comparison

N₂ with Quadrupole δ- δ- δ+ + Ar - No Quadrupole Ar + Strong specific interaction Weak non-specific interaction

Material-Specific Recommendations

Optimal adsorbate selection based on material properties

Use Nitrogen For:

  • Activated carbons (non-functionalized)
  • Silica gels (mesoporous)
  • Alumina (routine analysis)
  • Ceramics (non-polar)
  • Polymers (comparative studies)
  • Metal oxides (screening)
Note: Suitable when comparative data with historical measurements is needed

Use Argon For:

  • Zeolites (all types)
  • MOFs (metal-organic frameworks)
  • Microporous carbons
  • Clays and minerals
  • Functionalized materials
  • Reference materials
IUPAC: Recommended for microporous materials and heterogeneous surfaces

Consider Both When:

  • Developing new materials
  • Publishing research
  • Validating methods
  • Mixed micro/mesoporous
  • Unknown surface chemistry
  • Certification/standards
Best Practice: Report both N₂ and Ar data for comprehensive characterization

IUPAC Recommendations (2015)

Guidelines for Adsorbate Selection (Thommes et al., Pure Appl. Chem. 2015, 87, 1051–1069)

1. Microporous Materials (< 2 nm)

Primary recommendation: Argon at 87K
Reason: Absence of quadrupole moment eliminates specific interactions with surface heterogeneities. Provides more accurate pore size distributions and surface areas for zeolites, MOFs, and microporous carbons.

2. Mesoporous Materials (2-50 nm)

Primary recommendation: Nitrogen at 77K acceptable
Alternative: Argon at 87K for materials with surface functional groups
Note: For purely mesoporous materials without significant microporosity, N₂ remains suitable.

3. Ultra-microporous Materials (< 0.7 nm)

Recommendation: CO₂ at 273K or Ar at 87K
Reason: Better accessibility to narrow pores, faster equilibration at higher temperatures.

4. Standards and Reporting

Standards: ISO 9277:2022 (BET), ISO 15901-2:2022 (pore size)
Reporting: Must specify adsorbate, temperature, and cross-sectional area used.

Note on BET areas: IUPAC (2015) stresses that for microporous solids the BET area is an apparent, “BET-equivalent” area rather than a true geometric surface area. It is still useful for comparison when the Rouquerol criteria are applied and the adsorbate and temperature are reported.

Practical Considerations

Equipment Requirements

Nitrogen at 77K

  • Standard Dewar with liquid nitrogen
  • Readily available coolant
  • Simple temperature control
  • Lowest running cost

Argon at 87K

  • Option 1: Liquid argon Dewar
  • Option 2: Cryostat with N₂ cooling
  • Temperature control required
  • Higher running cost if liquid Ar is used

Measurement Protocols

Sample Preparation

  • Same outgassing procedures apply
  • Temperature: 150-350°C typical
  • Vacuum: < 10⁻³ torr
  • Time: 4-12 hours

Equilibration Times

  • Set by the instrument’s pressure-stability criterion
  • Narrow micropores with N₂ at 77 K can take much longer per point
  • Use equilibration criteria, not fixed times

Adsorbate Selection Decision Tree

Start Primary pore size? < 2 nm Surface chemistry? Polar/Heterogeneous? Use ARGON 2-50 nm Application purpose? QC or Research? QC/Routine Use NITROGEN Research Use BOTH < 0.7 nm Ultra-microporous CO₂ (273K) or Ar Additional Considerations • Zeolites, MOFs → Argon preferred • Publishing research → Report both N₂ and Ar • Historical comparison → Match previous adsorbate Recommendation Key: Argon - Most accurate for micropores Nitrogen - Standard for routine analysis Both - Comprehensive characterization CO₂/Ar - Ultra-micropore analysis

Summary and Best Practices

When to Use Nitrogen

  • Routine quality control testing
  • Mesoporous materials (2-50 nm)
  • Non-polar surfaces
  • Comparative studies with historical data
  • Cost-sensitive applications
  • Quick screening analyses

When to Use Argon

  • Microporous materials (< 2 nm)
  • Zeolites, MOFs, microporous carbons
  • Materials with surface heterogeneity
  • IUPAC compliance required
  • Research and development
  • Reference material certification

Universal Best Practices

  1. Sample Preparation: Use identical outgassing conditions for both adsorbates
  2. Cross-sectional Area: Always report the value used (0.162 nm² for N₂, 0.138 nm² for Ar)
  3. Temperature Control: Maintain stable temperature throughout measurement
  4. Equilibration: Use pressure-based criteria, not fixed time intervals
  5. Reporting: Clearly specify adsorbate, temperature, and analysis conditions
  6. Validation: Use certified reference materials when available

Continue with adsorbate selection

For most mesoporous materials, nitrogen at 77 K remains the default. For narrow micropores below ~1 nm, argon at 87 K resolves the pore-size distribution more cleanly. The decision criteria above and the BET calculator both feed back into the gas-adsorption methodology page.

Gas Adsorption Methodology BET Calculator