1、 Sample pre-processing: the core process that determines data accuracy

1. Degassing conditions (temperature/time/vacuum degree)

• Low temperature/short time/insufficient vacuum degree: impurities are not completely removed, adsorption sites are occupied,The measured specific surface area is relatively smallThe deviation of micropore detection results is the largest;

• High temperature: Some samples (such as lithium-ion silicon-based negative electrodes and porous organic materials) undergo thermal decomposition/sintering, and the pore structure is damaged,Pseudo decrease in specific surface areaThe aperture distribution is abnormal.

  ✅ Control points: Customize degassing conditions based on sample characteristics (such as activated carbon at 200~300 ℃, lithium iron phosphate at 120~150 ℃, organic materials at ≤ 80 ℃), with a recommended vacuum degree of ≤ 10Pa and a degassing time of 4~12h (for easily adsorbed samples, extend to 16h).

2. Sample grinding and sieving

• Excessive grinding: Sample particles are crushed, resulting inPseudo microporesThe measured specific surface area is higher than expected;

• Insufficient grinding/uneven particle size: The sample particles are too large for gas to enter the internal pores,Reduced effective adsorption areaThe measured value is low;

• Unfiltered: Large particle aggregates exist, pore channels are blocked, and data reproducibility is poor.

  ✅ Control points: brittle inorganic materials shall be lightly ground to 200~325 meshes, soft/easily agglomerated samples (such as nano powder) shall be dispersed with anhydrous ethanol and sieved to avoid dry grinding agglomeration.

3. Sample weighing

• Weighing too little (<0.1g): The adsorption signal is weak, and the instrument detection error is amplified,Extremely poor data repeatability；

• Excessive weighing (>2g): The sample is stacked too densely, and gas diffusion is hindered, making it difficult to adsorb evenly,The measured specific surface area is relatively low.

  ✅ Control points: Adjust the weighing value according to the specific surface area of the sample (high specific surface area samples such as activated carbon 0.1-0.5g, low specific surface area samples such as ceramic powder 1-2g), ensuring that the adsorption amount is within the instrument detection range (0.1-10mmol/g).

4. Sample contamination

• Contact tentacles, filter paper, containers: introducing impurities such as oil and fibers, occupying adsorption sites, resulting in low measurement values;

• Failure to test in a timely manner after degassing: The sample re adsorbs moisture/CO ₂ from the air, resulting in secondary pollution.

  ✅ Control points: Use drying tweezers throughout the process, use the dried glass sample tube, and immediately transfer it to the testing position after degassing is completed.

2、 Test operating parameters: key variables for instrument settings

1. Adsorbent selection

• Conventional samples with N ₂ (kinetic diameter 0.364nm): suitable for mesoporous (2-50nm) and macroporous detection,Unable to enter micropores (<2nm)；

• Ar/Kr (smaller kinetic diameter) is used for microporous samples: N ₂ is prone to capillary condensation in micropores, resulting in larger pore size measurements. Ar/Kr is more suitable for precise detection of micropores;

• Polar samples (such as metal oxides and hydroxylated materials) undergo strong interactions between CO ₂: N ₂ and polar groups on the surface of the sample, resulting in distorted adsorption isotherms and better compatibility with CO ₂.

  ✅ Control points: Ar is selected for microporous samples (such as molecular sieves and activated carbon), N ₂ is selected for mesoporous/macroporous samples, and CO ₂ is preferred for polar samples.

2. Liquid nitrogen bath level and temperature

• The liquid nitrogen bath liquid level has not reached the sample tube: the temperature in the sample adsorption zone is higher than 77K (boiling point of liquid nitrogen),Reduced gas adsorption capacityThe measured value is low;

• Delayed replenishment of liquid nitrogen: liquid level drops, temperature fluctuations, jagged adsorption isotherms, poor data reproducibility;

• Liquid nitrogen contains impurities (such as liquid oxygen): the temperature deviates from 77K, and the adsorption equilibrium is disrupted.

  ✅ Control points: During the test, keep the liquid level of the liquid nitrogen bath 2-3cm above the adsorption zone of the sample tube, replenish liquid nitrogen in real time, and use high-purity liquid nitrogen (≥ 99.999%).

3. Balance time setting

• The equilibrium time is too short: the gas has not reached adsorption equilibrium on the sample surface,The adsorption measurement value is too smallThe isotherm is not smooth;

• Long equilibrium time: meaningless, only increases testing time (the equilibrium time of microporous samples needs to be slightly longer to ensure gas diffusion into the micropores).

  ✅ Control points: The equilibrium time for mesoporous samples is 3-5 minutes, for microporous samples it is 5-10 minutes, and for macroporous samples it is 2-3 minutes.

4. Distribution of testing pressure points

• Too few pressure points (P/P ₀, relative pressure): low fitting accuracy of adsorption isotherms,Large deviation in aperture distribution calculation；

• Unencrypted low-pressure point (P/P ₀=10 ⁻⁶~10 ⁻ ²) for microporous detection: unable to capture microporous adsorption signal, misjudged as a sample without micropores;

• Unencrypted pressure points (P/P ₀=0.2~0.95) in mesoporous detection: distorted capillary condensation peak and shifted peak pore size distribution.

  ✅ Control points: The pressure points for routine testing should be ≥ 20, the low pressure points for microporous samples should be increased to 8-10, and the pressure points for mesoporous samples should be increased to 10-12.

3、 Characteristics of the sample itself: the influence of inherent material properties

1. Adsorption activity of the sample

• The surface contains strong polar groups (such as - OH, - COOH): it undergoes chemical adsorption with N ₂, and the adsorption isotherm deviates from the physical adsorption law,The surface area measurement is falsely high；

• The sample has magnetic/conductive properties: the adsorption probe is interfered with, resulting in large signal acquisition errors.

  ✅ Key points for handling: Replace the adsorbate (such as CO ₂) for polar samples, use non-magnetic sample tubes for magnetic samples, and perform insulation treatment for conductive samples.

2. Pore types and connectivity of samples

• High proportion of closed/blind holes: gas cannot enter,The detection is only based on the specific surface area of open poresThere is a deviation from the true value (which needs to be corrected by mercury intrusion method);

• Ink bottle type pores: During desorption, the pores are blocked, and the desorption isotherm shows hysteresis loop anomalies, resulting in distorted pore size distribution calculations.

3. Agglomeration of samples

• Nanoparticles (such as TiO ₂, graphene) are easy to agglomerate:Formation of pseudo macropores in gapsThis leads to an overestimation of the measured volume of large pores, masking the true pore structure.

  ✅ Key points: Use dispersants (anhydrous ethanol, n-hexane) for ultrasonic dispersion before testing to eliminate agglomeration gaps.

4、 Equipment status: Basic guarantee for instrument hardware and calibration

1. instrument calibration

• Non regular calibration of dead volume/standard sample: The measurement deviation of dead volume (blank volume of sample tube+pipeline) will be directly added to the adsorption capacity calculation,Systematically high/low specific surface area；

• Calibration failure of standard samples: Without verification with standard substances (such as BET standard carbon black, SiO ₂), instrument detection errors have not been corrected.

  ✅ Control points: Calibrate the dead volume monthly (measured with helium gas), calibrate quarterly with national standard substances, and the calibration deviation should be ≤ ± 5%.

2. Hardware sealing

• Leakage of pipelines/sample tubes/valves: During the testing process, pressure fluctuations and adsorption equilibrium are disrupted,The adsorption measurement value is too smallThe isotherm shows an abnormal decrease;

• The sealing of the liquid nitrogen Dewar flask is not tight: the liquid nitrogen evaporates too quickly and the temperature fluctuates greatly.

  ✅ Control points: Conduct air tightness testing before testing (maintain pressure for 10 minutes, pressure drop ≤ 0.5%), replace aging gaskets and pipelines, and regularly maintain valves.

3. Detector accuracy

• Aging of pressure sensor: large detection error in low pressure section (P/P ₀ ＜ 0.01), distortion of micro hole data;

• Flow controller malfunction: unstable gas flow rate, prolonged adsorption equilibrium time, poor data repeatability.

  ✅ Control points: Regularly calibrate pressure sensors (accuracy ≤ ± 0.1%), replace aging flow controllers, and ensure that instrument resolution meets detection requirements.

4. Data processing software

• Choosing the wrong calculation model: such as using BJH method to calculate micropores (BJH only applies to mesopores),Distortion of aperture distribution；

• The software parameters have not been modified by default, such as incorrect settings for dead volume, sample density, and adsorbate cross-sectional area, resulting in calculation deviation.

  ✅ Control points: Select the corresponding model based on the pore type (micropores: t-plot/DR method, mesopores: BJH method, specific surface area: multi-point BET method), and manually check the basic parameters of the sample and adsorbate in the software.

5、 Other secondary influencing factors

1. laboratory environmentTemperature fluctuations (＞ ± 2 ℃) and high humidity (＞ 60%) can cause temperature changes in instrument tubing, dead volume deviation, and easy secondary adsorption of samples;

2. Sample tube consistencyThe volume and glass thickness of different sample tubes vary, and there are differences in dead volume. Batch testing requires selecting the same batch of sample tubes and calibrating the dead volume separately;

3. Operator proficiencyWhen manually operating the instrument (such as connecting sample tubes and replenishing liquid nitrogen), human error can be greatly avoided by fully automatic instruments.

Core Summary: Key Principles for Accurate Data

1. customizationCustomize pre-treatment and testing parameters based on the material, pore type, and thermal stability of the sample, and reject a one size fits all approach;

2. standardizationFixed operating procedures (grinding, sieving, degassing, weighing) to ensure experimental reproducibility;

3. CalibrationRegularly calibrate the instrument with dead volume and standard samples to ensure hardware accuracy;

4. ValidationConduct parallel sample testing on new samples (at least 2 groups), with a relative deviation of ≤ ± 5% for parallel samples, otherwise retest.