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(1) The dynamic method, also known as continuous flow chromatography, involves conducting nitrogen adsorption in a flowing atmosphere containing nitrogen at liquid nitrogen temperature. This process allows the system to reach a dynamic equilibrium of adsorption under varying nitrogen partial pressures. If the sample tube is removed from the liquid nitrogen and allowed to warm up to room temperature, all the adsorbed nitrogen will be completely desorbed. In this method, the sample tube must be inserted into and removed from the liquid nitrogen cup for each pressure measurement, making it a time-consuming but precise technique.
(2) Another variation of the dynamic method uses helium as the carrier gas, since helium is not adsorbed at liquid nitrogen temperatures. By adjusting the ratio of nitrogen to helium or controlling their flow rates, different nitrogen partial pressures can be achieved without altering the total pressure of the gas mixture. The total pressure remains close to atmospheric, and the nitrogen partial pressure can range from 0 to full atmospheric pressure. This eliminates the need for vacuum conditions, making the process more efficient and user-friendly.
(3) Continuous flow chromatography employs a thermal conductivity detector to measure nitrogen adsorption. The sample tube is placed in series between the reference and measuring arms of the detector. When the sample adsorbs or desorbs nitrogen, the nitrogen concentration changes, causing an imbalance in the thermal conductivity cell. This imbalance generates an electrical signal, producing an adsorption or desorption peak. The area under the peak is proportional to the amount of nitrogen adsorbed or desorbed. Since helium and nitrogen have significantly different thermal conductivities, even small changes in nitrogen concentration lead to noticeable shifts in the detector’s output, enhancing sensitivity.
(4) One of the key advantages of the dynamic surface area meter is its ability to directly compare specific surface areas using a standard sample with a known surface area. By comparing the desorption peak area of the sample to that of the standard at a fixed nitrogen/ helium ratio (typically 0.2), the surface area can be calculated. This method is fast and ideal for real-time monitoring on production lines. However, it does not account for differences in adsorption behavior between materials, which may lead to significant errors when the samples differ greatly from the standard.
(5) The BET method addresses these limitations by allowing for the adjustment of nitrogen partial pressure and reaching a steady state. A dynamic BET-specific surface instrument can also use the direct comparison method by setting the nitrogen partial pressure to 0.2 or 0.3 and using a standard sample. This approach ensures more accurate results, especially when dealing with materials that have different adsorption characteristics. Overall, the dynamic method offers a reliable and versatile way to measure surface area with high precision. Test - lowercase jpg Kaixin micro test Test probe P100-M3 0805 side white