Dynamic chromatography to test specific surface area - Database & Sql Blog Articles

(1) Dynamic method, ie continuous flow chromatography, is to carry out nitrogen adsorption in a flowing nitrogen-containing atmosphere at liquid nitrogen temperature, and achieve dynamic equilibrium of adsorption under different nitrogen partial pressures, if the sample tube is left from liquid nitrogen And when it rises to room temperature, the sample will completely desorb the adsorbed nitrogen. The dynamic nitrogen adsorption instrument needs to make the sample tube enter and exit the liquid nitrogen cup every time a pressure point is measured; (2) the dynamic method is based on the use of helium gas. Carrier gas (because helium is not adsorbed at liquid nitrogen temperature), by adjusting the nitrogen/rhodium ratio, or by controlling the nitrogen/helium flow rate, different nitrogen partial pressures are obtained without changing the pressure of the mixed gas. The pressure of the mixed gas is approximately atmospheric pressure, the partial pressure of nitrogen = the flow rate of nitrogen / the total flow of helium plus nitrogen, the partial pressure of nitrogen can be varied from 0 to an atmospheric pressure, and vacuum conditions are not required; (3) continuous flow chromatography is adopted The thermal conductivity detector in chromatography is named for measuring the amount of nitrogen adsorption. The sample tube is connected in series between the reference arm of the thermal conductivity detector and the measuring arm, in the reference arm and measuring arm of the thermal conductivity detector. The nitrogen concentration is not simultaneously due to adsorption (or desorption) of the sample, the thermal conductivity cell loses equilibrium and outputs an electrical signal, resulting in a so-called adsorption peak (or desorption peak) whose peak area is proportional to the adsorption of nitrogen (or The amount of nitrogen adsorption or desorption can be obtained by a standard amount. By the way, since the thermal conductivity of helium and nitrogen differ greatly, the change of nitrogen concentration in the helium/nitrogen mixture will be significant. Change the equilibrium state of the thermal conductivity cell to improve the sensitivity of the thermal conductivity detector; (4) The direct comparison of the specific surface by the direct contrast method is a major advantage of the dynamic surface ratio surface meter, that is, using a standard sample with a known specific surface as The calibration substance can be calculated by comparing the desorption peak area of ​​the sample with the desorption peak area of ​​the standard sample at a fixed partial pressure of nitrogen (generally a mixture of nitrogen/氦 = 0.2). Than the surface. This method has a fast test speed and is suitable for on-line rapid detection of a production line. The disadvantage is that the difference in adsorption characteristics of the material is not considered. Therefore, when the adsorption characteristics of the sample to be compared with the standard are large, the test result may have a large deviation; (5) The BET specific surface measurement method can overcome the above limitations of the direct comparison method. The key to the dynamic method to achieve the BET specific surface measurement is to be able to adjust the partial pressure of nitrogen and reach a steady state. The dynamic BET specific surface measuring instrument can also measure the specific surface by direct comparison method by setting the nitrogen partial pressure to 0.2 or 0.3 and mounting the standard specific surface sample.