Engineering
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Quantitative Analysis by Thermogravimetry-Mass Spectrum Analysis for Reactions with Evolved Gases
Chapters
Summary October 29th, 2018
Precise determination of the evolved gases' flow rate is key to study the details of reactions. We provide a novel quantitative analysis method of equivalent characteristic spectrum analysis for thermogravimetry-mass spectrum analysis by establishing the calibration system of the characteristic spectrum and relative sensitivity, for obtaining the flow rate.
Transcript
This method can help answer key questions in the energy, chemistry, and the metallurgy field about how to identify reaction kinetic parameters and evolved gas compositions. To me, an advantage of this technique is that the mass fluid of individual gases evolved from reactions can be qualitatively and quantitatively precisely determined. Through this method, can provide insight into reactions in the energy, chemistry, metallurgy system, and so on.
It can also be applied to other systems such as food, pharmacy, or materials. To calibrate a characteristic spectrum, prepare the evolved gases to be calibrated, modulating the gas pressure at 0.15 megapascals. Use a stainless steel tube to connect each gas cylinder to the thermogravimetry mass spectrum, or TG-MS system, and purge all the evolved gases into the TG-MS system at a 100 milliliters per minute flow rate.
Monitor the mass spectrum of each individual gas, carefully watching and comparing the characteristic peaks of the gases to be calibrated and any possible impurities within the gases. To calibrate the relative sensitivity of the gases, purge the reference gas at 300 milliliters per minute flow rate into the TG-MS system for 20 minutes to clean the system. Next, synchronously purge each of the calibrated gases with the reference gas into the TG-MS system at a 100 milliliter per minute flow rate.
Then calculate the relative sensitivity of each gas according to the known flow rate and the mass spectrum as indicated in the equation. To prepare the samples, collect 10 grams of calcium carbonate with an average diameter of 15 micrometers, 10 grams of a white block of hydromagnesite or 20 grams of Zhundong coal. Break the hydromagnesite block into less than three millimeter pieces and grind the pieces with a machine stirred mill to approximately 10 micrometers.
Then dry all of the samples for 24 hours in a 105 degree Celsius oven, breaking and grinding the coal in a mill the next day to obtain a particle size range of 180 to 355 micrometers. To test the thermal reactions of the samples, purge the TG-MS system with helium as the carrier gas for two hours to expel the air and moisture and heat the instrument to around 500 degree Celsius. When the system is cooled back to room temperature, use mass spectrometry to monitor the atmosphere for 20 minutes, carefully watching and comparing the characteristic carbon dioxide and helium peaks and the impurity peaks of the oxygen, nitrogen, and water gases.
Weigh 10 milligrams of the sample of interest on a precision electronic balance, and add the weighed sample to an aluminum oxide crucible. Place the crucible with the sample into the TG system and close the furnace. Then set the appropriate operating parameters for the sample being tested.
So a reference gas in the calibration must be the same as that in the sample testing process and must never react with the evolved gases. We recommend using helium as the carrier gas in both the calibration and the test. For qualitative and quantitative analysis of the sample data, load the 3D mass spectrum data onto the computer connected to the TG-MS system and use the equivalent characteristic spectrum analysis, ECSA, method to calculate the actual sample parameters based on the previously determined calibrated characteristic peak and the relative sensitivity of the sample.
The thermal reaction can then be analyzed according to the actual sample parameters. After calibrating the characteristic peak and relative sensitivity of carbon dioxide to the carrier gas, helium, the actual mass flow rate of carbon dioxide evolved by thermal decomposition of calcium carbonate can be calculated by the ECSA method and compared with the actual mass loss. In this representative analysis, there was a good agreement between the mass flow rate of carbon dioxide and the mass loss data by digital thermogravimetry over the entire measurement process.
Comparison of the thermal decomposition process of hydromagnesite by ECSA and the calibration of carbon dioxide and water revealed that these data were also in good agreement with the experimental digital thermogravimetry data. Combining both the electron ionization and photoionization measuring modes, this representative pyrolysis of Zhundong coal revealed the presence of 16 different volatile gases. After detailed determination of the mass spectrum and sensitivity of each identified gas to the carrier gas, the mass flow rate of each gas was calculated and used to compare the mass ion data for each gas based on the same operating parameters.
While attempting this procedure, it's important to remember to build the composition fixture and the relative sensitivity of the gases before testing. Following this procedure, a method like differential thermal analysis combined ECSA can be performed to answer additional questions about the features of the reactions without evolved gases. After its development, this technique paved the way for the researchers in the field of energy, chemistry, metallurgy, and so on, to explore using gas reactions and mechanisms in energy conversion and advanced materials development.
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