Simulation Of Gas Flow In High-Pressure Sampling Device For Mass Spectrometer

Based on the determination of molecular formula,relative molecular mass and internal structure,mass spectrometry is widely used in various fields such as chemistry,environment,life-science and so on.The mass spectrometer continues to expand the range of applications of mass spectrometry because of its high degree of automation,fast analysis,wide measurement range,stable instrumentation,high reliability and sensitivity,and simultaneous separation and identification.A mass spectrometer consists of several components of the sampling system,ion source,mass analyzer,and computer output system.In the entire structural components of the mass spectrometer,the sampling system is the most important component,which determines the sensitivity and accuracy of the sample.Till this moment,many experts and scholars have studied the flow field in the mass spectrometer sampling device,but there are few simulations or experimental studies on the mass spectrometer high-pressure sampling technology.The high-pressure sampling technology of the mass spectrometer is still a short technology.The current mass spectrometer sampling mainly relies on other devices to process the sample into a vacuum state,and then enters the mass spectrometer for detection,which greatly limits the use and development of the mass spectrometer.Based on the computational fluid dynamics(CFD)method,this paper simulates the gas flow process in the basic structure of the mass spectrometer high-pressure sampling device used ANSYS Fluent software.The optimized structure of the high-pressure sampling device grid that can be directly used in the mass spectrometer is proposed,and the selection of specific structural parameters is determined by the simulation results.Then this paper designs the structure and specific parameters of a two-stage sampling device of the mass spectrometer and determines the pressure of the first vacuum chamber.The main results of this paper are concluded as follows:(1)Through the calculation of the relevant formula,the maximum mass flow and volume flow that can be achieved theoretically by the capillary flow are obtained.Based on this parameter,the internal flow field of the high-pressure sampling device is simplified by simulation software Fluent.The correctness of the simulation parameter setting is verified by the clogging theory and the comparison with the theoretical volume flow and mass flow.Then grid-independent and boundary-independent verification is carried out to illustrate the correct setting of the numerical simulation in the model and the feasibility of using this method to study the subject,and to prove the reliability of the final simulation results.(2)When the inlet pressure is atmospheric pressure,the smaller the outlet pressure,the larger the volume flow rate at the outlet.Under the same outlet pressure conditions,as the length of the capillary increases,the volume flow rate and mass flow rate of the model are reduced.Therefore,under the premise that the volume flow rate at the outlet meets the miniature floating scroll vacuum pump speed(0.6L/s)of the first vacuum chamber,the outlet pressure is reduced as much as possible to ensure the vacuum degree of the first vacuum chamber.The results show that when the diameter of the capillary is 0.2mm,the inlet pressure is atmospheric pressure,and the outlet pressure is 800Pa,when the length of the capillary is 0.8 mm,the outlet volume flow rate is 0.6L/s,which is the closest to the first vacuum chamber pump speed.(3)An optimized sampling device structure with grids,baffles and transition zones is proposed.And then the effects of grids number,grid shape,baffle number,baffle diameter,and transition zone width on the flow of the gas flow are discussed.The following conclusions were obtained:①For the same outlet pressure condition,the outlet volume flow decreases with the increase of the number of grids and the number of baffles,and the trend of volume flow decreases with the increase of the number of baffles.When the number of grid plates is 3 and the number of baffles is 2,it is a turning point where the volume flow rate decreases.②Under the same outlet pressure condition,the volume flow of the flow field outlets under different grid shapes from the largest to the smallest is the right cone grid plate,the left cone grid plate,and the straight grid plate structure.And in the left cone grid structure and the right cone grid structure model,the gas flow has a very obvious shock phenomenon,and it is more suitable to select the straight grid structure.③Under the same outlet pressure condition,the diameter of the baffle has a certain influence on the volume flow of the outlet,and the volume flow of the outlet under the width of different transition zones remains basically unchanged.④Three straight grids with a diameter of 0.2mm,two circular baffles with a diameter of 10mm,and a transition zone width of 6.5mm can maximize the performance requirements of the mass spectrometer sampling device.(4)In order to better play the role of the sampling device and reduce the sample pressure to a lower level,the secondary structure of the sampling device is proposed,and the basic structure and the grid structure are simulated the gas flow characteristics.The following conclusions were obtained:①For the secondary structure,the model outlet pressure is 5Pa,the inlet pressure changes,and the damper theoretical conditions are not met.The model outlet volume flow is proportional to the mass flow and is also proportional to the inlet pressure.②The primary structure adopts the grid structure and the secondary structure adopts the basic structure to obtain the most suitable first vacuum chamber pressure,so that the performance of the mass spectrometer sampling device reaches an optimum value.Therefore,the pressure of the first vacuum chamber is 550～650Pa,the volume flow rate of the first vacuum chamber is 0.5～0.6L/s,and the volume flow rate of the second vacuum chamber is 0.12～0.16L/s,and the working state of the vacuum pump is also most suitable.