Development Of Electron Impact Ion Source Control System For Mass Spectrometer

Electron Impact ion source widely used in Mass Spectrometers,with benefits of mature,wide application,complete spectrum,fully ionization and standard NIST compared,consisting of Filament,Ionization Chamber,Electron Repulsive Electrode,Electron Receiver and Lens System.The control system of ion source is a prerequisite for ionizing sample molecules,affecting the efficiency of ionization.The existing control system of EI source commonly apply PWM technology to control current of filament,but the matter is that current ripple will be increased,and stability is unreliable.The current ripple of single-pulse control is about 250m A,and the double-pulse is about 120m A.This article,had supported by the Magnetic Mass Spectrometer created by Chinese Academy of Geological Sciences,developed a new whole control system based on MCU which has the superiority of acute precision and well stability.The main contents is as follows:（1）Design current control system of filament.For enhancing current stability and improving accuracy,The control system of filament based on STM32F103 was developed which has advantages of high precision,good stability and wide adjustment range.It includes design system scheme,develop hardware circuit and simulate the key circuits.The result shows that the resolution of the buffer circuit can reach 0.1m V,R²≥0.9999 and Error≤0.1m V;the V/I convert circuit’s R²≥0.9999,Error≤0.1m V,and accuracy less than 0.0033A.That can meet the requirements of filament parameters.（2）Design voltage control system of electron repulsion.For meeting the requirements of research under different ionization energies,The control system of voltage ranging from-100V to 0V was developed.Firstly,the system consist of two parts,One is booster module consisting DC/AC converter,rectifier filter,feedback collector as well as transformer;The other is voltage control module includes PWM generator and switch tube driver.Secondly,the key circuits simulate and verify its feasibility.It turns out:the optimal filter capacitance is 20u F;While voltage steadily reach-119.173V,the ripple is less than 100m V at about 4μs;The driven circuit can amplify PWM signal current from 23.0μA to 135m A.The results show that the voltage control system can meet the voltage parameters required by the repulsive electrode.（3）Design detection system of electronic current.For reflecting ionization efficiency of ion source,the electronic current detection system is developed to meet requirements.Compared with the existing weak signal detection methods,the differential amplification which can resist zero drift and common mode interference is determined as the main method.Hardware includes range switching circuit,direct-coupled differential amplifier,LPF circuit,A/D feedback circuit,mainly focusing on three differential amplifier as well as Butterworth LPF.Four-layer PCB board reasonably laid out and taken anti-interference measures to improve accuracy of signal transmission.The simulation verifies and analyzes the main circuit.The results show that R²≥0.999Error≤±0.0489%based on I-V amplifier;Low pass filter can effectively filter 50Hz and 30k Hz noise interference.Above analysis results show that the parameter required by electronic current detection can be reached.（4）Circuit integration and test applications.Based on above these circuits,the design of the PCB accomplished.Upper computer controller based on Lab VIEW applied as a operating system to control MCU.Finally,the key circuit tested and results show that:1.Filament current:0～4000m A,error≤20m A,load capacity:0～2Ω,adjustment:10m A;2.Voltage output range:-100V～0V,voltage is stable;3.The electronic current can be detected within the range of 10^-6～10^-10A,and R²≥0.999 of I-V convert.Accuracy measurement results:Error≤2%,precision measurement results:RSD≤2%;Low pass filter effectively block50Hz and 30k Hz frequency noise signal.The test result of the integrated ion source control system show that control system can work on the ion source of the mass spectrometer in this project,which lays a foundation for analysis of application.