High-field Asymmetric Waveform Ion Mobility Spectrometry Technology

High-field asymmetric waveform ion mobility spectrometry(FAIMS) is a broad spectrum analysis technology which has the advantages of high speed, high sensitivity, easy miniaturization, etc. FAIMS technology has broad application prospect in many fields such as environmental monitoring, drugs and explosives detection etc. Ion mobility has different variation in high strength electric fields for the difference between their natural parameters, and the non-linear dependence of mobility values on high strength electric fields can be used for ions identification. Ions can be separated and only one specific type of ions pass through the filter region in the suitable electric fields.Ion movement in the electric field is affected by many factors which results in the high complexities of the movement. Recently, the research of the dependence of ion movement on ion diffusion and carrier gas velocity distribution in high strength electric field has achieved preliminary results. However, there is little work on the effect of compensating voltage waveform and deflecting voltage intensity which affect the sensitivity and resolving power of FAIMS. In FAIMS tube, the distance between filter region and detection region is small and RF electric field with high frequency and high strength is applied to the filter electrodes, which leads to the strong electric field interference in detection region and brings difficulty to the detection of weak current signal. Moreover, in specific RF electric field, the compensating voltages of two kinds of ions may be close to each other and it is difficult to separate and identify them.Considering the problems aforementioned, the paper is focused on four parts to study the FAIMS:key performance index and influence factor, drift tube and FAIMS prototype design, effect of control parameters, experiment of explosives, drugs and volatile phenolic compounds. At first, about the FAIMS key performance index and influence factor, two effects:one is the effect of ion diffusion, carrier gas velocity distribution, compensating voltage waveform on ion transmission rate in filter region, the other is the effect of deflecting voltage on the ion transmission in the detection region, are considered to present the relational model between FAIMS sensitivity and influence factors which contains of carrier gas flow rate, RF voltage magnitude, etc. The model of full-width-at-half-maximum(FWHM) and resolving power is presented after analyzing the effect of ion diffusion, carrier gas velocity distribution, compensating voltage waveform. Then, about the design of drift tube and FAIMS prototype, the drift tube containing of main and assistant shielding electrodes is designed to reduce the electric field interference between filter region and detection region, and the dimensions of the electrodes are optimized according to the simulation results. The drift tube manufacture processes based on the MEMS technology are proposed, and the processes based on thick-film technology are also proposed to avoid leakage current in borosilicate glass at high temperature. The control system of FAIMS is designed for the accuracy control of parameters. Then, about the effect of control parameters, the effect of carrier gas flow rate, RF voltage magnitude, deflecting voltage on FAIMS performance is tested and the proposed performance models are verified experimentally, then the three parameters are optimized according to the experiment results. Finally, the experiments of TNT, herion and five volatile phenolic compounds are carried out and the spectrometry of them is obtained. Considering the compensating voltage(CV) values of two kinds of ions may be close to each other in specific RF voltage, the method of 2-D spectrometry which is obtained by sweeping CV under different RF voltage values is proposed, which increases the detection accuracy.Here, we focused on the control parameters optimization and the relational model between FAIMS performance and influence factors. The key original contributions in this paper can be summarized as follows:1. The effect of deflecting voltage waveform on FAIMS performance is analyzed for the first time, and the relational model between FAIMS sensitivity, resolving power and influence factors is proposed. Thus the theoretical foundation is laid to the optimization of the control parameters.2. The FAIMS drift tube is designed, in which there are two pairs of main and assistant shielding electrodes. The simulation results show that the electric field interference between filter region and detection region with shielding electrodes decreases to 0.32‰of that without shielding electrodes.3. The drift tube manufacture processes based on thick-film technology are proposed for the first time to avoid leakage current in borosilicate glass at high temperature. The experiment results show that the leakage current in drift tube manufactured by thick-film technology is very little, which is suitable for the fast real time detection at high temperature. 4. Considering the CV values of two kinds of ions may be close to each other in specific RF electric field, the method of 2-D spectrometry which is obtained by sweeping CV under different RF voltage values is proposed, which increases the detection accuracy.