High Resolution High-Field Asymmetric Waveform Ion Mobility Spectrometry

High field asymmetric waveform ion mobility spectrometry (FAIMS) has evolved as a powerful technique for quick ion-separation and detection under ambient pressure. It can be deployed in a wide range of application scenarios, such as security screening, food quality inspection, medical diagnostics, and environmental analysis etc. As a special kind of ion mobility spectrometry, FAIMS separates ions based the nonlinear behavior of ion mobility under relatively high electric field rather than relying on the absolute value of each ion species’s ion mobility. In this way, FAIMS will show some unique advantages, such as alpha function independent with ion mobility, orthogonality with other ion separation or detection techniques, continuous detection capability and miniaturization. FAIMS shows a promising future in the field of in-situ investigation.Resolution of analysis and detection instrument respresents the ability to distinguish slightly different peaks in spectrum. The resolution performance of a FAIMS will have a great impact on the instrument’s detection ability and its possible application field. So far, the FAIMS systems have not really achieved really high resolution because of the following reasons. Firstly, ion recognition mainly depends on the function of peak positions. Thus, drift of the peak positions due to the chip structure and controlling conditions can cause a lot of problem. Secondly, FAIMS spectrum peak deviation of some chemicals is very small. Thus it requires a very high and well-tuned separation voltage to achieve a relatively high resolution. In fact, so much work has been done by researchers worldwide in an effort to improve the resolution of a FAIMS system, including FAIMS drift tube structure optimization and the improvement of machining accuracy; optimizing the measurement conditions to increase the peak of compensation voltage; fully tapping the FAIMS spectrum information and so on. In addition, researchers improved the FAIMS resolution through hyphenated methods, such as combine FAIMS with gas chromatography (GC), mass spectrometry (MS), and drift-time ion mobility spectrometers (DTIMS).However, current techniques to improve FAIMS resolution have the following deficiencies:Firstly, in the improvement of structure and condition, although researchers have done a lot of work, but there is still a huge space to improve. The relationship between the performance of FAIMS and the resolution is not clear;The theoretical analysis is not complete or there is a big error. Environment, carrier gas, ion source, migration tube, high voltage source, weak signal detection and control system and other aspects of the imperfect system exists.Secondly, The most common hyphenated methods are GC-FAIMS and FAIMS-MS. In these schemes, FAIMS is no longer an independent analysis instrument but an accessory of other instruments. The integration of FAIMS with DTIMS actually has great development potential. In this design, we can take advantage of both low field ion mobility and high field nonlinear coefficient function simultaneously. Due to the huge structural differences of each instrument, typical integration method is combining two instruments directly. In this case, ion loss become a serious problem and the system is no longer small. In addition, the increased complexity of the gas path can not be ignored.In order to address these challenges, the main work of this paper focuses on the following three aspects to improve the identification ability of the FAIMS.Firstly, by integrating a pair of oscillating electrodes, we obtain ion mobility characteristics under low electric field, to increase the dimensions of recognition for FAIMS identification. Based on the advantages of present hyphenated methods of FAIMS, we bulit new FAIMS tube based on a new and improved structure. By introducing of the new structure, low field ion mobility can be used in FAIMS. The resolution of low field ion mobility in this system is about10, which contribute to FAIMS resolution tenfold. This method based on the basic structure of FAIMS tube, which is better than direct tendam FAIMS-DTIMS on integration. Furthermore, the method can be used for attenuation of specific peaks, especially when some peaks partial overlapping, improving signal to noise ratio. At the same time, the method also can be used for identify of clusters in FAIMS.Secondly, By doping water vapor in the carrier gas, using the behavior of ion in alternating electric field, we can greatly improve the degree of separation of material ions. We have studied on the influence of carrier gas mixing water vapor to resolution of FAIMS. Through the experiments of different samples, alcohols, ketones, aromatichydrocarbons, a total of9substances, and the reserchs on several aspects directly affect the identification ability of FAIMS as the peak position, half peak width, peak number etc. The carrier gas doping will affect the FAIMS nonlinear coefficient function, and the influence of different material is different. Therefore, by controlling the moisture content of dopant, we can effectively improve the differences of nonlinear coefficient between different types of Ions including polarity and non polarity, so as to improve the identification ability of FAIMS.Thirdly, a reliable measurement and control system has been established. Base on the precise control of various experimental parameters and conditions, we can improve the stability and reliability of the system. High performance, such as high frequency and high voltage power supply, is conducive to the improvement of ion separation degree of separation. We can achieve better separation and identification.1. Atmospheric metastable desorption ionization (AMDI) was built for FAIMS. It is convenient for fast sampling and detection of solid-state condensation samples. The performance of the ion source directly affects the detection ability and application scope of FAIMS.2. High precision and highly integrated FAIMS drift tube was built. It has good mechanical, thermal, and electrical properties while ensuring the excellent flatness and air tightness of the drift tube.3. A flyback generator for asymmetric high voltage generation was built. The generator is stable and easy to miniaturize, which is favorable for field detection applications. The peak-to-peak vlotage achieved is1800V@1MHz, which is good for better separation results.4. Optimization design of a weak charge detection system. A large gain transconductance amplifier with ultra low noise current was constructed. According to our experiment, the limit of detection (LOD) for DMMP sample can reach as low as10PPB.