Ion Mobility Spectrometry (IMS), having an advantage in real-time and on-line detection, is an atmospheric pressure detecting technique. LA-IMS (Laser Ablation Ion Mobility Spectrometry) uses Nd-YAG laser as ionization source, whose energy is high enough to ionize metal.
In this work, we tested the signal in different electric field intensity by a home-made ion mobility spectrometer, using silicon wafers the sample. The transportation of metal ions was match with the formula: Td = d/K • 1/E, when the electric field intensity is greater than 350v/cm. The relationship between signal intensity and collection angle (the angle between drift tube and the surface of the sample) was studied. With the increasing of the collection angle, signal intensity had a significant increase; while the variation of incident angle of the laser had no significant influence. The signal intensity had a 140% increase when the collection angle varied from 0 to 45 degree, while the angle between the drift tube and incident laser beam keeping the same as 90 degree. The position of ion gate in LA-IMS(Laser Ablation Ion Mobility Spectrometry) is different from the traditional ones for the kinetic energy of the ions is too big, if the distance between ion gate and sampling points less than 2.5cm the ion gate will not work, the ions could go through ion gate when it closed. The SNR had been improved by define the signal when the ion gate is closed as background signal, the signal noise including shock wave and electrical field perturbation produced during the interaction between laser beam and samples is eliminated when the signal that the ion gate opened minus the background signal.
Ion mobility spectrometry (IMS) is a key trace detection technique for toxic pollutants and explosives in the atmosphere. Ultraviolet radiation photoionization source is widely used as an ionization source for IMS due to its advantages of high selectivity and non-radioactivity. However, UV-IMS bring problems that UV rays will be launched into the drift tube which will cause secondary ionization and lead to the photoelectric effect of the Faraday disk. So air is often used as working gas to reduce the effective distance of UV rays, but it will limit the application areas of UV-IMS. In this paper, we propose a new structure of curved drift tube, which can avoid abnormally incident UV rays. Furthermore, using curved drift tube may increase the length of drift tube and then improve the resolution of UV-IMS according to previous research. We studied the homogeneity of electric field in the curved drift tube, which determined the performance of UV-IMS. Numerical simulation of electric field in curved drift tube was conducted by SIMION in our study. In addition, modeling method and homogeneity standard for electric field were also presented. The influences of key parameters include radius of gyration, gap between electrode as well as inner diameter of curved drift tube, on the homogeneity of electric field were researched and some useful laws were summarized. Finally, an optimized curved drift tube is designed to achieve homogenous drift electric field. There is more than 98.75% of the region inside the curved drift tube where the fluctuation of the electric field strength along the radial direction is less than 0.2% of that along the axial direction.
Laser-induced breakdown spectroscopy (LIBS) is characterized as a powerful tool in in-situ online analysis with its fast and multiple detecting abilities. But in the area of detecting trace sample in aqueous solution of low concentration, the turbulence, scattering, absorbance and cooling effect of liquid medium limits its performance. Traditional method includes liquid jet, liquid-solid transformation and pre-concentration on other medium, yet the procedure of sample preparation is complicated and time consuming. In this work, we propose a new method to achieve pre-concentration, in which filter paper and electro-spray ionization (ESI) are used. In our experiment, we choose MnSO4 as sample. The surface of filter paper is sprayed with MnSO4 aqueous solution of different concentration by an ESI. The pulsed laser is focused on the surface of filter paper and the plasma is formed in the focusing area. Through an optical fiber the spectrum of plasma is detected by a spectrometer. The ESI system, pulses generator system and the UI on PC are home-made. The spectra lines of Mn at 257.6nm, 259.4nm and 260.6nm are analyzed. Results show that the limit of detection at 257.6nm is sub-ppb and the R2 of calibration curve is more than 0.93. Compared with traditional method, like soak and drip processing, our method can increase the concentration of the sample by simply expanding spraying time, achieving a higher signal-to-noise ratio (SNR) and a lower limit of detection (LOD). In addition, the consumption of sample solution is as low as several hundred μl in each detection.
Ion mobility spectrometry (IMS) is widely used in the field of chemical composition analysis. Faraday cup is the most classical method to detect ions for IMS in the atmospheric pressure. However, the performance of Faraday plate was limited by many kinds of factors, including interfering electromagnetic waves, thermal(Johnson) noise, induced current , gain bandwidth product, etc. There is a theoretical limit in detection of ions at ambient condition which is approximately 106 ions per second. In this paper, we introduced a novel way using laser-induced fluorescence (LIF) to bypass the limitation of Faraday plate. Fluorescent ions which were selected by IMS get excited when they fly through the laser excitation area. The fluorescence emitted by the excited ions was captured exponentially and amplified through proper optoelectronic system. Rhodamine 6G (R6G) was selected as the fluorochrome for the reason that excitation wavelength, emission wavelength, and fluorescence quantum yield were more appropriate than others. An orthometric light path is designed to eliminate the adverse impact which was caused by induced laser. The experiment result shows that a fluorescence signal from the sample ions of the IMS could be observed. Compared with Faraday plate, the LIF-IMS may find a potential application in more system at the atmosphere condition.