The Negative Corona Discharge Electron Source For Ion Mobility Spectrometry

The ion mobility spectrometry (IMS) is a new detection technology. It is convenient,fast and of high sensitivity. The ionization/electronic sources of IMS include radioactivesources, corona discharge sources, photoionization sources, surface ionization sources andelectrospray ionization source. The corona discharge ionization sources have advanstage inthe ion yield and the applicability. But its utilization of electrons and ions is extremely low,and the factors influencing its electron/ion yield have not been fully studied. In the otherhand, proprietary research on IMS has just started in China, so the actual experience issorely lacking. In this paper, a home-built IMS based on the negative corona dischargeelectron source is studied. The main contents and conclusions are as follows:Three negative corona discharge electron sources are designed and tested. They arebased on the needle-grid DC discharge, the needle-grid pulsed discharge and the needle-ringpulsed discharge, respectively. The discharge is stable in each souce. The electronic signalsfrom the Faraday plate are measured. The results show that the electronic signal currentproducted by both needle-grid discharge sources is lower than1nA, which is very hard tobe measured accurately; The the electronic signal current producted by the needle-ringdischarge source could be as high as severalμA, and the signal-noise ratio is very good.The noise signals are of high-frequency and power-line frequency. High-frequencynoise current is as high as10μA in the initial experimental design. It has two main sources.First is the electromagnetic interference on the oscilloscope and the probe by the pulsedvoltage; second is the electromagnetic interference on the Faraday plate. The first solution isto lengthening the coaxial cable, and then the high-frequency noise current can be reducedto100nA. Another solution is to use filter capacitors between drift rings and ground, andthen the high-frequency noise disappear. In addition, the measured waveforms arepost-processed via the Origin8software, thus the measurement result is more accurate.The electric fields inside the dirft tubes are simulated via Ansoft Maxwell12software.The simulation results could offer the basis and starting point for analyzing the measurementresults. Ultimately, the voltages of every part of our home-built IMS are determined by thesimulation result, which can produce a uniform drift electric field. Based on the electric field simulation and the experimental measurement, the effectiveelectrons yield and the electronic efficiency of needle-ring source are studied. The resultsshow that the home-built needle-ring discharge source can produce2×109to14×109effective electrons in one single pulse, and the electron effective rates are of0.3%to3.6%.In addition, the factors (drift field strength, ring inner diameter and discharge voltage) on theamount of effective electrons are studied. The main results are as follows:(1) the rise in thedrift voltage significantly reduces the electron loss, thereby increasing the quantity ofeffective electrons and the effective electron rate;(2) the increase in the ring inner diametermainly decreases the proportion of the discharge towards the inner side of the ring (metalstructure), thus improving the quantity of electrons passing through the ring and thereby thequantity of effective electrons and the effective electron rate;(3) the rise in the dischargevoltage increases the total strength of the corona discharge, thereby increasing the quantityof effective electrons, but the rise in the discharge voltage simultaneously decreases thequantity of electrons passing through the ring and increases the electron loss, thus leading toa lower effective electron rate.On the basis of the above study, the optimized design of our IMS experimental systemis acquired. The parasitic capacitance of the measurement circuit is about260pF. The RCtime constant could cause the distortion of signal waveform, but its integral value remainsunchanged. Therefore, this circuit can only be used to measure the amount of electrons. Theions of air are measured, when the discharge gas is pure nitrogen. The signal peak of air ionsis as high as24nA, the pulse width is20ms, and the drift time is6ms. This signal is highbut the resolution is low, thus it can not meet the requirement of the analytical measurement.The reasons of this error are discussed and proven by the simulation results via Fluentsoftware; the direction of the further research is presented.