Research On Quadrupole Mass Spectrometry Under A Low-vacuum Environment

As an advanced product in scientific instruments,mass spectrometer plays an important role in many areas.Miniature ion trap MS system has the advantages of small dimension,light weight,low cost and easy to use,which make it a good candidate for fast,onsite chemical detection.Recently,lot efforts have been put on the miniature ion trap MS system development.However,due to the restraints of vacuum systems,it is difficult to reduce the size of a miniature mass spectrometer furthermore.Performing mass analysis in a low-vacuum environment can avoid the use of a bulky turbo pump,which will significantly reduce the size,weight,cost and power of the instrument.Moreover,this technology could be used to develop next generation hand-portable instruments and bring mass spectrometry to new applications.Aiming at low-vacuum mass spectrometry,theoretical analysis,method simulation,platform construction and experimental verification were adopted to investigate low-vacuum mass spectrometry methods.Firstly,as a platform foundation,a cylindrical ion trap mass spectrometer was built.The platform used an electron impact source as the ionization source,a custom cylindrical ion trap as the mass analyser,and an electron multiplier as the ion detector.Its working parameters such as the background gas pressure,mass-to-charge ratio of the ion,mass of the background gas molecule,initial kinetic energy of the ion,frequency of the radio frequency(RF)voltage,size of the ion trap,were optimized aiming at high-vacuum firstly,and the mass spectrometry of methyl salicylate was successfully performed.However,with the increase of background pressure,the performance of MS began to decrease.To realize the MS in low-vacuum,the new mass analysis methods need to be carried out.Secondly,in order to achieve the stability regions at low vacuum,the equation of ion motion with resistance was derived.Pressure effects were treated by adding a drag term to the Mathieu function which were calculated based on fifth-order RungeKutta in MATLAB.The effects of gas pressure on stability regions and ion motion were described.A numerical method for getting stability regions was proposed.The first,the second and the higher stability regions in the presence of the damping force were given.The results showed that the damping force caused by gas pressure enlarge the stability regions.The initially separate stability regions merged together for higher pressure.In low vacuum conditions,mass analysis can be performed by altering operation mode.The pressure effects can be reduced by increasing the frequency.In addition,we measured and analysed the mass deviation of methyl salicylate with the changes in the background gas pressure and RF voltage frequency in the CIT platform.The results experimentally verified the theory that the stability regions expand with increasing pressure for the first time.Thirdly,for simulation purposes,a method that uses Langevin collision theory to simulate the trajectory of ions in a low-vacuum environment was developed.The introduction of Langevin collision theory generate more accurate descriptions of the movement of discrete ions,and the results can be used to evaluate the influence of different operational models on the performance of MS.The sensitivity and resolution of the instrument were simulated under different operation modes.The adoption of a small mass background gas,a higher frequency for the radio frequency voltage,a smaller ion trap size and a higher temperature allowed mass analysis to be conducted under a low vacuum.In the customized CIT platform,by optimizing mass analysis methods according to the simulation results,the mass spectrometry of methyl salicylate was successively performed at helium buffer gas pressures up to 2 Pa.The spectrum had stronger signal intensities and maintained a similar resolution to that acquired under a high vacuum.This pressure of 2 Pa was dozens of times higher than the upper pressure limit of mass spectrometers with conventional ion traps and was 2000 times that used on the platform under a high vacuum.More importantly,this pressure can be obtained by using a single pump,avoiding the use of a bulky turbo pump.The methods we explored could be used to develop next generation handportable instruments and bring mass spectrometry to new applications.Finally,aiming at higher-pressure mass spectrometry,a mass analysis method based on drift gas in quadrupole mass filter(QMF)was proposed.In this method,the drift gas provided the kinetic energy to the ions.Thus,the ions had enough energy to reach the detector.As components for high pressure mass spectrometry,a novel quadrupole mass filter and a novel ion detector were proposed.In the novel QMF,rectangular electrodes were employed instead of traditional hyperboloid or cylindrical electrodes.The simulation results showed that the performance of novel quadrupole mass filter turned out to be acceptable.For the low cost,simple geometry and easy to be assembled,it had a great prospect in the field of miniature,portable and low-level mass spectrometer.To eliminate the turbulence near the ion detector when the drift gas flow through the detector,a novel ion detector with a double layer structure was constructed.The simulation results indicated that the novel ion detector created a homogenous gas under at high inlet flow rate while turbulence was evident for the traditional ion detector.The simulation results showed that,by using this method,the obtained spectral peak was superior to those without drift gas flow in the intensity and the width of the spectral peak.A simplified experimental platform was built to verify the proposed method.Under the pressure of 20 Pa,a picoammeter was used to measure the current of the ion detector at different(a,q).The experimental results were in agreement with the theoretical analysis and simulation which show that the proposed method is suitable for the mass analysis in high pressure.