Study On Methods Of Unidirectional Ion Ejection In A Linear Ion Trap

Mass spectrometer,with high resolution,high sensitivity and accuracy is one of the most popular modern analytical instruments.It use electric field or magnetic field to make ionized analytes separate in area of space,time or frequency,by which we can attain mass spectrum.The information of the molecular weight,molecular formula and molecular structure of the analytes can be obtained from the mass spectrum.At the present time,mass spectrometer has played an important role in many areas such as food safety,environmental protection,medical diagnosis,proteomics,genomics research,aerospace and military.Among these analyzers,ion trap has unique advantages such as a simple structure,a compact size,high sensitivity,relatively low requirement of vacuum and the ability of tandem mass spectrometry independently,and thus become the first choice for mass spectrometer miniaturization.However,the high accuracy of the hyperbolic structure of conventional 3D ion trap is harsh on machining and assembly tough,and 3D ion trap is also limited by its low ion capacity and low ion capture efficiency.In order to overcome these weaknesses and make ion trap more desirable for developing miniaturized and inexpensive mass spectrometers,recently,many linear(2D)ion traps with simplified structure have been built.One of the most typical is Rectilinear Ion Trap(RIT).It was invented by Cooks and was mainly composed of two pairs of rectangular electrodes with planar-shaped surface instead of hyperbolic-shaped electrodes as that used in a conventional LIT.Compared with the precise hyperbolic electrodes in the traditional linear ion trap,it is inevitable to introduce harmful high-order field components into the ion trap and reduce its analytical performance.Thus,Jiang et al.invented the printed circuit board(PCB)ion trap mass analyzer(hereinafter referred to as PCB ion trap).The electrodes on this linear ion trap are divided into different areas by the insulating material(peek).The analysis performance can be significantly improved by adjusting the area ratio of the different regions or the radio frequency(RF)voltage ratio across the regions.My laboratory has also independently developed a new type of linear ion trap with simplified structure:Linear Ion Trap featuring half round rod electrodes(HreLIT).Because the electrode surface of HreLIT is round,the machining procession is simpler and the machining precision is also improved compared with the traditional linear ion trap.In addition,the harmful high-order field components in the ion trap significantly reduced and its analytical performance is also significantly improved compared with RIT.Sensitivity is critical for mass spectrometer in real application.For an ion trap,the ion capacity,ion ejection efficiency and ion detection efficiency have impacts on the ultimate sensitivity.For existed linear ion traps,ions were ejected from both sides along the x axis,which means that the ion detection efficiency would not be higher than 50%if only one ion detector was used,as it usually does.To solve this problem,two ion detectors were used in commercialized LIT mass spectrometer.However,this configuration requires relatively large vacuum chamber,and apparently higher cost,which is undesirable for developing miniaturized mass spectrometers.In order to address the issue on ion detection efficiency of miniaturized linear ion trap mass spectrometer,we propose two methods of achieving unidirectional ion ejection in a linear ion trap:construct asymmetric geometries of linear ion traps and configure asymmetric RF voltages.These methods introduced odd-order fields into the ion trap,which displayed the center of the trapping(RF)field,and then induced unidirectional ion ejection.Ideally,ion detection efficiency could be greatly increased and mass resolution was almost no loss by thy way even if only one detector was used.Thus,these methods not only increase the sensitivity of the mass spectrometer,but also facilitate the miniaturization of the ion trap mass spectrometer.All the above taken into consideration,we put forward three specific implementations based on HreLIT or PCB ion trap:(1)built two types of asymmetric HreLIT(A-HreLIT);(2)Different RF potential ratios are arranged on the two sets of discrete electrodes in the ion exit direction of PCB ion trap,while the RF partial pressure ratio of the two sets of discrete electrodes in the other direction remains constant;(3)configure directly different proportions of RF voltage at a pair of electrodes in direction of ions ejection.For these two specific implementations,their feasibility were demonstrated by means of simulation and experiment in this study.This paper presents the specific experimental methods and processes,simulation and experimental results both showed:these three specific implementations all meet needs.The experimental results indicate that the two methods of constructing asymmetric geometries and configuring asymmetric RF voltages are feasible and have significant advantages in the field of miniaturized mass spectrometers.