印刷线路板分压离子阱的离子单向出射性能研究

印刷线路板(Printed-Circuit-Board,PCB)分压离子阱是一种新型质量分析器,其突出优点在于内部电场可通过调节射频分压比进行优化.本实验在PCB分压离子阱离子出射方向的两组离散电极上配置了非对称的射频分压,以引入奇次阶场成分,使得射频电场的场中心(即离子运动中心)发生偏移,从而实现离子单向出射.通过数值计算软件SIMION和AXSIM分析了射频分压比差值与其内部电场分布的关系,并模拟离子运动轨迹,得到离子出射情况和模拟质谱峰.模拟结果表明,当两组离散电极的射频分压比差值为20％时,在合适的AC频率条件下,对于m/z=609 Th的离子,PCB分压离子阱的离子单向出射率可达90％以上,且质量分辨率大于2500.本研究可使PCB分压离子阱在基本不损失质量分辨率和使用单检测器模式下,大幅提高离子检测效率,因而在小型化质谱仪应用中具有显著优势.     

Numerical observation of preferred directionality in ion ejection from stretched rectilinear ion traps

We report on numerical investigations of directionality of ion ejection in stretched rectilinear ion traps (RIT). Three 4-electrode trap geometries have been investigated. In all cases, one pair of electrodes has slits at their center and the other pair has no slits. The studied traps include the RIT-S, in which the mass analyzer electrodes are symmetrically positioned around the central axis; the RIT-X, in which the mass analyzer has a stretch in the direction of the electrodes which have slits (labeled as x-direction); and the RIT-Y, in which the mass analyzer has a stretch in the direction of the electrodes which have no slits (labeled as y-direction).Our analysis has been carried out on two-dimensional (2D) fields at the centre of an infinitely long mass analyzer. The boundary element method (BEM) has been used for field computations. The trajectory of ion motion has been generated using Runge Kutta fourth order integration.Three sets of simulations have been carried out on each of the RIT-S, the RIT-X and the RIT-Y to check for directionality of ion ejection. In the first, we numerically obtain the stability region on the potential (U_dc– V_rf) axes. In the second we generate an escape velocity plot with U_dc=0 for different values of V_rf. In the third, we simulate the mass selective boundary ejection experiment on a single ion.In the symmetric RIT-S, as expected, all three simulations show that there is an equal probability of ion reaching the trap boundary in either of the x- or y-directions. For the stretched traps, however, the results are dramatically different. For the RIT-X, all three simulations suggest that ion destabilization at the stability boundary occurs in the x-direction. Similarly, for the RIT-Y, ions preferentially get destabilized in the y-direction. That is, ions reaching the trap boundary overwhelmingly prefer the stretch direction.     

阶梯电极离子阱质量分析器的结构与性能

本研究从理论上优化了一种新型结构的线型离子阱质量分析器-阶梯电极离子阱质量分析器,它是由2对阶梯电极与1对端盖电极组成。与传统平板电极矩形离子阱长阶梯电极离子阱相比,具有调节电场分布的优点,同时在几何结构设计上更接近于双曲面电极结构,但比双曲面电极更容易加工。通过改变阶梯电极结构的高度、宽度、场半径比例等几何参数,实现了对离子阱内部电场分布的优化,从而实现离子阱性能的优化。理论模拟研究结果表明,根据几何结构和电场分布优化获得的阶梯电极离子阱质量分析器(X0×Y0=9 mm×5 mm),可以在225 Da/ s 扫速下获得10150的质量分辨率。阶梯电极离子阱结构简单,分辨能力明显高于矩形离子阱。初步的实验结果表明,阶梯电极离子阱具有较好的串级质谱分析性能。     

Improved ion extraction from a linear octopole ion trap: SIMION analysis and experimental demonstration

Externally generated ions are accumulated in a linear octopole ion trap before injection into our 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer. Such instrumental configuration has previously been shown to provide improved sensitivity, scan rate, and duty cycle relative to accumulated trapping in the ICR cell. However, inefficient ion ejection from the octopole currently limits both detection limit and scan rate. SIMION 7.0 analysis predicts that a dc axial electric field inside the linear octopole ion trap expedites and synchronizes the efficient extraction of the octopole-accumulated ions. Further SIMION analysis optimizes the ion ejection properties of each of three electrode configurations designed to produce a near-linear axial potential gradient. More efficient extraction and transfer of accumulated ions spanning a wide m/z range promises to reduce detection limit and increase front-end sampling rate (e.g., to increase front-end resolution for separation techniques coupled with FT-ICR mass analysis). Addition of the axial field improves experimental signal-to-noise ratio by more than an order of magnitude.     

Linear ion trap with added octopole field component: the property and method

It is well known that superimposition of some positive octopole field will benefit the performance of ion trap mass analyzer. In the radial‐ejection linear ion trap (LIT), adding some octopole field component to the main quadrupole field is usually accomplished by stretching the ejection rod pair. In this study, the effect of octopole potential and some other higher order potential on the performance of LIT mass analyzer is investigated. A simple and effective method, which is to add some octopole component by building a LIT with a pair of rectangular electrodes and a pair of semi‐circular electrodes, is reported. Its properties were studied by numerical simulations and experiments. The results showed that a certain amount of positive octopole component could be produced by simply adjusting the position and width of the rectangular electrodes. A resolution of over 1200 at m/z 609 (~1600 Da/s) was observed in this type of LIT. They also performed tandem mass spectrometry well. The device with optimum geometry for ion ejection from rectangular electrodes provided comparable performance to that for ion ejection from semi‐circular electrodes. This type of LIT design is easy for fabrication and assembly. Copyright © 2015 John Wiley & Sons, Ltd.     

Novel linear ion trap mass analyzer composed of four planar electrodes

A novel linear ion trap mass analyzer was developed using just four elongated planar electrodes, mounted in parallel, and employing an RF potential for ion trapping in the radial and axial directions. Mass analysis was achieved using the mass-selective instability scan with ion ejection in the radial direction. The performance of this new device was characterized in comparison with the 6-electrode rectilinear ion trap (RIT) from which it is derived. The 4-electrode trap gives optimum performance in an asymmetric geometry, just like the original optimized 6-electrode RIT. The strong RF fringing fields at the ends of the RF rods account for axial ion trapping without use of extra electrodes or an axial DC voltage. Field calculations and simulations have been carried out to study the trapping potential inside RITs with various configurations. Demonstrated capabilities include analysis of externally injected ions with mass resolution in excess of 1000 and a mass/charge range of 650 Th as well as tandem mass spectrometry capabilities. The geometric simplicity and performance characteristics of the 4-electrode RIT make it particularly attractive in the development of next generation miniaturized mass spectrometers.     

稳定图的测定与矩形离子阱质谱性能分析

从理论上讲, 离子阱质谱仪的性能是由阱内电场分布决定的,而电场分布又是由组成离子阱的电极几何结构和离子阱工作电压决定的. 对于矩形离子阱, 即使不考虑其几何结构的偏差, 其阱内的电场分布一般也很复杂. 在矩形离子阱内, 除四极电场外, 还包含多种成分的其他各种高阶场, 它们直接影响离子在阱内的运动轨迹和离子阱质谱的性能. 由于各种电场成分对离子阱内离子运动的影响非常复杂, 还很难从数学上给出精确的解析解, 使得目前从理论上还无法预测高阶场成分对质谱性能的影响. 本工作通过测定不同几何结构的矩形离子阱的稳定图, 从实验上比较了不同场半径, 即不同电场分布条件下的离子阱质谱性能的差别. 实验中, 通过改变离子阱的几何比例结构, 详细测定了不同结构的矩形离子阱的稳定图特征, 并与实验测得的质谱分析结果进行比较. 同时, 我们还详细介绍了矩形离子阱质谱的稳定图的测定方法, 并根据得到的不同情况下的稳定图结构分析了离子阱的质谱性能. 研究结果表明： 可以通过比较试验得到的稳定图结构来判断其离子阱质谱仪的性能如质量分辨能力等. 此外, 实验结果还发现： 对于y方向拉伸结构的矩形离子阱, 其实验绘制得到的是不完整的稳定图. 但根据稳定图边界的特点, 通过采用四极直流电压调制的方法, 可以对y方向拉伸结构的矩形离子阱的性能进行改善, 极大地提高了阱的质量分辨能力.     

Development and Investigation of a Mesh-Electrode Linear Ion Trap (ME-LIT) Mass Analyzer

A mesh-electrode linear ion trap (ME-LIT) mass analyzer was developed and its performance was primarily characterized. In conventional linear ion trap mass analyzers, the trapped ions are mass-selected and then ejected in a radial direction by a slot on a trap electrode. The presence of slots can strongly affect the electric field distribution in the ion trapping region and distort the mass analysis performance. To compensate for detrimental electric field effects, the slot is usually designed and fabricated to be as small as possible, and also has very high mechanical accuracy and symmetry. A ME-LIT with several mesh electrodes was built to compensate for the effects caused by slots. Each mesh electrode was fabricated from a plate electrode with a relatively large slot and the slot was covered with a conductive mesh. Our preliminary experimental results show that the ME-LIT could considerably diminish the detrimental electric field effects caused by slots, and increase the mass resolving power and ion detection efficiency. Even with 4-mm-wide slots, a mass resolution in excess of 600 was obtained using the ME-LIT. Mass resolution could be remarkably improved using mesh electrodes in ion traps with asymmetric electrodes. The stability diagram of the ME-LIT was mapped, and highly efficient tandem mass spectrometry was demonstrated. The ME-LIT was qualified as a LIT mass analyzer. The ME-LIT can improve the mass resolution and decrease the requirements of mechanical accuracy and symmetry of slots, so it shows potential for a wide range of practical uses.

Origin of mass shifts in the quadrupole ion trap: dissociation of fragile ions observed with a hybrid ion trap/mass filter instrument

A novel hybrid tandem mass analyzer, coupling a quadrupole ion trap with a quadrupole mass filter, has been constructed to permit mass analysis of ions ejected from the ion trap. The initial application of this instrument is the investigation of the origin of mass shifts in the ion trap due to ion fragility. We hypothesize that fragile ions undergo mass shifts, characterized by peak fronting, due to early ejection from the quadrupole ion trap. As these ions come into resonance with the ejection frequency, they gain kinetic energy, collide with buffer gas molecules and thus can dissociate to produce fragment ions. These fragment ions will not be stable within the ion trap as they are situated past the stability boundary at q(z) = 0. 908. Consequently the fragment ions are ejected prematurely. This results in an apparent mass shift due to peak fronting. The experiments reported here clearly document the production of fragment ions as the origin of mass shifts during the resonant ejection of fragile ions. Copyright 2000 John Wiley & Sons, Ltd.     

新型三角形电极圆环离子阱的理论模拟研究

圆环离子阱由于其离子储存能力明显优于相同体积下的三维离子阱,近年来被认为是离子阱小型化发展的另一个重要方向。为进一步优化圆环形离子阱的质谱性能,特别是质量分辨能力,本研究提出了一种由三角形电极构建的新型圆环离子阱,它由两个完全等同的、截面为三角形的圆环电极及两个大小不等的圆筒型电极所组成,离子通过共振激发方式弹出。通过理论模拟和对电极结构的优化,获得了具有非对称性的三角形电极结构,通过改善圆环结构,优化电场分布,提高了离子引出效率和离子阱的质量分辨能力,其中一种最优化结构的圆环离子阱对m/z 609离子的质量分辨率达到1486。     

Characterization of electrode surface roughness and its impact on ion trap mass analysis

With the recent trend towards mass spectrometer miniaturization, the fabrication of mass analyzers and other ion optical components is being performed at scales where critical dimensions range from several millimeters to several micrometers. Depending on the sizes of the objects and the nature of the fabrication method used, electrode surface roughness can become non‐negligible and affect the analytical performance of the mass analyzer. In this work, a method of characterizing surface roughness is introduced through the concept of spatial roughness frequency. The roughness of a given surface is quantitatively described using spatial roughness components at a series of frequencies and with characteristic intensities. Based on this concept, an analytical method has been developed to describe the electromagnetic field inside an electrode assembly including consideration for the electrode roughness. The methodology is applied in simplified form to cylindrical and rectilinear ion trap analyzers. Four types of surface finishes were applied to ion trap electrodes of various sizes to illustrate the surface roughness effects on the high‐order fields and to compare the analytical performance of the ion traps. Application of this method to arrays of large numbers of micro‐scale ion traps has enabled the impact of fabrication methodology to be evaluated in terms of mass resolution for the ion trap arrays. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetic energy effects in an ion ensemble subjected to mass-selective isolation and resonance excitation: A simulation study

A simulation study is described of the behaviour of ions confined in a quadrupole ion trap during each of two separate operations of a tandem mass spectrometric experiment. The two operations are those of mass-selective ion isolation and mass-selective resonance excitation to the point of ion ejection from the ion trap. The method of mass-selective ion isolation simulated is that of consecutive ion isolation. Simulation data indicate that the collisional history of the ions prior to the isolation process can greatly influence the degree to which ions survive this process. Simulation data for mass-selective resonance ejection are compared with experimental data obtained with a Finnigan-MAT ion trap mass spectrometer. In each operation, the facility with which ions absorb energy from the field within the ion trap, whether this field is derived from the R.F. drive potential or a supplementary potential, can determine the extent to which ions are retained within the ion trap during the two mass-selective operations described.     

Ion trajectory simulation for electrode configurations with arbitrary geometries

A multi-particle ion trajectory simulation program ITSIM 6.0 is described, which is capable of ion trajectory simulations for electrode configurations with arbitrary geometries. The electrode structures are input from a 3D drawing program AutoCAD and the electric field is calculated using a 3D field solver COMSOL. The program CreatePot acts as interface between the field solver and ITSIM 6.0. It converts the calculated electric field into a field array file readable by ITSIM 6.0 and ion trajectories are calculated by solving Newton's equation using Runge-Kutta integration methods. The accuracy of the field calculation is discussed for the ideal quadrupole ion trap in terms of applied mesh density. Electric fields of several different types of devices with 3D geometry are simulated, including ion transport through an ion optical system as a function of pressure. Ion spatial distributions, including the storage of positively charged ions only and simultaneous storage of positively/negatively charged ions in commercial linear ion traps with various geometries, are investigated using different trapping modes. Inelastic collisions and collision induced dissociation modeled using RRKM theory are studied, with emphasis on the fragmentation of n-butylbenzene inside an ideal quadrupole ion trap. The mass spectrum of 1,3-dichlorobenzene is simulated for the rectilinear ion trap device and good agreement is observed between the simulated and the experimental mass spectra. Collisional cooling using helium at different pressures is found to affect mass resolution in the rectilinear ion trap.     

Space-charge effects with mass-selective axial ejection from a linear quadrupole ion trap

Methods to reduce mass shifts caused by space charge with mass‐selective axial ejection from a linear quadrupole ion trap are investigated. For axial ejection, dipole excitation is applied to excite ions at q ≈ 0.85. The trapping radiofrequency (rf) voltage is scanned to bring ions of different m/z values into resonance for excitation. In the fringing field at the quadrupole exit, excited ions gain axial kinetic energy, overcoming the trapping potential, and are ejected from the trap. Space charge causes the frequencies of ion oscillation to decrease. Thus, greater rf voltages are required to bring ions into resonance for excitation and ejection, and the ions shift to higher apparent masses in a mass spectrum. At the same time, the peaks broaden, lowering resolution. The effects of injection q value, ejection q value, excitation amplitude, quadrupole dc voltages applied to the electrodes, applying an rf voltage to the exit lens, and scan speed, on mass shifts have been studied experimentally. Most experiments were done with only ions of protonated reserpine (m/z 609.3 and its isotopic peaks) in the trap. Some experiments were done with ions of protonated reserpine and ions of m/z 622 in the trap. In general, the mass shifts are reduced with higher ejection q values, higher excitation amplitudes, with quadrupole dc applied, and at higher scan speeds. The application of quadrupole dc appears to increase the ion cloud temperature, which lowers mass shifts. Thus, a proper choice of operating conditions can reduce, but not eliminate, mass shifts caused by space charge. Copyright © 2011 John Wiley & Sons, Ltd.     

High performance fourier transform ion cyclotron resonance mass spectrometry via a single trap electrode

An open-ended cylindrical cell with a single annular trap electrode located at the center of the excitation and detection region is demonstrated for Fourier transform ion cyclotron resonance mass spectrometry. A trapping well is created by applying a static potential to the trap electrode of polarity opposite the charge of the ion to be trapped, after which conventional dipolar excitation and detection are performed. The annular trap electrode is axially narrow to allow the creation of a potential well without excessively shielding excitation and detection. Trapping is limited to the region of homogeneous excitation at the cell centerline without the use of capacitive coupling. Perfluorotributylamine excitation profiles demonstrate negligible axial ejection throughout the entire excitation voltage range even at an effective centerline potential of only ?0.009 V. High mass resolving power in the single-trap electrode cell is demonstrated by achievement of mass resolving power of 1.45 × 10⁶ for benzene during an experiment in which ions created in a high pressure source cubic cell are transferred to the low pressure analyzer single-trap electrode cell for detection. Such high performance is attributed to the negligible radius dependent radial electric field for ions cooled to the center of the potential well and accelerated to less than 60% of the cell radius. An important distinction of the single-trap electrode geometry from all previous open and closed cell arrangements is exhibition of combined gated and accumulated trapping. Because there is no potential barrier, all ions penetrate into the trapping region regardless of their translational energy as in gated trapping, but additional ions may accumulate over time, as in accumulated trapping. Ions of low translational kinetic energy are demonstrated to be preferentially trapped in the single-trap electrode cell. In a further demonstration of the minimal radial electric field of the single-trap electrode cell, positive voltages can be applied to the annular trap electrode as well as the source cell trap electrode to achieve highly efficient transfer of ions between cells.     

Design of a new multi‐turn ion optical system ‘IRIS’ for a time‐of‐flight mass spectrometer

A new multi‐turn ion optical system ‘IRIS’ has been designed for use with a high‐performance time‐of‐flight (TOF) mass spectrometer, which satisfies the new design concepts of time focusing and phase space stability. It has an elliptical flight path composed of four toroidal electric sectors, with a flight path length for one lap of 0.974 m. Dimensions and voltages of sector electrodes have been optimized to satisfy theoretical requirements by simulations using surface charge method. Generally, multi‐turn instruments require an injection and ejection system to inject and eject ions. On the basis of this ion optical study, we have designed an injection and ejection ion optical system, which achieves time focusing for the total system. Furthermore, we have designed novel field‐adjusting electrodes (FAEs) for the perforated sectors in the injection and ejection systems, which accurately correct the electric potential around the perforated sector's hole. We have also used simulations to evaluate mass resolving power and ion transmissions for various lap numbers or flight path lengths. Through these we have confirmed that mass resolving powers of over 100 000 can be achieved with reasonable ion transmissions for a given set of initial conditions. Usually a multi‐turn TOF mass spectrometer with a closed optic axis has mass range limitations from overtaking ions. To solve this problem, a TOF segmentation method is proposed that identifies all peaks in a TOF spectrum, including those from overtaking ions. Copyright © 2008 John Wiley & Sons, Ltd.     

A secondary ion microprobe ion trap mass spectrometer

An ion trap mass analyzer has been attached to an organic secondary ion microprobe. A pressure differential >100 can be maintained between the ion trap and microprobe. The well-focused secondary ion beam can transit a small (2 mm) diameter tube, but gas flow from ion trap to microprobe is impeded. This pressure differential allows the microprobe to retain imaging capability. Ion trap and microprobe data systems are integrated by taking advantage of the highly reproducible periodicity of the ion trap operating in resonant ejection mode and asynchronous signal and data acquisition afforded by commercially available interface cards. Secondary ion mass spectra and images obtained indicate an approximately 10-fold improvement in sensitivity, although preliminary evidence indicates low (<1%) trapping efficiency. Image data acquisition using the ion trap for mass analysis requires at least 10 times as much time compared to using a quadrupole mass filter because the mass-selected instability mode is used for mass analysis, i.e., mass resolution in the ion trap is not continuous as it is in the quadrupole.     

Plasma source ion trap mass spectrometry: Enhanced abundance sensitivity by resonant ejection of atomic ions

An experimental study of resonant ion excitation in an rf quadrupole ion trap is reported. Atomic ions are generated in an inductively coupled plasma and injected into the ion trap where, after collisional cooling, they are irradiated by a low-voltage, dipole coupled waveform. Single frequency, narrowband, and broadband excitation pulses have been used. Absorption lineshapes (plots of observed ion signal versus excitation frequency) are shown for variations in buffer gas pressure and the amplitude and duration of the single frequency pulses. The absorption lineshapes are usually asymmetric and tail toward lower frequencies. At sufficiently low buffer gas pressure or potential well depth, the lineshapes broaden and become more asymmetric to the point that absorption by ions with adjacent mass-to-charge ratios overlaps. This overlapping absorption reduces the selectivity with which a single mass-to-charge ratio ion can be excited and ejected relative to nearby mass-to-charge ratio ions. The rate of ion ejection is different on the low versus high frequency edges of the absorption lines. This difference in ejection rates provides an important key to understanding the shape of the absorption lines. All of these observations are explained in terms of the known kinematic behavior of ions in real traps, that is, traps with substantial higher order symmetry components in the trapping field (“nonlinear” fields). The importance of the nonlinearity of the trapping field in understanding the observed lineshapes and their time dependencies is discussed. We also report resonant ejection results obtained using multiple frequency (narrow or broad bandwidth) excitation. Multiple frequency excitation allows ions with different mass-to-charge ratio values to be ejected from the trap using one excitation waveform. The finite ion storage capacity of the ion trap is thereby reserved for the ion(s) of interest. We show that ejection of ⁸⁹Y ions can be ～ 10⁵ times more efficient than ejection of ions at either m/z 88 or 90.     

Ion motion in the rectangular wave quadrupole field and digital operation mode of a quadrupole ion trap mass spectrometer

A quadrupolar electric field driven by a rectangular wave voltage can be used for mass-selective storage and analysis. The ion motion in such an electric field is derived, and the stability of ions is presented in the a-q diagram that is commonly used for sinusoidal wave quadrupole mass spectrometry in association with the solution of the Mathieu equation. The pseudo-potential well is discussed in an approximation that leads to the relation of secular frequency to operating parameters. A scheme for a digital ion trap mass spectrometer is described, based on this theory. An ion optics simulation was performed to check the theory of resonant ejection, and to prove the feasibility of the mass scan method for a practical ion trap of such geometry.     

Structure and Performance of Polygonal Electrode Linear Ion Trap

The polygonal electrode linear ion trap (PeLIT) can produce quadrupolar electric field plus some higher order field, which balances the relationship between mass resolution and electrode manufacturing difficulties. The electrodes of PeLIT are relatively simple, but have a good mass resolving power. This study investigated the relationship between the electric field distribution and the ion trap structures, and the performances of PeLIT through theoretical simulation and experimental study. Research results of simulation showed that the polygonal electrode linear ion traps with different structures had different electric field distributions and mass analysis performances. The negative decapole field distorted the performances significantly. The experimental results showed that the mass resolution of reserpine ions (m/z 609) was more than 2500 using a polygonal electrode ion trap. At the same time, mass selective excitation and tandem mass spectrometry experiments were also carried.