Prediction of quadrupole mass filter performance for hyperbolic and circular cross section electrodes

A computer model has been developed that is able to predict the performance of a quadrupole mass spectrometer (QMS) for any constant cross section electrode geometry. It has been used to predict the performance of QMS systems with both hyperbolic and circular cross section electrodes. The predictions confirm the limited previous work that indicates QMS performance is poorer when circular cross section electrodes are used rather than hyperbolic ones. There is also an indication that use of circular electrodes causes a movement of the peak position from the expected one and produces an extended tail on the low mass side of the peak. Copyright 2000 John Wiley & Sons, Ltd.     

A Quadrupole mass spectrometer for resolution of low mass isotopes

The qualitative and quantitative identification of low mass isotopes in the mass range 1–6 u poses certain difficulties when attempting to achieve the required resolution with an instrument suitable for deployment within a process environment. Certain adjacent species present in the process sample (HT and D₂) require a resolution greater than 930 to achieve an accurate measurement. We demonstrate here through simulation techniques that this level of performance required is unachievable using commercially available instruments. Using previously reported simulation techniques, this article demonstrates how the required performance for resolving the low mass isotopes can be achieved by a quadrupole mass spectrometer (QMS), which incorporates a quadrupole mass filter (QMF) constructed from hyperbolic electrodes and operated in zone 3 of the Mathieu stability diagram.     

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.     

Effect of an axial magnetic field on the performance of a quadrupole mass spectrometer

We consider the case of a quadrupole mass spectrometer (QMS) in which a static magnetic field is applied axially in the z-direction along the length of the mass filter. The theoretical approach assumed in the model is that the QMS contains hyperbolic rods as electrodes and that the magnetic field acts over the full length of the mass filter assembly. Initial experimental results with argon and helium for a low-resolution instrument confirm the predicted theoretical trends. The analysis also predicts for which values of operating parameters an enhancement of the instrument resolution is achieved when an axial magnetic field is applied. The model predicts instrument resolution R > 3000 for a QMS with a 200 mm long mass filter via application of an axial magnetic field.     

Asymmetrical features of mass spectral peaks produced by quadrupole mass filters

The individual mass spectral peaks produced by a quadrupole mass spectrometer (QMS) are asymmetric; they exhibit a 'tail' on the low mass side. In some cases a definite structure is observed in the tail. We show that the tail structure is a consequence of the use of circular electrodes. An extreme case of an experimentally observed QMS mass peak with a distinct tail structure is shown and the general form is reproduced using our numerical model. The effect of instrument resolution, length, operating frequency, ion energy, mass and ion source aperture upon the tail structure are considered. Results show that extensive long tails originate mainly from ions that enter the mass filter at a relatively large distance from the QMS axis; also no significant tail is produced in the case of ideal hyperbolic form electrodes of finite length.     

Optimization of a quadrupole ion storage trap as a source for time‐of‐flight mass spectrometry

Designs of a quadrupole ion trap (QIT) as a source for time‐of‐flight (TOF) mass spectrometry are evaluated for mass resolution, ion trapping, and laser activation of trapped ions. Comparisons are made with the standard hyperbolic electrode ion trap geometry for TOF mass analysis in both linear and reflectron modes. A parallel‐plate design for the QIT is found to give significantly improved TOF mass spectrometer performance. Effects of ion temperature, trapped ion cloud size, mass, and extraction field on mass resolution are investigated in detail by simulation of the TOF peak profiles. Mass resolution (m/Δm) values of several thousand are predicted even at room temperature with moderate extraction fields for the optimized design. The optimized design also allows larger radial ion collection size compared with the hyperbolic ion trap, without compromising the mass resolution. The proposed design of the QIT also improves the ion–laser interaction volume and photon collection efficiency for fluorescence measurements on trapped ions. Copyright © 2011 John Wiley & Sons, Ltd.     

QMS in the Third Stability Zone with a Transverse Magnetic Field Applied

We report here a study using a quadrupole mass spectrometer (QMS) in which a static magnetic field is applied transversely to the body of the mass filter operating in stability zone 3. Significant improvement in QMS performance was obtained under certain magnetic field conditions, and these have been explained in terms of our theoretical model. The theoretical approach assumed in the model is that the QMS contains hyperbolic rods as electrodes and that the magnetic field acts over the full length of the mass filter assembly. Our latest analysis also predicts for what values of operating parameters an enhancement of the quadrupole resolution is achieved when a transverse magnetic field is applied. The model predicts instrument resolution R > 5000 for Ar with a 100 mm long mass filter and R > 3500 for a HT and D₂ mixture with a 200 mm long mass filter via application of a transverse magnetic field.     

离子阱阵列的理论模拟研究

采用电脑模拟的方法对一种新型的离子阱质量分析器——离子阱阵列进行了电极结构的优化.该离子阱质量分析器为一种多通道质量分析器,可以同时对不同的离子进行储存和质量分析.本实验主要研究了该离子阱质量分析器的性能和电极结构之间的关系.通过对离子运动轨迹的计算分析,可以得到模拟的离子质谱峰.通过对模拟离子质谱峰的分析,可以区分出使得离子阱性能较优的电极结构.在对模拟质谱峰的分析中,峰形和离子弹出效率都作为性能指标被考虑.有部分模拟的数据与实验结果进行了对比.     

Two-dimensional separations with electrospray ionization ambient pressure high-resolution ion mobility spectrometry/quadrupole mass spectrometry

The coupling of ion mobility spectrometry (IMS) instruments with mass spectrometers has been described since early in IMS development, most commonly with quadrupole mass analyzers. The recent development of IMS with time-of-flight (TOF) instruments has demonstrated that the time compatibility (IMS milliseconds and TOFMS microseconds) of the two techniques enables rapid two-dimensional separations to be performed, theoretically in the order of seconds for a complete analysis. This study presents a unique way to operate a traditional IMS/QMS system to attain separations similar to those achieved with IMS/TOF. For this new approach, the quadrupole was slowly scanned in the single-ion monitoring mode while IMS spectra were continually embedded in each m/z step. In this way, two-dimensional separations (IMS drift times and m/z) were obtained using the traditional IMS/QMS arrangement. An example of a five amino acid separation (quadrupole scan of 40 m/z values at a rate of approximately 7 steps/min) led to a complete two-dimensional analysis within 6 min, comparable to rapid chromatographic separations with mass spectrometry. Proposed approaches to reduce the analysis time are discussed and a reduction in the analysis time to less than 1 min is feasible when the IMS/QMS separation conditions are optimized.     

Quadrupole mass filter operation under the influence of magnetic field

This work demonstrates resolution enhancement of a quadrupole mass filter (QMF) under the influence of a static magnetic field. Generally, QMF resolution can be improved by increasing the number of rf cycles an ion experiences when passing through the mass filter. In order to improve the resolution, the dimensions of the QMF or the operating parameters need to be changed. However, geometric modifications to improve performance increase the manufacturing cost and usually the size of the instrument. By applying a magnetic field, a low‐cost, small footprint instrument with reduced power requirements can be realized. Significant improvement in QMF resolution was observed experimentally for certain magnetic field conditions, and these have been explained in terms of our theoretical model developed at the University of Liverpool. This model is capable of accurate simulation of spectra allowing the user to specify different values of mass spectrometer dimensions and applied input signals. The model predicts enhanced instrument resolution R>26 000 for a CO₂ and N₂ mixture with a 200‐mm long mass filter operating in stability zone 3 via application of an axial magnetic field. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Modelling mass analyzer performance with fields determined using the boundary element method

Computer modelling is widely used in the design of mass analysers to evaluate proposed designs and determine the effects of manufacturing imperfections. For quadrupole mass filters and ion traps, the models require accurate values of the electric field throughout the regions of the analyser in which ions travel. Most published results using models to predict mass analyser behaviour use electric fields computed with finite element (FE) or finite difference (FD) method. However, the boundary element method (BEM) is capable of achieving the same, or higher, accuracy with both computation times and memory requirements that are at least an order of magnitude less than those required by FE and FD methods. In this paper, electric field evaluation is performed using the BEM formulated in a manner described by previous workers; modifications to their method are described, which lead to higher accuracy field values. Simultaneous equation solution techniques are incorporated, which avoid solutions that are physically not realistic. The performance of linear quadrupole mass spectrometers with hyperbolic, circular and planar section electrodes has been determined using fields computed using these methods and compared with previous results obtained by alternative field computation techniques and with experiment. Behaviour of an ion trap mass spectrometer with circular symmetry has also been investigated. The results demonstrate that in each case using the BEM to determine the fields produces the observed behaviour. Copyright © 2010 John Wiley & Sons, Ltd.     

三角形电极离子阱的理论模拟及性能优化

作为一种新型结构的线性离子阱,三角形电极离子阱( TeLIT)具有简单的电极结构与良好的分析性能。为进一步提高TeLIT的质谱性能,本研究考察了TeLIT的性能与电极结构的关系。利用模拟软件SIMI-ON和AXSIM分析TeLIT场半径比与其内部电场分布的关系,并模拟离子运动轨迹,得出模拟离子质谱峰。理论模拟结果表明：优化场半径比可以改善内部电场分布,并能显著提高TeLIT的质量分辨率。最终模拟得出场半径比rx/ry=5.75：5时为最优结构,在该结构下,m/z为1892 Th的离子在扫描速率为1500 Th/s时,质量分辨率可以达到8287;扫描速率降为300 Th/s时,最高质量分辨率可达23000。     

Design and construction of glow discharge-ion trap mass spectrometer (GD-ITMS) and its application for the analysis of volatile organic samples

In this study, we present a newly designed see-through type hollow cathode glow discharge (St-HCGD) cell developed for the analysis of volatile organic materials in an ion trap mass spectrometer. The cell was interfaced with a homemade ion trap mass spectrometer by adopting skimmer and sampler in an optimized dimensions based on the computer simulation done by SIMION software. The St-HCGD cell has a relatively small size (4×4×7 cm) with the diameter of the inner tube of 0.25′′. The anode and cathode were made of stainless steel-304 and helium was used as a buffer gas for discharge to enhance the Penning ionization process rather than sputtering process. Mass spectra of volatile organic samples such as benzene, toluene, cyclohexane were obtained by using the St-HCGD-ITMS.     

Effects of the source gap on transmission efficiency of a quadrupole mass spectrometer

Rationale

Recent trends towards miniature and portable quadrupole mass spectrometry (QMS) entail challenges in instrumental sensitivity, which is influenced by 3D fringe field effects on ion transmission in the Quadrupole Mass Filter (QMF). The relationship of these effects with the gap from the ion source to the QMF entrance (source gap) is significant and little explored. We examine transmission characteristics experimentally and use the results to test the predictive accuracy of a recently developed 3D QMF simulation model. The model is then applied to directly investigate optimal transmission m/z ranges across multiple source gaps.

Methods

A portable single filter quadrupole mass spectrometer is used to analyse transmission characteristics across a range of common gases. We use an experimental approach originally proposed by Ehlert, enhanced with a novel method for absolute calibration of the transmission curve. Custom QMF simulation software employs the boundary element method (BEM) to compute accurate 3D electric fields. This is used to study the effects of the source gap on transmission efficiency.

Results

Experimental findings confirm a centrally peaked transmission curve; simulations correctly predict the optimal transmission location (in m/z) and percentage, and extend the experimental trend. We compare several methods for determining fringe field length, demonstrating how the size of the physical source gap influences both the length and the intensity of the fringe field at the QMF entrance. A complex relationship with ion transmission is revealed in which different source gaps promote optimal transmission at differing m/z ranges.

Conclusions

The presented results map the relationship between the source gap and transmission efficiency for the given instrument, using a simulation method transferrable to other setups. This is of importance to miniature and portable quadrupole mass spectrometers design for specific applications, for the first time enabling the source gap to be tailored for optimal transmission in the desired mass range.

     

Interference peaks in mass analysed ion kinetic energy spectra

When mass analysed ion kinetic energy spectroscopy is performed on ions of low abundance and low mass compared with the rest of the ions present in the ion chamber, severe interference may be observed. The major form taken by the interference is of narrow peaks at non-integral mass numbers. Many such peaks can be observed in mass analysed ion kinetic energy spectra and they are mainly attributed to decompositions of metastable ions occurring in the first field free region of the reversed geometry double focusing mass spectrometer.     

Mapping the Stability Diagram of a Quadrupole Mass Spectrometer with a Static Transverse Magnetic Field Applied

Previous experimental and theoretical work identified that the application of a static magnetic (B) field can improve the resolution of a quadrupole mass spectrometer (QMS) and this simple method of performance enhancement offers advantages for field deployment. Presented here are further data showing the effect of the transverse magnetic field upon the QMS performance. For the first time, the asymmetry in QMS operation with B _x and B _y is considered and explained in terms of operation in the fourth quadrant of the stability diagram. The results may be explained by considering the additional Lorentz force (v x B) experienced by the ion trajectories in each case. Using our numerical approach, we model not only the individual ion trajectories for a transverse B field applied in x and y but also the mass spectra and the effect of the magnetic field upon the stability diagram. Our theoretical findings, confirmed by experiment, show an improvement in resolution and ion transmission by application of magnetic field for certain operating conditions.

Using accurate mass electrospray ionization-time-of-flight mass spectrometry with in-source collision-induced dissociation to sequence peptide mixtures

Although data-dependent LC-MS-MS with database searching has become au courant for identifying proteins, the technique is constrained by duty-cycle inefficiency and the inability of most tandem mass analyzers to accurately measure peptide product ion masses. In this work, a novel approach is presented for simultaneous peptide fragmentation and accurate mass measurement using in-source collision-induced dissociation (CID) on electrospray ionization (ESI)-time-of-flight (TOF) MS. By employing internal mass reference compounds, mass measurement accuracy within +/-5 ppm for tryptic peptide precursors and +/-10 ppm for most sequence-specific product ions was consistently achieved. Analysis of a complex solution containing several digested protein standards did not adversely affect instrument performance.