Detailed study of the quadrupole mass analyzer operating within the first, second, and third (intermediate) stability regions. II. Transmission and resolution

Theoretical aspects of ion separation in imperfect fields of the quadrupole mass analyzer operating within the first, second, and third stability regions are applied to simulate transmission and resolution by using an analytical approach. A mathematical simulation based on statistical mechanics reveals in analytical form that the region of beam capture, i.e., the transmission, is inversely proportional to the relative values of the mass analyzer distortions to the 1.18th power in log scale. Otherwise, taking into account tails of peaks from unstable ion trajectories shows that the maximum attainable resolution is directly proportional to the number of cycles the ions spend in the field to the 1.33rd power. Because the ion current is amplitude modulated by the frequency of the alternating component of the field, transmission losses because of a parasitic modulation increase more than tenfold as the resolution and distortion increase. These losses are reduced to a minimum by applying a heterogeneous standing wave voltage to the mass analyzer with a linear distribution of amplitude along the ion transit axis. This additional standing wave increases the transmission tenfold. This procedure has the additional advantage of ion injection at zero phase in each cycle of the radiofrequency (rf) field. Experimental verification of the techniques used to avoid transmission losses caused by field distortions indicates the validity of the simulation results. The coarse approximation by means of the operating surface of electrodes in the form of rings instead of a solenoid to create a heterogeneous standing wave voltage applied to the mass analyzer with a linear distribution of amplitude along the drift axis increases the transmission by a factor of 5 compared with a traditional coupling mass filter with pre- and post-filters. Such a comparison proves the advantages of ion injection into the mass analyzer at zero phase in each cycle of an rf field. This reduces the mechanical tolerances of the mass analyzer by an order of magnitude and creates prospects for an increase in attainable resolution by using electrodes of circular profile.     

The gated electrostatic mass spectrometer (GEMS): Definition and preliminary results

GEMS is a new type of time-of-flight mass spectrometer based on an electrostatic energy analyzer. Mass resolution equals the energy analyzer kinetic energy resolution, which is set by its slit size. In GEMS, monochromatic ions enter the entrance slit at random times, and the gated ion deflection produced by the electrostatic field in the analyzer rejects ions that are inside the analyzer at gate onset, detecting those entering the analyzer after gate onset. This provides mass separation while overcoming the temporal and spatial spread problems typical of TOF applications. Paradoxically, GEMS works because all ion masses follow identical trajectories. GEMS is easily multiplied into two-dimensional arrays to increase sensitivity in space applications, requires relatively low voltages, and uses only a few electrical connections. Thus, it is easy to package GEMS as a small, low-power instrument for applications in harsh environments. A disadvantage of GEMS is that its output is the integral of the TOF spectrum and the derivative of the raw data must be taken, a procedure that is likely to add noise. A version of GEMS detecting un-deflected ions (u-GEMS) has been tested to demonstrate the time-integrated feature of the raw data but without the benefit of energy analysis. This paper describes GEMS implemented with the small deflection energy analyzer (SDEA), a compact version of the parallel plate energy analyzer. SDEA is described both analytically and with ion trajectory simulations using the ion trajectory simulation software SIMION; the results are then used to describe GEMS and compute its performance.     

A New Approach to Increasing the Resolution of a Mass Spectrometer with Wedge-Shaped Reflectors

The paper describes the investigation of the ion-optical properties of a laser TOF mass spectrometer including two successively positioned wedge-shaped ion mirrors. Some specific properties of the configuration of ion trajectories near their reflection in the second ion reflector are found. The dependence of aberrations on ion energy acquired toothed shape for the resolution of the analyzer higher than 3000–5000. The approximation of the dependence gave a 15^th degree polynomial. The calculation of polynomial coefficients showed a great contribution to the duration of ion packets for aberrations of higher order. The discovered features allowed us to suggest a way of the local correction of nearby trajectories in the total ion flux. By correcting the local motion of individual groups of ions, we could reduce temporary aberration to 1–1.6 ns, depending on ion energy. For the time of ion flight ~35 μs, such duration limits the resolution of the analyzer by a value not less than 10000. The real length of ion drift path was about 30 cm. The total overall sizes of the ionoptical system were ~24 × 19 × 5 cm.     

Lithium analysis in Li-ion battery materials via a scanning electron microscopy-based approach

We performed Li analysis by reflection electron energy loss spectroscopy (REELS) with a scanning electron microscopy-based apparatus. It was possible to distinguish between Li compounds containing different transition metal elements spatially, via the spectrum imaging scheme of REELS spectra. We also acquired the Li spectrum for an Li-inserted graphite negative electrode. REELS measurements with hemispherical analyzer were performed to obtain high-quality spectra with sufficient energy resolution and compared with those from cylindrical mirror analyzer measurements; the former provided a more detailed chemical state evaluation of Li.     

Spherical FAIMS: comparison of curved electrode geometries

High-field asymmetric waveform ion mobility spectrometry (FAIMS) can operate at atmospheric pressure to separate gas-phase ions on the basis of a difference in the mobility of an ion at high fields relative to its mobility at low field strengths. Several novel cell geometries have been proposed in addition to the commercially available planar and cylindrical designs. Nevertheless, there is still much to explore about three-dimensional (3-D) curved cell geometries (spherical and hemispherical) and comparison to two-dimensional (2-D) curved geometries (cylindrical). The geometry of a FAIMS cell is one of the essential features affecting the transmission, resolution, and resolving power of FAIMS. Electric fields in a spherical design allow advantages such as virtual potential wells that can induce atmospheric-pressure near-trapping conditions and help reduce ion losses. Curvature of electrodes enables the ions to remain focused near the gap median, which help to improve sensitivity and ion trapping at higher pressures. Here we detail the design and characterization of a novel FAIMS cell having spherical electrode geometry and compare it to hemispherical and cylindrical cells. These FAIMS cells were interfaced with a quadrupole ion trap mass spectrometer in this study. Several structural classes of common explosives were employed to evaluate the separation power of these geometries. FAIMS spectra were generated by scanning the compensation voltage (CV) while operating the mass spectrometer in total ion mode. The identification of ions was accomplished through mass spectra acquired at fixed values of CVs. The performance of FAIMS using cylindrical, hemispherical, and spherical cells was compared and trends identified. For all trials, the best transmission was obtained by the spherical FAIMS cell while hemispherical FAIMS provided the best resolution and resolving power.     

A novel tandem quadrupole mass analyzer

A new “tandem mass analyzer” is described. Two quadrupole mass filters are operated in series. Each is operated at low resolution and a small mass offset is introduced between the two quadrupoles so that the pair operate together to give higher resolution. The resolution of the tandem analyzer can be changed by changing the mass offset. The transmission is highest when the quadrupoles are operated as close together as possible with the poles aligned, with no intervening ion lens, and with the radio frequency (rf) voltages phase locked. A phase shift between the rf voltages applied to the quadrupoles also improves the ion transmission. For a given resolution the tandem analyzer has transmission comparable to that of a single quadrupole. Results obtained with operation of the quadrupoles in the first, second, and third stability regions are described. Operation in the third stability region is particularly advantageous because the tandem analyzer exhibits good abundance sensitivity on the low and high mass sides under conditions where a single quadrupole produces a long peak tail on at least one side. It is also shown that scattering losses in the tandem analyzer are about half of those of a conventional quadrupole. The results suggest that it may be possible to build a low cost tandem analyzer that has relatively poor mechanical precision and yet that produces satisfactory peak shape and resolution.     

Why are electron capture negative ion mass spectra not reproducible? An ion source problem

Differences in the electron capture negative ion mass spectra of environmentally related organic compounds acquired on a VG 30-250 triple quadruple mass spectrometer and on an HP 5985B gas chromatography/mass spectrometry system were investigated with respect to the ion formation process. Neither ion source temperature nor pressure was responsible for the differences. The populations of thermal electrons in both ion sources were experimentally determined and found to be similar, suggesting that electron capturing reactions should proceed with comparable efficiencies in both ion sources. The ion extraction efficiencies of the two instruments were examined by monitoring the transmission profiles of low- and high-mass ions as a function of lens potentials. Results indicated that the HP 5985B extraction lens significantly suppressed low-mass ions. Further, theoretical evaluation of ion trajectories using SIMION suggested that on the HP 5985B, low-mass ions entered the mass analyzer as a defocused beam, but high-mass ions entered the analyzer as a well-collimated beam. On the VG 30–250, low- and high-mass ions were transmitted to the analyzer with equal efficiency by the ion extraction system.     

High resolution dissociative electron attachment to gas phase adenine

The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed.     

Design of electron energy analyzers for electron impact studies

The design and construction of an electron energy analyzer for the study of electron impact processes in atoms, molecules and solids is described. The analyzer incorporates a 180° hemispherical deflector and five-element entrance optics. Focusing characteristics and angular behavior of the analyzer have been investigated by using the electron-ray tracing simulation program, SIMION. The entrance lens system to the hemispherical deflector has been designed to have high collection efficiency for low-energy electrons. The fringing field correction has been done by tilting the input beam angle outward for real aperture configuration.     

He I ultraviolet photoelectron spectroscopy of benzene and pyridine in supersonic molecular beams using photoelectron imaging

We performed He I ultraviolet photoelectron spectroscopy (UPS) of jet-cooled aromatic molecules using a newly developed photoelectron imaging (PEI) spectrometer. The PEI spectrometer can measure photoelectron spectra and photoelectron angular distributions at a considerably higher efficiency than a conventional spectrometer that uses a hemispherical energy analyzer. One technical problem with PEI is its relatively high susceptibility to background electrons generated by scattered He I radiation. To reduce this problem, we designed a new electrostatic lens that intercepts background photoelectrons emitted from the repeller plate toward the imaging detector. An energy resolution (ΔE/E) of 0.735% at E = 5.461 eV is demonstrated with He I radiation. The energy resolution is limited by the size of the ionization region. Trajectory calculations indicate that the system is capable of achieving an energy resolution of 0.04% with a laser if the imaging resolution is not limited. Experimental results are presented for jet-cooled benzene and pyridine, and they are compared with results in the literature.     

Novel tandem quadrupole-acceleration-deceleration mass spectrometer for neutralization-reionization studies

A new tandem mass spectrometer of the quadrupole-acceleration lens-deceleration. lens-quadrupole (QADQ) configuration is described. The instrument is designed for neutralization-reionization studies and consists of a 2000-u quadrupole mass analyzer as MS-I, an acceleration electrostatic lens, a series of three differentially pumped collision cells, and an electrostatic deceleration lens, energy filter, and another 2000-u quadrupole mass analyzer as MS-II. The ion optical system achieves high total ion transmission for 5–9-keV ions. Unit mass resolution in neutralization-reionization mass spectra of aromatic compounds is demonstrated. Mass, kinetic energy, and linked scans at various levels of mass resolution and sensitivity are described.     

Improving the sensitivity of the end-cap reflectron time-of-flight mass spectrometer

A miniaturized time-of-flight (TOF) mass spectrometer utilizes an end-cap reflectron to achieve high kinetic energy focusing and improved mass resolution. However, the coaxial geometry gives rise to considerable losses in sensitivity resulting from reflected ion trajectories close to the center. These trajectories were modeled, using initial ion velocity distributions in the radial direction up to 300 m s(-1), and the portion of the active area of the detector that is utilized was evaluated experimentally using a variable diameter iris diaphragm. The sensitivity was improved by modification of the reflectron by tilting the end-cap electrode 4 degrees and redirecting the ions to a portion of the detector active area. Sensitivity was then measured as 3 fmol of the peptide substance P.     

A Fourier transform time-of-flight mass spectrometer. A SIMION calculation approach

A new kind of approach to time-of-flight type spectrometers is presented on the basis of SIMION calculations. The detector studied is a short cylindrical tube capacitor closed with parallel plates at both ends. The main principle of operation is to force ions of equal energy to circulate in the volume between the two tubes on a path of equal radius and measure their flight times pro revolution which corresponds to the frequency of oscillation. By performing spectral analysis on the received signal through transformation from the time domain to frequency space the different masses can be detected.To study the expected performance of the FT-TOF detector, calculations of ion trajectories have been made by varying the dimensions and electric potentials of the electrodes. The effect of the beam position, variations in the angle of entrance and ion energy to the trajectories was calculated to monitor the resolution that is achievable.     

An improved ion guide for external ion injection in glow discharge—fourier transform ion cyclotron resonance mass spectrometry

To improve the existing ion transport optics of our glow discharge (GD)-Fourier transformion cyclotron resonance (FT-ICR) mass spectrometer, we simulated several ion trajectories between the GD source region and the ICR analyzer cell. These calculations suggested that a number of simple improvements, including the use of an ion flight tube and an electrically isolated conductance limit, would increase the efficiency of ion transfer through the fringing fields of the FT-ICR superconducting magnet and into the ICR analyzer cell. Ion beam intensity was monitored as a function of the distance between the GD source and the analyzer cell before and after implementing these improvements. A twentyfold improvement in the transport efficiency, as well as a fifteenfold enhancement in detected ET-ICR signals, was observed.     

宽离子能量检测范围垂直引入反射式飞行时间质谱仪的研制

本实验室研制了国内首台宽离子能量检测范围飞行时间质谱仪。仪器采用紧凑式电子轰击源设计,配合离子透镜系统有效的调制离子流,飞行时间质量分析器采用了离子垂直引入式,双场加速和双场反射以及大尺寸MCP检测装置设计。仪器单离子信号半峰宽约2 ns,仪器分辨率优于1600FWHM,检测实际样品质量范围为1~127 amu(仪器理论质量检测上限优于800 amu),可检测离子能量范围优于2个数量级(3~140 eV)。若该TOF质量分析器与短瞬高压脉冲放电离子源耦合联用,可广泛应用于高能离子束的快速检测,如真空阴极放电对制备薄膜、离子注入材料的表征,导电材料的离子电荷态分布以及离子扩散速度的测定等。     

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.     

Determination of high-energy fragmentation of protonated peptides using a beqq hybrid mass spectrometer

A hybrid tandem instrument of BEqQ geometry was used to determine high-energy decomposition of protonated peptides, such as side-chain fragmentation yielding d _n and w _n ions. The transmission through both E and Q of such product ions, formed in the second field-free region, permits improved mass resolution and confident mass assignment. The experimental technique may involve synchronous scanning of E and Q, or, for the purpose of identification of specific products, limited-range scanning of either E or Q with the other analyzer fixed. These techniques are not equivalent, with respect to product ion transmission, to the double focusing of product ions achieved with four-sector instruments but nevertheless represent a critical improvement over conventional mass-analyzed ion kinetic energy spectrometry analyses. Fragmentation of protonated peptides occurring in the second field-free region inside and outside the collision cell were distinguished by floating the collision cell above ground potential. Mass filtering using Q confirmed the mass assignments. The data indicate that product ions resulting from spontaneous decomposition are in some instances quantitatively more significant than those resulting from high-energy collisional activation. Furthermore, the differentiation of the products of low- and high-energy processes should facilitate spectral interpretation.     

Elimination of the helium requirement in high-field asymmetric waveform ion mobility spectrometry (FAIMS): beneficial effects of decreasing the analyzer gap width on peptide analysis

Cylindrical geometry high-field asymmetric waveform ion mobility spectrometry (FAIMS) focuses and separates gas-phase ions at atmospheric pressure and room (or elevated) temperature. Addition of helium to a nitrogen-based separation medium offers significant advantages for FAIMS including improved resolution, selectivity and sensitivity. Aside from gas composition, ion transmission through FAIMS is governed by electric field strength (E/N) that is determined by the applied voltage, the analyzer gap width, atmospheric pressure and electrode temperature. In this study, the analyzer width of a cylindrical FAIMS device is varied from 2.5 to 1.25 mm to achieve average electric field strengths as high as 187.5 Townsend (Td). At these electric fields, the performance of FAIMS in an N(2) environment is dramatically improved over a commercial system that uses an analyzer width of 2.5 mm in 1:1 N(2) /He. At fields of 162 Td using electrodes at room temperature, the average effective temperature for the [M+2H](2+) ion of angiotensin II reaches 365 K. This has a dramatic impact on the curtain gas flow rate, resulting in lower optimum flows and reduced turbulence in the ion inlet. The use of narrow analyzer widths in a N(2) carrier gas offers previously unattainable baseline resolution of the [M+2H](2+) and [M+3H](3+) ions of angiotensin II. Comparisons of absolute ion current with FAIMS to conventional electrospray ionization (ESI) are as high as 77% with FAIMS versus standard ESI-MS.