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.     

Determination of quaternary ammonium pesticides by liquid chromatography-electrospray tandem mass spectrometry

A method for the direct determination of paraquat, diquat, chlormequat and difenzoquat in water samples, using an on-line solid-phase extraction-liquid chromatography-tandem mass spectrometry system was developed. No sample preparation was required and the detection limits were below the European Union maximum residue levels. The chromatographic separation was performed using an XTera MS C8 column. The concentration of the ion pair reagent, the pH and the gradient elution were optimized to give high recoveries and good chromatographic resolution between quats. The detection was carried out using an ion trap as mass analyzer. Parameters such as the magnitude and duration of the resonant excitation voltage and the magnitude of the trapping RF voltage for full scan tandem mass spectrometry (MS-MS) experiments were studied to establish the optimal experimental conditions. Moreover, the accurate optimization of these parameters allowed MS-MS experiments of low mass ions, below m/z 200, providing unambiguous peak identification. Finally, the reproducibility of the proposed method was shown by good run-to-run and day-to-day precision values and its applicability to the determination of quats in drinking water was evaluated using spiked samples.     

Multiplexed MS/MS in a Miniature Rectilinear Ion Trap

A multiplexed method for performing MS/MS on multiple ions simultaneously in a miniature rectilinear ion trap (RIT) mass spectrometer has been developed. This method uses an ion encoding procedure that relies on the mass bias that exists when ions are externally injected into an RIT operated with only a single phase rf applied to one pair of electrodes. The ion injection profile under such conditions ions is Gaussian-like over a wide range of rf amplitudes, or low mass cutoff (LMCO) values, during ion accumulation. We show that this distribution is related to ion m/z and is likely caused by ions having an optimal range of pseudo-potential well depths for efficient trapping. Based on this observation, precursor ion intensity changes between two different injection LMCO values can be predicted, and these ion intensity changes are found to be carried through to their corresponding product ions, enabling multiplexed MS/MS spectra to be deconvoluted.     

Selective ion isolation/rejection over a broad mass range in the quadrupole ion trap

Techniques are presented for mass-selective ion manipulation over a wide mass range in a three-dimensional quadrupole. The methods use an auxiliary, low-amplitude radio-frequency signal applied to the endcap electrodes. This signal is either held at a single frequency as the fundamental radio-frequency trapping amplitude is ramped or swept over a frequency range while the fundamental radio-frequency trapping amplitude is held at a fixed level. Ion isolation and ejection are demonstrated for ions formed within the ion trap using electron ionization and for ions injected into the ion trap formed either by an air-sustained glow discharge or by electrospray. Mass-selective ion ejection is used to reduce matrix-ion-induced space charge during ion injection, thereby producing signal enhancement for the detection of 2, 4, 6-trinitrotoluene in air. Mass-selective isolation of ions with mass-to-charge ratios above the normal operating range (m / z 650) for the ion trap is also demonstrated after injection of myoglobin ions formed via electrospray.     

How far can ion trap miniaturization go? Parameter scaling and space‐charge limits for very small cylindrical ion traps

A broad effort is underway to make radiofrequency (RF) ion trap mass spectrometers small enough for portable chemical analysis. A variety of trap geometries and fabrication approaches are under development from several research groups. A common issue is the reduced trapping capacity in smaller traps, with the associated reduction in sensitivity. This article explores the key variables that scale with trap size including RF voltage, frequency, electrical capacitance, power and pseudopotential well depth. High‐field electric breakdown constrains the maximum RF voltages used in smaller ion traps. Simulations show the effects of space charge and the limits of trapping capacity as a function of trap dimensions for cylindrical ion traps down to the micrometer level. RF amplitudes that scale as the1/3, 1/2 and 2/3 power of trap radius, r₀, were studied. At a fixed level of performance, the number of analyzable ions scales as r₀ⁿ, with n ranging from 1.55 to 1.75 depending on the choice of voltage scaling. The implications for miniaturized ion trap mass spectrometry are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Comparison of experimental and calculated peak shapes for three cylindrical geometry FAIMS prototypes of differing electrode diameters

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates ions at atmospheric pressure and room temperature based on the difference of the mobility of ions in strong electric fields and weak electric fields. This field-dependent mobility of an ion is reflected in the compensation voltage (CV) at which the ion is transmitted through FAIMS, at a given asymmetric waveform dispersion voltage (DV). Experimental CV, relative peak ion intensity, and peak width data were compared for three FAIMS prototypes with concentric cylindrical electrodes having inner/outer electrode radii of: (1) 0.4/0.6 cm, (2) 0.8/1.0 cm, and (3) 1.2/1.4 cm. The annular analyzer space was 0.2 cm wide in each case. A finite-difference numerical computation method is described for evaluation of peak shapes and widths in a CV spectrum collected using cylindrical geometry FAIMS devices. Simulation of the radial distribution of the ion density in the FAIMS analyzer is based upon calculation of diffusion, electric fields, and the electric fields introduced by coulombic ion-ion repulsion. Excellent agreement between experimental and calculated peak shapes were obtained for electrodes of wide diameter and for ions transmitted at low CV.     

Rapid analysis of polychlorinated biphenyls in the gas phase with resonance-enhanced two-photon ionization: optimal injection of ions into the ion-trap storage/time-of-flight mass spectrometer.

LI-IT-TOFMS (laser ionization/ion-trap storage/time-of-flight mass spectrometry) is expected to be a powerful tool for environmental monitoring. In the research reported here, real-time LI-IT-TOFMS measurements were carried out on gaseous 2-4 chlorinated PCBs in order to evaluate the applicability of an environmental monitoring method. With respect to ion-trap storage for PCBs, we found that the effect was due to the driving RF voltage on the ring electrode in the ion trap. For PCBs ions produced by laser irradiation, we observed that it was more efficient to reach the center of the ion trap by using a gated RF voltage rather than by using a continuous RF voltage. The ion trajectories in the ion trap were simulated by SIMION 7.0. We found that the voltage of the exit end cap electrode affected both the number of ions trapped and the orbit of ions inside the trap cell. Optimization of this parameter was performed using both simulated and experimental results. The achievable PCBs sensitivity for real-time (1 min) measurement using the LI-IT-TOFMS method was found to be in the pptV range (<0.01 mg/m3N) by means of a comparison with the conventional gas sampling/GS-MS method. A satisfactory proportional relationship was confirmed between the laser-based and conventional results.     

Matrix methods for the calculation of stability diagrams in quadrupole mass spectrometry

The theory of the computer calculation of the stability of ion motion in periodic quadrupole fields is considered. A matrix approach for the numerical solution of the Hill equation and examples of calculations of stability diagrams are described. The advantage of this method is that it can be used for any periodic waveform. The stability diagrams with periodic rectangular waveform voltages are calculated with this approach. Calculations of the conventional stability diagram of the 3-D ion trap and the first six regions of stability of a mass filter with this method are presented. The stability of the ion motion for the case of a trapping voltage with two or more frequencies is also discussed. It is shown that quadrupole excitation with the rational angular frequency omega = Nomega/P (where N, P are integers and omega is the angular frequency of the trapping field) leads to splitting of the stability diagram along iso-beta lines. Each stable region of the unperturbed diagram splits into P stable bands. The widths of the unstable resonance lines depend on the amplitude of the auxiliary voltage and the frequency. With a low auxiliary frequency splitting of the stability diagram is greater near the boundaries of the unperturbed diagram. It is also shown that amplitude modulation of the trapping RF voltage by an auxiliary signal is equivalent to quadrupole excitation with three frequencies. The effect of modulation by a rational frequency is similar to the case of quadrupole excitation, although splitting of the stability diagram differs to some extent. The methods and results of these calculations will be useful for studies of higher stability regions, resonant excitation, and non-sinusoidal trapping voltages.     

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.     

Computer simulation of the gap-tripole ion trap with linear injection, 3D ion accumulation,and versatile packet ejection

The behavior of a completely new ion trap is shown with SIMION 7.0 simulations. The simulated trap, which was a mix of a linear and a 3D trap, was made by axially setting two ion guides with a gap between them. Each guide consisted of three rods with three symmetrically delayed radio frequency (rf) voltages (tripole). The "injected" ions were linearly contained by pulsed potentials on the entrance and exit plates. Then the three-dimensional (3D) rf field in the gap, which was created by the tripole special rod arrangement, could trap the ions when the translational energy was dampened by collisions with low-pressure nitrogen. Because the injected ions were trapped in the small gap, the trapping cycle could be repeated many times before ion ejection, so a high concentrated ion cloud could be obtained. This trapping and accumulation methodology is not possible in most conventional multipole linear traps with even number of poles. Compared with quadrupole linear trap at the same rf amplitude, tripole lost more ions due to strong charge repulsion in the ion cloud. However, tripole could catch up the ions at higher voltage. Radial and axial mass-independent ejection of the ions localized in the tripole gap was very simple, compared with conventional linear ion traps that need extra and complicated electrodes for effective axial ejection.     

Investigations into the use of a reverse scan in a quadrupole ion trap mass spectrometer

The forward scan (i.e. an increasing RF voltage ramp for the mass-selective instability scan) is commonly used as an analytical scan for ion detection with quadrupole ion trap instruments. A number of phenomena have been observed while using this scan technique. These include space charge effects resulting in the delayed ejection of ions from the ion trap, and the fragmentation of fragile ions producing very broad peaks. Here the use of a reverse scan (i.e. a decreasing RF voltage ramp) is examined to determine the effect of the above phenomena in this acquisition method. With regard to space charge effects, the apparent reduction of the carbon isotope spacing below one Thomson (for singly charged ions) that is observed with the forward scan is now replaced by an apparent increase in this spacing. The reverse scan, which optimizes at lower axial modulation ejection voltages than the forward scan, allows for the intact ejection of fragile ions under its typical operating conditions whereas the forward scan results in fragmentation. Reducing the axial modulation voltage for the ejection of ions in the forward scan results in less dissociation of the fragile ions during ion ejection, but with the observation of ghost peaks due to incomplete ejection of all of the ions at the resonance ejection condition. While performing the reverse scan experiment, the formation of product ions from dissociation of the MH(+) ion has also been observed.     

Trapping of intact,singly-charged,bovine serum albumin ions injected from the atmosphere with a 10-cm diameter,frequency-adjusted linear quadrupole ion trap

High-resolution real-time particle mass measurements have not been achievable because the enormous amount of kinetic energy imparted to the particles upon expansion into vacuum competes with and overwhelms the forces applied to the charged particles within the mass spectrometer. It is possible to reduce the kinetic energy of a collimated particulate ion beam through collisions with a buffer gas while radially constraining their motion using a quadrupole guide or trap over a limited mass range. Controlling the pressure drop of the final expansion into a quadrupole trap permits a much broader mass range at the cost of sacrificing collimation. To achieve high-resolution mass analysis of massive particulate ions, an efficient trap with a large tolerance for radial divergence of the injected ions was developed that permits trapping a large range of ions for on-demand injection into an awaiting mass analyzer. The design specifications required that frequency of the trapping potential be adjustable to cover a large mass range and the trap radius be increased to increase the tolerance to divergent ion injection. The large-radius linear quadrupole ion trap was demonstrated by trapping singly-charged bovine serum albumin ions for on-demand injection into a mass analyzer. Additionally, this work demonstrates the ability to measure an electrophoretic mobility cross section (or ion mobility) of singly-charged intact proteins in the low-pressure regime. This work represents a large step toward the goal of high-resolution analysis of intact proteins, RNA, DNA, and viruses.     

Electron capture dissociation of polypeptides in a three-dimensional quadrupole ion trap: Implementation and first results

The adverse influence of the radio frequency (RF) voltage on electrons has been the main obstacle for the implementation of electron capture dissociation (ECD) in three-dimensional quadrupole ion traps (3D QITs). Here we demonstrate that the use of axial magnetic field, together with the injection of low-energy (<5 eV) electrons, in the beginning of the positive RF semi-period achieves trapping of electrons for a period of time comparable with the semi-period duration. Importantly, the energy of the electrons remains low during most of the trapping period. With this technique, which we call "magnetized electrons, in-phase injection" (MEPhI), ECD and other ion-electron reactions have become possible in a 3D QIT. Initial ECD results, including single-scan data, were obtained with dications of Substance P. The observed secondary fragmentation of ECD fragments indicates that the trapped electrons are still somewhat hotter than desired.     

Accelerated Simulation Study of Space Charge Effects in Quadrupole Ion Traps Using GPU Techniques

Space charge effects play important roles in the performance of various types of mass analyzers. Simulation of space charge effects is often limited by the computation capability. In this study, we evaluate the method of using graphics processing unit (GPU) to accelerate ion trajectory simulation. Simulation using GPU has been compared with multi-core central processing unit (CPU), and an acceleration of about 390 times have been obtained using a single computer for simulation of up to 10⁵ ions in quadrupole ion traps. Characteristics of trapped ions can be investigated at detailed levels within a reasonable simulation time. Space charge effects on the trapping capacities of linear and 3D ion traps, ion cloud shapes, ion motion frequency shift, mass spectrum peak coalescence effects between two ion clouds of close m/z are studied with the ion trajectory simulation using GPU.     

Upper stability island of the quadrupole mass filter with amplitude modulation of the applied voltages

Modulation of the voltages applied to a quadrupole mass filter (QMF), either RF or RF and DC, leads to splitting of the stability region into islands of stability. The ion optical properties, such as transmission, resolving power and peak tails of the first upper stability islands have been investigated by numerical simulation of ion trajectories. The dependence of the location of this island on the amplitude of the modulation and the parameter nu = omega/Omega = Q/P where omega is modulation frequency, Omega is main angular radio frequency, and Q and P are integers, is calculated in detail. Different methods of adjusting the QMF resolution are examined. It is found that operation at the upper and lower tips of the stability islands created by amplitude modulation of the RF voltage is preferred, because of the technical simplicity of this method and a reduction of the required separation time. Amplitude modulation improves the performance of a QMF constructed with round rods, in comparison to perfect quadrupole fields. For example, with amplitude modulation of the RF, to reach a resolution of R(0.1) = 1200 requires only about 75 RF cycles of ion motion in a quadrupole field created by round rods.     

Simulations of ion trapping in a micrometer-sized cylindrical ion trap

We have performed detailed SIMION simulations of ion behavior in micrometer-sized cylindrical ion traps (r0 = 1 microm). Simulations examined the effects of ion and neutral temperature, the pressure and nature of cooling gas, ion mass, trap voltage and frequency, space-charge, fabrication defects, and other parameters on the ability of micrometer-sized traps to store ions. At this size scale voltage and power limitations constrain trap operation to frequencies about 1 GHz and rf amplitudes of tens of volts. Correspondingly, the pseudopotential well depth of traps is shallow, and thermal energies contribute significantly to ion losses. Trapping efficiency falls off gradually as qz approaches 0.908, possibly complicating mass-selective trapping, ejection, or quantitation. Coulombic repulsion caused by multiple ions in a small-volume results in a trapping limit of a single ion per trap. If multiple ions are produced in a trap, all but one ion are ejected within a few microseconds. The remaining ion tends to have favorable trapping parameters and a lifetime about hundreds of microseconds; however, this lifetime is significantly shorter than it would have been in the absence of space-charge. Typical microfabrication defects affect ion trapping only minimally. We recently reported (IJMS 2004, 236, 91-104) on the construction of a massively parallel array of ion traps with dimensions of r0 = 1 microm. The relationship of the simulations to the expected performance of the microfabricated array is discussed.     

A novel ion trap that enables high duty cycle and wide m/z range on an orthogonal injection TOF mass spectrometer

Although TOF analyzers with orthogonal ion injection provide the whole spectrum without scanning, their duty cycle is low compared with scanning analyzers in single ion monitoring mode. Typical duty cycle is in the range of 5% to 30% depending on the instrument geometry and ion m/z value. We present here a novel trapping/releasing setup, which offers the duty cycle near 100% over a wide range. Operation in the mass range from m/z 120 to almost 2000 is demonstrated. Ions are trapped in a short linear ion trap at the end of the collision cell in an axial pseudopotential well created by additional rf (“AC”) voltage applied to all four rods of the trap with the same amplitude and phase. The pseudopotential created by AC field is mass dependent, and by ramping down the AC voltage, ions can be released from the trap sequentially from high m/z to low, while all ions are gaining the same kinetic energy. Upon entering the TOF accelerator, ions with lower m/z catch up with heavier ions, and the AC ramp parameters can be selected to make all ions meet in the center of the TOF extraction region, resulting in sensitivity gains from 3 to 14 without loss of mass accuracy or resolution.     

飞行时间质谱仪四极杆射频信号放大器的研制

根据自制质谱仪需求研制了一种射频放大装置,由放大电路模块和耦合线圈构成,将其与射频信号发生装置结合用以驱动射频四极杆.该射频放大器可将射频信号放大30倍,为自制射频四极杆提供共振频率1.4 MHz,射频电压峰峰值电压Vp-p可调范围0～600 V的射频信号.自制四极杆用于传输大气压下离子源产生的离子从离子源至工作在高真...     

A novel fourier transform ion cyclotron resonance mass spectrometer with improved ion trapping and detection capabilities

A novel Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been developed for improved biomolecule analysis. A flared metal capillary and an electrodynamic ion funnel were installed in the source region of the instrument for improved ion transmission. The transfer quadrupole is divided into 19 segments, with the capacity for independent control of DC voltage biases for each segment. Restrained ion population transfer (RIPT) is used to transfer ions from the ion accumulation region to the ICR cell. The RIPT ion guide reduces mass discrimination that occurs as a result of time-of-flight effects associated with gated trapping. Increasing the number of applied DC bias voltages from 8 to 18 increases the number of ions that are effectively trapped in the ICR cell. The RIPT ion guide with a novel voltage profile applied during ion transfer provides a 3- to 4-fold increase in the number of ions that are trapped in the ICR cell compared with gated trapping for the same ion accumulation time period. A novel ICR cell was incorporated in the instrument to reduce radial electric field variation for ions with different z-axis oscillation amplitudes. With the ICR cell, called trapping ring electrode cell (TREC), we can tailor the shape of the trapping electric fields to reduce dephasing of coherent cyclotron motion of an excited ion packet. With TREC, nearly an order of magnitude increase in sensitivity is observed. The performance of the instrument with the combination of RIPT, TREC, flared inlet, and ion funnel is presented.     

Sensitivity and amplitude calibration of a Fourier transform 3D quadrupole ion trap mass spectrometer

With a three-dimensional (3D) quadrupole ion trap running in a Fourier transform operating mode, the detected signal is an image of the collective motion of the confined ions. Consequently, it is assumed that the image signal is the sum of the axial trajectories of the simultaneously confined ions. The resulting frequency spectrum after Fourier transformation comprises frequency peaks at the axial secular frequencies of the confined species according to their mass/charge ratio. With a singly confined species, the maximal amplitude of the image signal is proportional to the amplitude of the secular axial frequency peak. The matrix method is employed to express the axial trajectory sampled at the confinement field period. In that case, the expression of the image signal, as well as its maximal amplitude, is calculated as a function of the trap operating conditions and initial axial positions and axial velocities of the ions. The initial position and velocity distributions are connected to the injection mode. With the steady ion flow injection mode (SIFIM) and an initial phase of the confinement field equal to kπ, the maximal amplitude of the image signal is proportional to either the sum of the initial axial positions or the number of confined ions and the mean value of the initial axial positions. By simulation, amplitude fluctuation of the frequency peak is then calculated for a number of ions ranging between some tens to some thousands of ions injected by SIFIM. The peak amplitude fluctuations induced by the fluctuations of the number of ions are seven times greater than those induced by the fluctuations of the distribution of the initial axial positions.