Coaxial ion trap mass spectrometer: concentric toroidal and quadrupolar trapping regions

We present the design and results for a new radio-frequency ion trap mass analyzer, the coaxial ion trap, in which both toroidal and quadrupolar trapping regions are created simultaneously. The device is composed of two parallel ceramic plates, the facing surfaces of which are lithographically patterned with concentric metal rings and covered with a thin film of germanium. Experiments demonstrate that ions can be trapped in either region, transferred from the toroidal to the quadrupolar region, and mass-selectively ejected from the quadrupolar region to a detector. Ions trapped in the toroidal region can be transferred to the quadrupole region using an applied ac signal in the radial direction, although it appears that the mechanism of this transfer does not involve resonance with the ion secular frequency, and the process is not mass selective. Ions in the quadrupole trapping region are mass analyzed using dipole resonant ejection. Multiple transfer steps and mass analysis scans are possible on a single population of ions, as from a single ionization/trapping event. The device demonstrates better mass resolving power than the radially ejecting halo ion trap and better sensitivity than the planar quadrupole ion trap.     

Elimination of z-ejection in Fourier transform ion cyclotron resonance mass spectrometry by radio frequency electric field shimming

In Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry, coherent ion cyclotron orbital motion is produced by resonant radio frequency (rf) electric field excitation. However, because the excitation electrodes are of finite dimensions, the desired transverse (to the applied magnetic field) rf electric field is accompanied by an rf electric field component along the z- (magnetic field) direction, resulting in mass-dependent z-ejection and mass-dependent FT/ICR mass spectral peak relative magnitudes. Addition of several "guard wires" of voltage-divided rf amplitude allows the rf electric field to be "shimmed" to near-perfect uniformity. In this paper (see also the accompanying paper by Russell et al.), we introduce two types of rf-shimmed ion traps. In the first type, guard wires are placed only in front of the trapping electrodes. In the second type, guard wire rings are placed inside the detector and trapping electrodes. For either arrangement, simion simulations were used to adjust the rf voltages applied (by use of voltage dividers) to the guard wires or rings so as to produce an optimally uniform rf field within the trap. The virtual elimination of z-excitation is confirmed by plots of magnitude-mode relative peak height vs ICR orbital radius. Because the guard wires (or rings) tend to shield the ions from the trapping electrode potential, the shift in ICR frequency with trapping voltage is also reduced, but not as well as by a screened trap.(ABSTRACT TRUNCATED AT 250 WORDS)     

Trapping ring electrode cell: a FTICR mass spectrometer cell for improved signal-to-noise and resolving power

A novel FTICR cell called the trapping ring electrode cell (TREC) has been conceived, simulated, developed, and tested. The performance of the TREC is compared to a closed cylindrical cell at different excited cyclotron radii. The TREC permits the ability to maintain coherent ion motion at larger initial excited cyclotron radii by decreasing the change in radial electric field with respect to z-axis position in the cell. This is accomplished through postexcitation modulation of the trapping potentials applied to segmented trap plates. Resolving power approaching the theoretical limit was achieved using the novel TREC technology; over 420,000 resolving power was observed on melittin [M + 4H] (4+) species when employed under modest magnetic field strength (3T) and a data acquisition duration of 13 s. A 10-fold gain in signal-to-noise ratio is demonstrated over the closed cylindrical cell optimized with common potentials on all ring electrodes. The observed frequency drift during signal acquisition over long time periods was also significantly reduced, resulting in improved resolving power.     

A "screened" electrostatic ion trap for enhanced mass resolution, mass accuracy, reproducibility, and upper mass limit in Fourier transform ion cyclotron resonance mass spectrometry

Until now, it was thought that the optimal static electromagnetic ion trap for Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry should be designed to produce a quadrupolar electrical potential, for which the ion cyclotron frequency is independent of the ion's preexcitation location within the trap. However, a quadrupolar potential results in a transverse (to the magnetic field) electric field that increases linearly with distance from the center of the trap. That radially linear electric field shifts the observed ICR frequency, increases the ICR orbital radius, and ultimately limits the highest mass-to-charge ratio ion that can be contained within the trap. In this paper, we propose a new static electromagnetic ion "trap" in which grounded screens placed just inside the usual "trapping" plates produce a good approximation to a "particle-in-a-box" potential (rather than the quadrupolar "harmonic oscillator" potential). SIMION calculations confirm that the electric potential of the screened trap is near zero almost everywhere within the trap. For our screened orthorhombic (2.5 in. X 2 in. X 2 in.) trap, the experimental ICR frequency shift due to trapping voltage is reduced by a factor of approximately 100, and the experimental variation of ICR frequency with ICR radius is reduced by a factor of approximately 10 compared to a conventional (unscreened) 2-in. cubic ion trap.(ABSTRACT TRUNCATED AT 250 WORDS)     

Design and Construction of a Helical Resonator for Delivering Radio Frequency to an Ion Trap

Supplying high voltage radio frequency (RF) is a critical part of ion trapping system due to impedance mismatching between RF source and the ion trap. A helical resonator has been constructed in order to deliver narrow bandwidth and high voltage RF to the ion trap for stable confinement of ions. The performances of the helical resonator have been studied for different capacitive load of the ion trap. Both the resonant frequency and quality factor of the resonator show strong dependence on external capacitive loads.     

Rectilinear ion trap: concepts, calculations, and analytical performance of a new mass analyzer

A mass analyzer based on a rectilinear geometry ion trap (RIT) has been built, and its performance has been characterized. Design concepts for this type of ion trap are delineated with emphasis on the effects of electrode geometry on the calculated electric field. The Mathieu stability region was mapped experimentally. The instrument can be operated using mass-selective instability scans in both the boundary and resonance ejection versions. Comparisons of performance between different versions of the device having different dimensions allowed selection of an optimized geometry with an appropriate distribution of higher-order electric fields. Comparisons made under the same conditions between the performance of a conventional cylindrical ion trap and a RIT of 4 times greater volume show an improvement of 40 times in the signal-to-noise ratio resulting from the higher ion trapping capacity of the RIT. The demonstrated capabilities of the RIT include tandem mass spectrometry, a mass resolution in excess of 1000, and a mass/charge range of 650 Th, all in a simple structure that is only 3.5 cm(3) in internal volume.     

Ion funnel trap interface for orthogonal time-of-flight mass spectrometry

A combined electrodynamic ion funnel and ion trap coupled to an orthogonal acceleration (oa)-time-of-flight mass spectrometer was developed and characterized. The ion trap was incorporated through the use of added terminal electrodynamic ion funnel electrodes enabling control over the axial dc gradient in the trap section. The ion trap operates efficiently at a pressure of approximately 1 Torr, and measurements indicate a maximum charge capacity of approximately 3 x 10(7) charges. An order of magnitude increase in sensitivity was observed in the analysis of low concentration peptides mixtures with orthogonal acceleration (oa)-time-of-flight mass spectrometry (oa-TOF MS) in the trapping mode as compared to the continuous regime. A signal increase in the trapping mode was accompanied by reduction in the chemical background, due to more efficient desolvation of, for example, solvent related clusters. Controlling the ion trap ejection time was found to result in efficient removal of singly charged species and improving signal-to-noise ratio (S/N) for the multiply charged analytes.     

Frequency-scanning MALDI linear ion trap mass spectrometer for large biomolecular ion detection

This study presents the first report on the development of a matrix-assisted laser desorption ionization (MALDI) linear ion trap mass spectrometer for large biomolecular ion detection by frequency scan. We designed, installed, and tested this radio frequency (RF) scan linear ion trap mass spectrometer and its associated electronics to dramatically extend the mass region to be detected. The RF circuit can be adjusted from 300 to 10 kHz with a set of operation amplifiers. To trap the ions produced by MALDI, a high pressure of helium buffer gas was employed to quench extra kinetic energy of the heavy ions produced by MALDI. The successful detection of the singly charged secretory immunoglobulin A ions indicates that the detectable mass-to-charge ratio (m/z) of this system can reach ~385 000 or beyond.     

A parallel miniature cylindrical ion trap array

A small mass spectrometer array is described in which each element is a cylindrical ion trap (CIT). The array contains four CITs, each having an inner radius of 2.5 mm, arranged in parallel and operated using a single electronics system under common conditions for trapping and mass analysis. By using an array of identically sized traps, higher ion capacity can be achieved than with a single miniature CIT, but the advantage of lower power and voltage requirements associated with the smaller ion trap is maintained. Overall signal intensity of the array of four traps is compared with that of a two-element CIT array to demonstrate the increased ion capacity of larger arrays. Resolution for m-dichlorobenzene is shown to be approximately 180 (full width at half-maximum), with no significant loss in resolution as a result of using multiple CITs. The detection of 5 x 10(-9) Torr partial pressure of krypton in argon with a signal-to-noise ratio of approximately 30 for the most abundant isotope is shown, demonstrating the applicability of the device for process gas monitoring. Also, a preliminary spectrum from a 10-CIT array with each element having an inner radius of 1.5 mm is reported.     

Planar geometry for trapping and separating ions and charged particles

A planar quadrupole ion trap is proposed. We have demonstrated an extremely large operating range by trapping ions and particles with mass-to-charge ratio ranging from 10(2) to 10(9) at frequencies from 2.8 x 10(6) to 60 Hz at an operating pressure of 1.1 x 10(-4) to 760 Torr, respectively, using a trap radius of r1 = 1 mm. We have also performed mass spectrometry with a resolution of 1.2 amu with mass-to-charge range from 50 to 150. Our geometry is simple enough to be integrated into existing integrated circuits and microelectromechanical system devices, opening up the possibility of many novel hybrid applications and experiments.     

Photoelectron emission as an alternative electron impact ionization source for ion trap mass spectrometry

Electron impact ionization has several known advantages; however, heated filament electron sources have pressure limitations and their power consumption can be significant for certain applications, such as in field-portable instruments. Herein, we evaluate a VUV krypton lamp as an alternative source for ionization inside the ion trap of a mass spectrometer. The observed fragmentation patterns are more characteristic of electron impact ionization than photoionization. In addition, mass spectra of analytes with ionization potentials higher than the lamp's photon energy (10.6 eV) can be easily obtained. A photoelectron impact ionization mechanism is suggested by the observed data allowed by the work function of the ion trap electrodes (4.5 eV), which is well within the lamp's photon energy. In this case, the photoelectrons emitted at the surface of the ion trap end-cap electrode are accelerated by the applied rf field to the ring electrode. This allows the photoelectrons to gain sufficient energy to ionize compounds with high ionization potentials to yield mass spectra characteristic of electron impact. In this manner, electron impact ionization can be used in ion trap mass spectrometers at low powers and without the limitations imposed by elevated pressures on heated filaments.     

Design of the Ion Trap and Vacuum System for <Superscript>171</Superscript>Yb-ion Optical Frequency Standard

We are developing a frequency standard based on the ultra-narrow electric octupole transition of the ytterbium-ion (¹⁷¹Yb⁺), which is in the optical wavelength region. In this article, we describe optimized design of our end-cap type Paul trap which will be used for trapping single ions for precision frequency metrology. Selection of the materials for fabricating different parts of the trap assembly is also described. Customized design of the ultra-high vacuum chamber, which houses the ion trap, oven producing ytterbium atomic beam, compensation electrodes and high numerical aperture fluorescence collection lens together with four pairs of optical viewports is lastly described.     

离子在矩形波四极场中的运动与四极场离子阱质谱的数字化操作

矩形波驱动的四极场同样可用于对离子进行质量分析和存储。本文推导了离子在矩形波四极场中的运动规律,并以常用于正弦波四极场的马绍方程参量a,q来表出离子的稳定性。运用赝势阱概念探讨了离子在这种四极场中的本征振动,推导了本征振动频率的近似公式。在理论研究的基础上,提出了数字化离子阱质谱的概念,并运用离子光学模拟,研究了实际离子阱几何结构下的共振出射以及数字化质量扫描的可行性。     

Hydrodynamic tweezers: impact of design geometry on flow and microparticle trapping

Here we explore the role of microfabricated device geometry on frequency-dependent low Reynolds number steady streaming flow and particle trapping behavior. In our system, flow and particle trapping is induced near an obstruction or cavity located in an otherwise rectilinear oscillating flow of frequency ω and amplitude s in a fluid of kinematic viscosity ν. This work expands prior studies to characterize nine distinct obstruction/cavity geometries. The imaged microeddy flows show that the device geometry affects the eddy number, shape, structure, and strength. Comparison of measured particle trap locations with the computed eddy flow structure shows that particles trap closer to the wall than the eddy core. Trapping strength and location are controlled by the geometry and the oscillation frequency. In most cases, the trapping behavior is linearly proportional to the Stokes layer thickness, δ(AC) ~ O((ν/ω)(1/2)). We show that steady streaming in microfluidic eddies can be a flexible and versatile method for noncontact microparticle trapping, and hence we call this class of devices "hydrodynamic tweezers".     

Cylindrical ion trap array with mass selection by variation in trap dimensions

A mass spectrometer array is described in which each array element is a cylindrical ion trap (CIT) within which an approximately quadrupolar, time-varying, field is established. The individual traps are of different sizes, so that when the array is operated with a fixed rf potential, ions of different masses (or mass ranges) are stored in each trap. By choosing the dimensions of each CIT element in the array, a multiple ion monitoring experiment can be performed. For example, in a two-element array with elements having internal radii of 5 and 4 mm, the smaller trap selects for m/z 91 and the larger for m/z 57, corresponding to characteristic aromatic and aliphatic hydrocarbon ions. Ion storage using both rf/dc (apex) isolation and the stored waveform inverse Fourier transform method is demonstrated.The array reduces the complexity of the electronics needed to operate the ion trap, which should make it suitable for use in a miniature mass spectrometer system.     

Narrow-band collisional activation technique for ion trap mass spectrometers.

Narrow-bandwidth signals were applied to the end caps of an ion trap mass spectrometer to excite ions during collisional activation. Excitation waveforms were created from a single-frequency component and a random noise component using a multiplier circuit. Tandem and higher order mass spectrometry experiments (MS3) can be performed without optimization of the supplemental frequency applied to the end cap electrodes. The usefulness of this method of ion excitation is demonstrated using singly and multiply protonated peptide ions as well as sodium-cationized carbohydrates.     

A molecular dynamics simulation study on trapping ions in a nanoscale Paul trap

We found by molecular dynamics simulations that a low energy ion can be trapped effectively in a nanoscale Paul trap in both vacuum and aqueous environments when appropriate AC/DC electric fields are applied to the system. Using the negatively charged chlorine ion as an example, we show that the trapped ion oscillates around the center of the nanotrap with an amplitude dependent on the parameters of the system and applied voltages. Successful trapping of the ion within nanoseconds requires an electric bias of GHz frequency, in the range of hundreds of mV. The oscillations are damped in the aqueous environment, but polarization of water molecules requires the application of a higher voltage bias to reach improved stability of the trapping. Application of a supplemental DC driving field along the trap axis can effectively drive the ion off the trap center and out of the trap, opening up the possibility of studying DNA and other charged molecules using embedded probes while achieving a full control of their translocation and localization in the trap.     

Ion trap mass spectrometry of fluorescently labeled nanoparticles

Mass spectra of fluorescently labeled polystyrene nanoparticles have been obtained using a combined technique of matrix-assisted laser desorption/ionization (MALDI), laser-induced fluorescence (LIF), and a dual quadrupole ion trap mass spectrometer. The spectrometer is designed in such a way that the first trap serves as a trapping and mass-analyzing device, while the second trap serves to capture and concentrate the ions ejected from the first trap for fluorescence detection. An enhancement in the LIF signal by more than 3 orders of magnitude is achieved with the help of the second trap, making mass/charge (m/z) analysis of the nanoparticles possible. Additional unique features of this mass spectrometer include that frequency scan (0.5-50 kHz) at a constant voltage (200 V), instead of voltage scan at a constant frequency, is implemented to widen the spectral analysis range of the instrument. The implementation has allowed the spectrometer to operate at relatively high buffer gas pressures (50 mTorr), crucial for effective trapping of the nanometer-sized particles generated by MALDI. We present in this report the first mass spectra of fluorescently labeled nanoparticles with a size of 27 nm using this new mass spectrometric approach. The utility of this method in the study of biological macromolecules or particles is demonstrated with dye-labeled IgG.     

Handheld rectilinear ion trap mass spectrometer

A shoebox-sized, 10-kg, handheld mass spectrometer, Mini 10, based on a rectilinear ion trap mass analyzer has been designed, built, and characterized. This instrument has evolved from a decade-long experimental and simulation program in mass spectrometer miniaturization. The rectilinear ion trap has a simplified geometry and high trapping capacity, and when used with a miniature and ruggedized pumping system, it allows chemical analysis while the instrument is being carried. Compact electronics, including an air core RF drive coil, were developed to control the instrument and to record mass spectra. The instrument runs on battery power, consuming less than 70 W, similar to a laptop computer. Wired and wireless networking capabilities are implemented. The instrument gives unit resolution and a mass range of over m/z 500. Tandem mass spectrometry capabilities are implemented using collision-induced dissociation, and they are used to provide confirmation of chemical structure during in situ analysis. Continuous monitoring of air and solution samples is demonstrated, and a limit of detection of 50 ppb was obtained for toluene vapor in air and for an aqueous naphthalene solution using membrane sample introduction.     

Parallel ion parking of protein mixtures

The multiple charging phenomenon resulting from electrospray ionization of proteins, while useful for the ability to make several mass measurements on a single component, can lead to highly complex spectra when mixtures are analyzed, as each component can generate multiple ions of distinct mass-to-charge ratio. Ion/ion proton-transfer reactions can overcome this problem by reduction of all components to the +1 charge state, but this typically requires the ability to extend the mass range of the instrument well beyond that available in most commercial instruments. Furthermore, reduction of protein charge to +1 also results in a reduction in detector response. Here it is shown that application of a relatively high amplitude, low-frequency auxiliary ac signal to the end cap electrodes of a 3-D ion trap during an ion/ion reaction can slow the ion/ion reaction rates of ions over a broad m/z range, in a process termed HALF parallel ion parking. Adjustment of the frequency and amplitude of the applied voltage allows the mass range into which the initial ion signal is moved to be controlled, allowing for the simplification of multicomponent mixtures within a mass range that is more commonly available on commercial systems. In addition to decreasing spectral complexity, this is advantageous for mixtures with low-abundance components, as there is less compromise with detector response than in reduction to the +1 charge state. Preliminary evidence also suggests that the ion collision cross section may play an important role in determining which charge states are most significantly inhibited from further ion/ion reactions under a given set of ion parking conditions.