Constant-Momentum Acceleration Time-of-Flight Mass Spectrometry with Energy Focusing

Fundamental aspects of constant-momentum acceleration time-of-flight mass spectrometry (CMA-TOFMS) are explored as a means to improve mass resolution. By accelerating all ions to the same momentum rather than to the same energy, the effects of the initial ion spatial and energy distributions upon the total ion flight time are decoupled. This decoupling permits the initial spatial distribution of ions in the acceleration region to be optimized independently, and energy focus, including ion turn-around-time error, to be accomplished with a linear-field reflectron. Constant-momentum acceleration also linearly disperses ions across time according to mass-to-charge (m/z) ratio, instead of the quadratic relationship between flight time and m/z found in conventional TOFMS. Here, CMA-TOFMS is shown to achieve simultaneous spatial and energy focusing over a selected portion of the mass spectrum. An orthogonal-acceleration time-of-flight system outfitted with a reduced-pressure DC glow discharge (GD) ionization source is used to demonstrate CMA-TOFMS with atomic ions. The influence of experimental parameters such as the amplitude and width of the time-dependent CMA pulse on mass resolution is investigated, and a useful CMA-TOFMS focusing window of 2 to 18 Da is found for GD-CMA-TOFMS.

Practical considerations when using radio frequency-only quadrupole ion guide for atmospheric pressure ionization sources with time-of-flight mass spectrometry

Construction details and performance evaluation of a radio frequency (rf)-only quadrupole ion guide for use with an electrospray ionization time-of-flight mass spectrometer is presented in this paper. Angiotensin III and cytochrome c were used in these experiments to investigate the ion transmission properties of the rf-only quadrupole for different m/z species. In addition, influence of ion kinetic energies along with the characteristic fragmentation due to collision induced dissociation (CID) were studied. These experiments demonstrate that the transmissions of different m/z ions were not only dependent on the frequency and magnitude of the rf waveform, which is similar to a high vacuum rf-only quadrupole ion guide, but also on the pressure inside the quadrupole chamber. For the pressure range tested, low m/z ions are better focused with increasing pressure. As expected, transmission of ions are subject to space charge limitations when significant numbers of ions are focused on the axis of the quadrupole. It is also observed that CID results are related to transverse motion and longitude motion of ions inside the quadrupole region. Consequently, CID is useful for fragmentation of linear peptides and it is not effective (in present configuration) for large bulky proteins. The kinetic energy of ions that enter the repelling region of the TOFMS is ultimately determined by the ensemble effect resulting from the dc bias potential of the quadrupole (the dominant factor), skimmer-2, pressure inside the quadrupole chamber, and jet expansion. While this system is tested with an ESI source, the operational principle and design criteria are directly applicable for improving other atmospheric pressure ionization sources with time-of-flight mass analyzers such as an inductively coupled plasma ion source.     

Mapping the ion kinetic energy in a helium microwave plasma orthogonal acceleration time-of-flight mass spectrometer.

The ion kinetic energy of a helium microwave plasma is studied using an orthogonal acceleration time-of-flight mass spectrometer. The ions produced in the plasma are extracted into the mass spectrometer in an 'off-cone' mode (i.e. the helium plasma plume is off the sampler cone), and enter the repelling zone in the x-direction, which is perpendicular to the flight tube. The information of ion initial kinetic energy was obtained from both theoretical calculations and experimental results. The potential influence of two x-direction steering plates (X-steering plates) on the ion energy and signal intensity was examined. The influence of gas composition on the ion kinetic energy was also investigated. The calculated results show that ions with different m/z have different velocity and kinetic energy when they enter the ion modulation zone, and lighter ions have higher velocity and lower kinetic energy. The experimental results obtained demonstrate that the ion signals of different m/z produced with an 'off-cone' sampling helium microwave plasma show similar trends as calculated with the potential difference of the X-steering plates, revealing their narrow kinetic energy distribution in the x-direction. Under typical operating conditions, the x-direction kinetic energy of ions detected mostly range from about 14.9 eV for (7)Li(+) to 16.8 eV for (208)Pb(+).     

Restrained ion population transfer: a novel ion transfer method for mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometers function such that the ion accumulation event takes place in a region of higher pressure outside the magnetic field which allows ions to be thermally cooled before being accelerated toward the ICR cell where they are decelerated and re-trapped. This transfer process suffers from mass discrimination due to time-of-flight effects. Also, trapping ions with substantial axial kinetic energy can decrease the performance of the FTICR instrument compared with the analysis of thermally cooled ions located at the trap center. Therefore, it is desirable to limit the energy imparted to the ions which results in lower applied trap plate potentials and reduces the spread in axial kinetic energy. The approach presented here for ion transfer, called restrained ion population transfer or RIPT, is designed to provide complete axial and radial containment of an ion population throughout the entire transfer process from the accumulation region to the ICR cell, eliminating mass discrimination associated with time-of-flight separation. This was accomplished by use of a number of quadrupole segments arranged in series with independent control of the direct current (DC) bias voltage applied to each segment of the quadrupole ion guide. The DC bias voltage is applied in such a way as to minimize the energy imparted to the ions allowing transfer of ions with low kinetic energy from the ion accumulation region to the ICR cell. Initial FTICR mass spectral data are presented that illustrate the feasibility of RIPT. A larger m/z range for a mixture of peptides is demonstrated compared with gated trapping. The increase in ion transfer time (3 ms to 130 ms) resulted in an approximately 11% decrease in the duty cycle; however this can be improved by simultaneously transferring multiple ion populations with RIPT. The technique was also modeled with SIMION 7.0 and simulation results that support our feasibility studies of the ion transfer process are presented.     

Microsecond pulsed glow discharge time-of-flight mass spectrometer

A linear time-of-flight mass spectrometer (TOFMS) has been designed, constructed, and coupled with a glow discharge source in microsecond pulsed mode (MSPGD). Orthogonal acceleration, a DC quadrupole and deflecting pulse techniques are used to diminish kinetic distribution and the spatial distribution of ions, and for deflecting Ar⁺ ions in their flight path. Comparison was made in the same discharge source between MSPGD and DC discharge. The continuous ion current is only 0.2 nA in the DC discharge mode, while the peak ion current reaches over 100 nA in the MSPGD mode. In addition, the ratio of the repelled ions to total ions is much higher in MSPGD than with a DC discharge in TOFMS. The mechanism of MSPGD is discussed. A resolving power of 500 was achieved, which is excellent for elemental analysis. To the authors' knowledge, this is the first time that a MSPGD-TOFMS combination has been described. The system is now being further optimized to improve its performance.     

Tandem Time-of-Flight Mass Spectrometer with High Precursor Ion Selectivity Employing Spiral Ion Trajectory and Improved Offset Parabolic Reflectron

In this study, we have developed a tandem time-of-flight mass spectrometry (TOF/TOF) technique involving the use of a matrix-assisted laser desorption/ionization ion source that exhibits high precursor ion selectivity. An ion optical system with a 17 m spiral ion trajectory was used in the first time-of-flight mass spectrometer. High precursor ion selectivity was achieved by realizing a 15 m flight path, which is considerably longer than that of the conventional MALDI-TOF/TOF before the precursor ion selection by an ion gate; monoisotopic ions could be selected properly up to m/z 2500. Furthermore, the first time-of-flight mass spectrometer was composed of electrostatic sectors and could eliminate post-source decay (PSD) ions. Precursor ions with 20 keV kinetic energy were selected and injected into a collision cell, leading to the generation of fragment ions by high-energy collision-induced dissociation (HE-CID). The optimized second time-of-flight mass spectrometer included a post-acceleration region and an offset parabolic reflectron to record product ion spectra in the entire mass range. Our system could generate a simple HE-CID product ion spectrum because each fragment pathway could be observed as a single peak by the selection of monoisotopic ions of all precursor ions and HE-CID fragment pathways could be predominantly observed by the PSD ion elimination.     

Laser photo-induced dissociation using tandem time-of-flight mass spectrometry

A novel tandem time-of-flight (TOF) mass spectrometer has been developed for studying the photo-induced dissociation of large molecules and elemental clusters. It consists of a linear first stage TOF analyser for primary mass separation and precursor ion selection, and a second orthogonal reflecting field TOF analyser for product ion analysis. The instrument is equipped with a large volume throughput molecular beam source chamber allowing the production of jet-cooled molecules and molecular clusters, as well as elemental clusters, using either a pulsed laser vaporisation source (LVS) or a pulsed are cluster ion source (PACIS). A second differentially pumped chamber can be used with effusive sources, or for infrared laser desorption of large molecules, followed by laser ionisation. These primary ions can then be irradiated with a second, high energy laser to induce photodissociation. Detailed information about the fragmentation mechanisms can be deduced from the product ion mass spectra. Preliminary results on the photo-induced dissociation (PID) of the molecule ion of aniline at 266 nm are presented. In this case the molecule ions were generated via two-photon laser ionisation at 266 nm using an effusive source. Results for the collision-induced dissociation (CID) of the aniline molecule ion, using a commercial mass spectrometer equipped with an atmospheric pressure electrospray ionisation interface, are also presented. Copyright 2000 John Wiley & Sons, Ltd.     

Measurement of initial translational energies of peptide ions in laser desorption/ionization mass spectrometry

The initial kinetic energy distribution of [Arg]-vasopressin molecular ions generated by matrix-assisted UV laser desorption/ionization was measured using a delayed ion extraction, linear time-of-flight mass spectrometer. Energy distributions of the nicotinic acid matrix ions, with or without the presence of peptide, were also measured. These were compared with the kinetic energy distribution of gramicidin-S ions using IR laser desorption. The measured molecular ion kinetic energy distribution from vasopressin is much broader than that from gramicidin-S, and is characterized by a high-energy tail that most likely results from entrainment of anlayte ions in the higher velocity matrix ions and fragments as they leave the surface.     

Selective generation of charge-cependent/independent ion energy distributions from a heated capillary electrospray source

Retarding grid and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry variable trap potential measurements are performed to determine factors that contribute to the kinetic energy distribution of ions formed in an electrospray source that uses a heated capillary for desolvation. The control of ion kinetic energies is achieved by manipulating the skimmer position in the postcapillary expansion and by varying the potential appEed to the skimmer. The selective generation of either charge-dependent or charge-independent ion energy distributions is demonstrated. Charge-dependent energy distributions of electro-sprayed ions are created by sampling ions near the Mach disk of the supersonic expansion and by using a larger diameter skimmer orifice; the FTICR spectra acquired under these conditions exhibit mass-to-charge ratio-dependent mass discrimination determined by the potential used to trap the ions. Charge-independent energies of electrosprayed ions are created by positioning the capillary adjacent to the skimmer to sample thermal ions and by using a smaller skimmer orifice to reduce expansion cooling; under these conditions ion kinetic energy is determined primarily by the skimmer potential and no mass-to-charge ratio-dependence is observed in the selection of optimum FTICR trapping conditions. The ability to select between proteins of different conformation on the basis of kinetic energy differences is demonstrated. For example, a 0.4 V difference in trap potential is observed in the selective trapping of open and closed forms of the +10 charge state of lysozyme. Finally, it is demonstrated that by operating the source under conditions which deliver a beam of ions with charge-independent energies to the cell, it is possible to obtain precursor and product ion signal magnitudes in FTTCR spectra without charge-dependent mass discrimina-tion.     

Protein desolvation in UV matrix-assisted laser desorption/ionization (MALDI)

We describe experiments in MALDI-TOF and MALDI-TOF-TOF showing that the ejection of protein-matrix cluster ions and their partial decay in the source occur in MALDI. The use of radial beam deflection and small size detector in linear mode allows detection of ions with higher time-of-flight and kinetic energy deficit. MALDI-TOF-TOF experiments were carried out by selecting chemical noise ions at m/z higher than that of a free peptide ion. Whatever the selected m/z (up to m/z 300) the molecular peptide ion appeared as the main fragment. The production of protein-matrix clusters and their partial decay in the source was found to increase with the size of the protein (MW from 1000 to 150,000 u), although it decreases with increasing charge state. These effects were observed for different matrices (HCCA and SA) and in a large laser fluence range. Experimental results and calculation highlight that a continuous decay of protein-matrix cluster ions occurs in the source. This decay-desolvation process can account for the high-mass tailing and peak shifting as well as the strong noise/background in the mass spectra of proteins.     

异黄酮同分异构体的ESI-IT-TOF质谱特征及区别

采用电喷雾-离子阱-飞行时间串联质谱(ESI-IT-TOF)技术, 在正/负离子检测模式下对芒柄花素及其同分异构体7-甲氧基异黄酮的质谱裂解规律进行了系统研究. 实验结果表明, 该化合物在正、负离子模式下均得到了5级高分辨质谱. 结果显示, 二者在负离子模式下的碎片相同, 而在正离子模式下的碎片裂解不同. 根据正负离子模式的5级高分辨质谱推导了两者的可能裂解规律, 丰富了异黄酮的ESI-MSn数据, 为其它异黄酮类化合物的分析鉴定提供了有效的质谱方法.     

Double ionisation of ICN and BrCN studied by a new photoelectron–photoion coincidence technique

A double time-of-flight technique is described, whereby any number of electrons and ions produced by photoionisation can be detected in coincidence, with energy analysis for the electrons and mass analysis for the ions. Branching ratios, kinetic energy releases and mechanisms in doubly charged ion decays can be measured with selection of the initial double ionisation energy. The new technique, together with spectroscopy by the recently established time-of-flight photoelectron–photoelectron coincidence (TOF-PEPECO) method, is used to study the spectra and dissociation dynamics of ICN⁺⁺ and BrCN⁺⁺.     

Collision Energetics in a Tandem Time-of-Flight (TOF/TOF) Mass Spectrometer with a Curved-Field Reflectron

Collisions of fullerene ions (C(60) (+)) with helium and neon were carried out over a range of laboratory energies (3-20 keV) on a unique tandem time-of-flight (TOF/TOF) mass spectrometer equipped with a curved-field reflectron (CFR). The CFR enables focusing of product ions over a wide kinetic energy range. Thus, ions extracted from a laser desorption/ionization (LDI) source are not decelerated prior to collision, and collision energies in the laboratory frame are determined by the source extraction voltages. Comparison of product ion mass spectra obtained following collisions with inert gases show a time (and apparent mass) shift for product ions relative to those observed in spectra obtained by metastable dissociation (unimolecular decay), consistent with impulse collision models, in which interactions of helium with fullerene in the high energy range are primarily with a single carbon atom. In addition, within a narrow range of kinetic energies an additional peak corresponding to the capture of helium is observed for fragment ions C(50) (+), C(52) (+), C(54) (+), C(56) (+) and C(58) (+).     

Etude Structurale de Cations-Radicaux

Mass analysed ion kinetic energy spectrometry has been used to study the structure of radical cations from 26 precursor molecules. Metastable ion characteristics, viz. kinetic energy releases, abundance ratios, and isotope distribution in labelled compounds, show that all the metastable ions are isomerized into a common structure or similar mixture of structures before fragmentation. Collision induced dissociations and collisional ionization to doubly charged ions have been used to study stable ions whose lifetimes are greater than the time-of-flight from the source to the collector, and the results are interpreted on the basis of the occurrence of mixtures of initial and isomerized species.     

Positive and negative ion desorption from PVC as studied by electron stimulated desorption

With the aim of performing time-of-flight studies of electron stimulated ion desorption (ESID) from polymers bombarded with a variable energy electron beam source, an experimental set-up including a homemade time-of-flight mass spectrometer has been developed for positive and negative ion analysis. This system uses as a trigger for the experiments either the pulsed electron beam or the pulsed (positive/negative) extraction high voltage applied to the sample. Results for both positive and negative ion desorption from poly(vinyl chloride) (PVC) obtained in ESID measurements with these two different modes of operation will be presented and discussed.     

A collinear tandem time-of-flight mass spectrometer for infrared photodissociation spectroscopy of mass-selected ions

An apparatus based on collinear tandem time-of-flight mass spectrometer has been designed for the measurement of infrared photodissociation spectroscopy of mass-selected ions in the gas phase.The ions from a pulsed laser vaporization supersonic ion source are skimmed and mass separated by a Wiley-McLaren time-of-flight mass spectrometer.The ion of interest is mass selected,decelerated and dissociated by a tunable IR laser.The fragment and parent ions are reaccelerated and mass analyzed by the second time-of-flight mass spectrometer.A simple new assembly integrated with mass gate,deceleration and reacceleration ion optics was designed,which allows us to measure the infrared spectra of mass selected ions with high sensitivity and easy timing synchronization.     

Nonlinear ion acceleration for improved space focusing in time-of-flight mass spectrometry

The initial spatial distribution of gas-phase ions is one of the primary factors limiting the achievable mass resolving power in time-of-flight mass spectrometry. While the effect of the spatial distribution is minimized along the flight path at a location known as the space-focus plane, the use of a single, linear acceleration field to generate this focus represents only a first-order approximation of ideal space focusing. Alternatively, ideal space focusing is possible through the use of a nonlinear ion acceleration field. A computational model delineates the requirements of the nonlinear potential profile, and suggests that substantial improvements to resolving power can be achieved when using an ion source configuration where a first field approximates the optimal nonlinear field gradient and a second, linear accelerating field imparts additional kinetic energy. Experimental results using a novel, static-field ion source geometry designed to allow selective position-specific ionization through photoinduced dissociation indicate that a 10× improvement in focusing can be achieved using this configuration.     

Time-of-flight mass spectrometric analysis of ion formation in hypervelocity impact of organic polymer microspheres: comparison with secondary ion mass spectrometry, (252)Cf mass spectrometry and laser mass spectrometry

Unisized 1.6-microm polystyrene microspheres coated with PEDOT (polyethylene-dioxythiophene) were accelerated to speeds of 6-16 km/s and shot onto a silver target. Either positive or negative ions, both instantaneously formed by the impact process, have been analyzed by time-of-flight mass spectrometry (TOF). Apparently, the processes that control the formation of ions of either polarity depend on the impact velocity. Comparing the results with those of secondary ion mass spectrometry with primary ion energy in both the elastic and the inelastic ((252)Cf-MS) energy loss regimes, some reaction mechanisms of the polymer ions for different energy densities could be elucidated. Some aspects of ion formation are also related to those found in pulsed laser ion generation from these microspheres. This investigation was performed in order to further improve the method of analyzing the organic fraction of interstellar, interplanetary, and cometary dust particles impinging on the targets of the "CIDA" time-of-flight (TOF) mass spectrometers on-board the NASA comet missions "STARDUST" and "CONTOUR".     

Miniaturized time-of-flight mass spectrometer for peptide and oligonucleotide analysis

A matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometer was developed which uses a novel reflectron composed of a grounded cylinder and an adjustable endcap electrode to provide high-order kinetic energy focusing for a miniaturized mass analyzer. The nearly quadratic potential form of the reflecting field focuses ions desorbed from a source of very small dimensions formed by placing the sample probe within the centered hole of the coaxial dual channel plate detector. At the same time, the depth of the reflectron can be adjusted to accommodate a short drift length between the source/detector and the reflectron. For larger drift lengths, in particular to allow the addition of an XY stage for the analysis of sample arrays, endcap reflectron focusing can be combined with time-delayed ion extraction to achieve good mass resolution. The instrument has been used for the analysis of peptides digested with trypsin or carboxypeptidase, and also small DNA oligomers.     

The gas phase structure of some protonated and ethylated aromatic amines

The fundamental processes of protonation and ethylation, occurring in a methane chemical ionization source, have been investigated for a variety of aromatic amines. The positions of protonation and ethylation on the substrate amines were determined by generating isomeric ions either by protonation of neutral ethyl substituted amines or by ethylation of the amines themselves. The product ions were investigated for structural differences via collision induced dissociation and subsequent analysis via mass analysed ion kinetic energy spectrometry. Similarities and differences between mass analysed ion kinetic energy/collision induced dissociation spectra of these isomeric ions were used to determine protonation and ethylation sites for imidazole, benzimidazole, indazole, pyrrole, pyridine and aniline.