Multi-turn time-of-flight mass spectrometers with electrostatic sectors

The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi-turn or multi-passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi-turn TOF mass spectrometer with electrostatic sectors. The concept of 'perfect' focusing conditions is introduced. Furthermore, a new type of multi-turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 x 70 x 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi-turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi-turn TOF mass spectrometers is also discussed.     

Development of an Ultra-High Performance Multi-Turn TOF-SIMS/SNMS System “MULTUM-SIMS/SNMS”

A new system incorporating a multi-turn time-of-flight secondary ion/sputtered neutral mass spectrometer (TOF-SIMS/SNMS) with laser post-ionization was designed and constructed. This system consists of a gallium focused ion beam, femtosecond (fs) laser for post-ionization, and multi-turn TOF mass spectrometer. When laser post-ionization was used, the secondary ion signal strengths for several metals increased by up to 650 times, and were greater than the values obtained in conventional TOF-SIMS experiments. Use of the multi-turn mass spectrometer resulted in an increase in mass resolving power with increase in the total TOF. The mass resolving power reached to 23,000 after 800 multi-turn cycles, corresponding to a flight path length of 1040 m. These results indicated that this system is very effective for the analysis of valuable materials such as space samples with high sensitivity, high mass resolving power, and high lateral resolution.      

High-resolution time-of-flight spectra obtained using the MULTUM II multi-turn type time-of-flight mass spectrometer with an electron ionization ion source

This paper describes experiments demonstrating the high mass-resolving power of the MULTUM II multi-turn type time-of-flight (ToF) mass spectrometer with a 1.308-meter circuit controlled by four toroidal electric sector fields(1) and an electron ionization (EI) ion source. A mass resolution of 250,000 [full-width at half maximum: (FWHM)] was obtained for N(2)(+) after a flight time of 9.0 ms (flight cycles: 1,200, flight length: 1,500 M). A doublet of (12)C(5)H(5)(14)N and (13)C(12)C(5)H(6) (m/Deltam = 9,746; Deltam: mass difference of doublet, m: mass of lighter ion of doublet) was separated and a mass resolution of 91,000 (FWHM) was obtained. A doublet of CDCl(2) and CH(2)Cl(2) (m/Deltam = 54,162) was also separated. A mass resolution of 115,000 (FWHM) was then achieved. When one peak of these doublets was used as a calibrant, the mass of the other peak was determined within a few ppm by mass difference. The ToF depending on the square of m/z was significantly larger than the systematic errors in the ToF, so that good mass accuracy was obtained by one-point mass determination.     

Construction of a new multi-turn time-of-flight mass spectrometer

A new type of multi-turn time-of-flight mass spectrometer was constructed, consisting of four cylindrical electric sectors and 28 electric quadrupole lenses, the size of the vacuum chamber being 60 x 70 x 20 cm. It was demonstrated that the mass resolution can be increased according to the number of cycles of the ions through the ion optical system.     

Comparison of mass spectra of peptides in different matrices using matrix-assisted laser desorption/ionization and a multi-turn time-of-flight mass spectrometer, MULTUM-IMG.

The mass spectra of peptides obtained with different matrices were compared using a matrix-assisted laser desorption/ionization (MALDI) ion source and a multi-turn time-of-flight (TOF) mass spectrometer, MULTUM-IMG, which has been developed at Osaka University. Two types of solid matrices, alpha-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB), and a liquid matrix made from a mixture of 3-aminoquinoline and CHCA were used. When measuring the peak signal intensity of human angiotensin II [M+H]+ from a fixed sample position, the liquid matrix produced a stable signal over 1000 laser shots, while the signal obtained with CHCA and DHB decayed after about 300 and 100 shots, respectively. Significant differences in the mass resolving power were not observed between the spectra obtained with the three matrices. Signal peak areas were measured as a function of the cycle number in a multi-turn ion trajectory, i.e., the total flight time over a millisecond time scale. For both [M+H]+ of human angiotensin II and bovine insulin, the decay of the signal peak area was the most significant with CHCA, while that measured with DHB was the smallest. The results of the mean initial ion velocity measurements suggested that the extent of metastable decomposition of the analyte ions increased in order of DHB, the liquid matrix, and CHCA, which is consistent with the difference in the decay of the signal peak area as the total flight time increased.     

Development of a high-Performance MALDI-TOF mass spectrometer utilizing a spiral ion trajectory

A novel MALDI-TOF mass spectrometer that utilizes a spiral ion trajectory was developed. In this mass spectrometer, the ions sequentially passed through four toroidal electrostatic sectors and revolved along a figure-eight-shaped orbit on a particular projection plane. Each toroidal electrostatic sector had eight stories, and during multiple revolutions, the ion trajectory shifted perpendicular to the projection plane in every cycle, thereby generating a spiral trajectory. The flight path length of one cycle was 2.1 m; therefore, when the ions completed eight cycles, the total flight path length was 17 m. By adopting an ion optical system that had a flight path length five times longer than that in the commonly used reflectron ion optical system, the mass dependence on the mass resolving power was reduced, while improving the mass accuracy of the mass measurements. The basic performance of the system was tested by using standard peptides or the tryptic digest of bovine serum albumin. A mass resolving power of 80,000 (full width at half maximum) was achieved at m/z = 2564 (ACTH18-39). An improved mass accuracy less than 2 ppm was realized over a wide m/z range of 500 to 3000 by correction using one or two internal standard substances.     

The design and characteristic features of a new time-of-flight mass spectrometer with a spiral ion trajectory

A new time-of-flight (TOF) mass spectrometer with a corkscrew ion trajectory was designed and constructed. The spiral trajectory was realized by using four toroidal electrostatic sectors. Each had fifteen-stories made of sixteen Matsuda plates piled up inside a cylindrical electrostatic sector. The ions passed the four toroidal electrostatic sectors sequentially and revolved along a figure-eight-shaped orbit on a certain projection plane. During the multiple revolutions, each ion trajectory was shifted by 50 mm per cycle on a direction perpendicular to the projection plane, thus generating a spiral trajectory. The flight path length of one cycle was 1.308 m so that the maximum flight path length became approximately 20 m. The mass resolution, mass accuracy, and ion transmission were tested by utilizing an orthogonally coupled electron ionization source. A mass resolution of 35,000 (FWHM) for m/z greater than 300 was achieved. Even in a lower mass region, mass resolutions of more than 20,000 (FWHM) were confirmed with a doublet of (12)C(5)(1)H(5)(14)N(+) and (13)C(12)C(5)(1)H(6)(+). The mass accuracy was also improved such that it was better than 1 ppm with only one internal standard peak. An ion transmission of approximately of 100% was observed for 15 cycles.     

Optimizing signal and mass resolution for matrix-assisted laser desorption utilizing a linear time-of-flight mass spectrometer.

Results are presented for various instrumental configurations employed for matrix-assisted laser desorption mass spectrometry. Mass resolution is determined for a linear time-of-flight mass spectrometer for various lengths of the field-free region. A wire ion guide is utilized and is shown to improve ion transport efficiencies for longer field-free regions. It is also determined experimentally that a modest mass resolution increase is often obtained in configurations employing the wire ion guide when compared to the mass resolution obtained with the same geometry without the wire ion guide. Optimal applied potentials are determined for the wire ion guide. No mass dependence on the optimal applied potential (-100 V) for the wire ion guide is observed for samples of equine myoglobin (MW 16,951.5 Da) and a bacterial protease (MW 27,228.4 Da). The optimal applied voltage was also found to be identical (-100 V) for the singly through quadruply charged molecular ion species of rabbit gamma globulin (MW approximately 150,000 Da). It is shown that a 2 m flight tube with a wire ion guide provides better signal-to-noise mass spectra than a 1 m flight tube without the wire ion guide and can more than double the mass resolution obtainable. Utilization of a 4 m flight tube gives minimal mass resolution enhancement at the expense of signal-to-noise.     

A tandem time-of-flight mass spectrometer: combination of a multi-turn time-of-flight and a quadratic-field mirror

A tandem time-of-flight (ToF) mass spectrometer consisting of a multi-turn time-of-flight (ToF) and a quadratic-field ion mirror has been designed and constructed. The instrument combines the unique capabilities of both ToF instruments, namely high-resolution and monoisotopic precursor ion selection from the multi-turn ToF and temporal focus for fragment ions with different kinetic energies from the quadratic-field mirror. The first tandem mass spectra for this unique combination of ToF systems are presented.     

Performance evaluation of a miniature laser ablation time‐of‐flight mass spectrometer designed for in situ investigations in planetary space research

Key performance features of a miniature laser ablation time‐of‐flight mass spectrometer designed for in situ investigations of the chemical composition of planetary surfaces are presented. This mass spectrometer is well suited for elemental and isotopic analysis of raw solid materials with high sensitivity and high spatial resolution. In this study, ultraviolet laser radiation with irradiances suitable for ablation (< 1 GW/cm²) is used to achieve stable ion formation and low sample consumption. In comparison to our previous laser ablation studies at infrared wavelengths, several improvements to the experimental setup have been made, which allow accurate control over the experimental conditions and good reproducibility of measurements. Current performance evaluations indicate significant improvements to several instrumental figures of merit. Calibration of the mass scale is performed within a mass accuracy (Δm/m) in the range of 100 ppm, and a typical mass resolution (m/Δm) ~600 is achieved at the lead mass peaks. At lower laser irradiances, the mass resolution is better, about (m/Δm) ~900 for lead, and limited by the laser pulse duration of 3 ns. The effective dynamic range of the instrument was enhanced from about 6 decades determined in previous study up to more than 8 decades at present. Current studies show high sensitivity in detection of both metallic and non‐metallic elements. Their abundance down to tens of ppb can be measured together with their isotopic patterns. Due to strict control of the experimental parameters, e.g. laser characteristics, ion‐optical parameters and sample position, by computer control, measurements can be performed with high reproducibility. Copyright © 2012 John Wiley & Sons, Ltd.     

Digital Imaging Mass Spectrometry

Methods to visualize the two-dimensional (2D) distribution of molecules by mass spectrometric imaging evolve rapidly and yield novel applications in biology, medicine, and material surface sciences. Most mass spectrometric imagers acquire high mass resolution spectra spot-by-spot and thereby scan the object’s surface. Thus, imaging is slow and image reconstruction remains cumbersome. Here we describe an imaging mass spectrometer that exploits the true imaging capabilities by ion optical means for the time of flight mass separation. The mass spectrometer is equipped with the ASIC Timepix chip as an array detector to acquire the position, mass, and intensity of ions that are imaged by matrix-assisted laser desorption/ionization (MALDI) directly from the target sample onto the detector. This imaging mass spectrometer has a spatial resolving power at the specimen of (84 ± 35) μm with a mass resolution of 45 and locates atoms or organic compounds on a surface area up to ~2 cm². Extended laser spots of ~5 mm² on structured specimens allows parallel imaging of selected masses. The digital imaging mass spectrometer proves high hit-multiplicity, straightforward image reconstruction, and potential for high-speed readout at 4 kHz or more. This device demonstrates a simple way of true image acquisition like a digital photographic camera. The technology may enable a fast analysis of biomolecular samples in near future.     

Orthogonal trap time-of-flight mass spectrometer using a collisional damping chamber

We report a new hybrid mass spectrometer, which is a combination of a quadrupole ion trap and an orthogonal time-of-flight (TOF) mass spectrometer. This new configuration consists of a collisional-damping chamber (CDC) inserted between an MSn-capable ion trap and a high-mass-accuracy orthogonal-TOF mass spectrometer. Because the CDC converted an ion packet into an energy-focused and quasi-continuous beam, a high mass resolution of over 10,000 and a high mass accuracy of better than 3 ppm were achieved. Moreover, the ratio of the maximum detectable m/z to the minimum detectable m/z, which is referred to here as the mass window, was improved to more than 10.     

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

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

Fragmentation characteristic of glutathione conjugates activated by high-energy collisions

Product ion spectra of fifteen monoglutathione and diglutathione conjugates have been measured using activation by 6000-eV collisions with helium in the third field-free region of a four-sector tandem mass spectrometer of EBEB configuration. Fragmentation patterns in the cation spectra have been analyzed for decompositions of the glutathione moiety that would permit recognition of an unknown as a glutathione conjugate. Five spectra from an earlier study of high-energy collisional activation on a BEEB four-sector instrument have also been included in this analysis. A suite of appropriate ions was found to occur consistently, including ions of m/z 307 comprising the glutathione tripeptide and the complementary ion [MH — 307]⁺ or the ion radical [MH — 306]^+’.     

Simulation of Duty Cycle-Based Trapping and Ejection of Massive Ions Using Linear Digital Quadrupoles: the Enabling Technology for High Resolution Time-of-Flight Mass Spectrometry in the Ultra High Mass Range

Duty cycle-based trapping and extraction processes have been investigated for linear digitally-driven multipoles by simulating ion trajectories. The duty cycles of the applied waveforms were adjusted so that an effective trapping or ejection electric field was created between the rods and the grounded end cap electrodes. By manipulating the duty cycles of the waveforms, the potentials of the multipole rods can be set equal for part of the waveform cycle. When all rods are negative for this period, the device traps positive ions and when all are positive, it ejects them in focused trajectories. Four Linac II electrodes[1] have been added between the quadrupole rods along the asymptotes to create an electric field along the symmetry axis for collecting the ions near the exit end cap electrode and prompt ejection. This method permits the ions to be collected and then ejected in a concentrated and collimated plug into the acceleration region of a time-of-flight mass spectrometer (TOFMS). Our method has been shown to be independent of mass. Because the resolution of orthogonal acceleration TOFMS depends primarily on the dispersion of the ions injected into the acceleration region and not on the ion mass, this technology will enable high resolution in the ultrahigh mass range (m/z > 20,000).     

Formation of doubly positively charged diatomic ions of Mo2(2+) produced by Ar+ sputtering of an Mo metal surface

Long-lived metastable doubly positively charged diatomic ions of Mo2(2+) have been produced by Ar+ bombardment of a molybdenum metal surface. These exotic molecular dications, such as for example 92,95Mo2(2+) at m/z 93.5, could be observed in positive ion mass spectra for ion flight times of approximately 17 micros in a Cameca IMS-3f secondary ion mass spectrometer, when the ion extraction field was adjusted for detection of ions that are formed in the gas phase several micrometers in front of the sputtered surface. Mo2(2+) was observed at high primary current densities for projectile ions of Ar+, but could not be detected under very similar bombarding conditions for projectile ions of Xe+. Such a dependence of ion production by inert gas sputtering on the primary ion species [ionization energies: IP1(Ar) = 15.76 eV and IP1(Xe) = 12.13 eV] is unusual. It is shown that formation of Mo2(2+) dications takes place by resonant charge transfer in grazing gas-phase collisions between incoming projectile ions of Ar+ and sputtered molecular ions of Mo2+. The efficiency for such a resonant electron capture (Mo2+ + Ar+ --> Mo2(2+) + Ar) is of the order of 10(-5) for the bombarding conditions in our mass spectrometer and corresponds to a cross section of a few 10(-15) cm2.     

Design of a new multi‐turn ion optical system ‘IRIS’ for a time‐of‐flight mass spectrometer

A new multi‐turn ion optical system ‘IRIS’ has been designed for use with a high‐performance time‐of‐flight (TOF) mass spectrometer, which satisfies the new design concepts of time focusing and phase space stability. It has an elliptical flight path composed of four toroidal electric sectors, with a flight path length for one lap of 0.974 m. Dimensions and voltages of sector electrodes have been optimized to satisfy theoretical requirements by simulations using surface charge method. Generally, multi‐turn instruments require an injection and ejection system to inject and eject ions. On the basis of this ion optical study, we have designed an injection and ejection ion optical system, which achieves time focusing for the total system. Furthermore, we have designed novel field‐adjusting electrodes (FAEs) for the perforated sectors in the injection and ejection systems, which accurately correct the electric potential around the perforated sector's hole. We have also used simulations to evaluate mass resolving power and ion transmissions for various lap numbers or flight path lengths. Through these we have confirmed that mass resolving powers of over 100 000 can be achieved with reasonable ion transmissions for a given set of initial conditions. Usually a multi‐turn TOF mass spectrometer with a closed optic axis has mass range limitations from overtaking ions. To solve this problem, a TOF segmentation method is proposed that identifies all peaks in a TOF spectrum, including those from overtaking ions. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

Acetone and perdeuterated acetone in UV-IMS

Measuring a mixture of acetone and perdeuterated acetone (acetone-d6) with an ultra-high resolution drift time ion mobility spectrometer (resolving power of R_p?=?235) and ultraviolet ionization (10.6 eV) at ambient pressure reveals three separated peaks. Two of the peaks can easily be associated with acetone and perdeuterated acetone. In a former publication several findings indicated an exchange of a methyl group and the formation of a H₃COCD₃ related peak. In this work the formed ion species were analyzed with a high resolution drift time ion mobility time of flight mass spectrometer. The mass spectra clearly show the formation of three proton-bound dimer peaks whereas the peak between acetone and acetone-d6 is a proton-bound mixed dimer consisting of one acetone and one acetone-d6 molecule.