Ion Mirror Research Articles

A Simulation Study of the Interaction Between a Fast Shock and the Heliopuase

In a one-dimensional hybrid simulation of a fast shock impinging on the heliopause (HP), it is found that two regions of increased plasma density are formed between the transmitted and reflected shocks. The region downstream of the shock, on the interstellar side of the heliopause, has typically twice the original plasma density. The density increase is dependent on the impinging shock speed. This region has a greatly enhanced ion temperature anisotropy T(subscript ┴)/T(subscript ∥)-100 and a plasma beta β~0.1-1.0, which may lead to the generation of ion cyclotron waves and mirror waves. The region downstream of the reflected shock, on the solar wind side of the heliopause, has typically a 25% increase in plasma density. The enhanced temperature anisotropy is T(subscript ┴)/T(subscript ∥)~1.2, but the plasma beta is very high, β-20, which may also lead to the generation of mirror waves and ion cyclotron waves.

Magnetosheath dynamics downstream of low Mach number shocks

The dissipation of the flow kinetic energy of solar wind ions at the quasi‐perpendicular bow shock results in the formation of ion distributions that have large perpendicular temperature anisotropies. These anisotropies provide free energy for the growth of Alfvén ion cyclotron (A/IC) and mirror waves. The waves then make the ion distributions gyrotropic and substantially reduce their anisotropy. Although differences exist, many of the mechanisms governing wave generation and particle isotropization operate at both high and low Mach number shocks. These mechanisms are easier to study at low Mach number shocks because they proceed relatively slowly and the turbulence level is lower. Also, in general, the plasma beta is lower at the low Mach number bow shock, allowing for situations in which minority ion species like He++ can suppress proton cyclotron waves and thus play a larger role in sheath dynamics than they do at high Mach number shocks. For these reasons, we use two‐dimensional hybrid simulations to model the heating of H+ and He++ ions at the low Mach number bow shock and examine wave excitation and ion isotropization in the magnetosheath downstream. In agreement with observations and theory and in contrast to high Mach numbers, we find that the magnetosheath turbulence mostly consists of A/IC waves. Without helium ions, the A/IC wave activity is dominated by parallel propagating proton cyclotron waves. When helium ions are included at a low density, they tend to absorb these waves, leaving obliquely‐propagating A/IC waves dominant, though at a lower intensity level. Proton heating at the shock is dominated by bulk perpendicular heating of the core, while the helium ions are heated only slightly. Instead, initially they gyrate around field lines downstream as a coherent, nongyrotropic bunch. Downstream, the protons are slowly isotropized through pitch‐angle scattering by A/IC waves. The helium ions undergo perpendicular heating through absorption of proton waves and become more gyrotropic. The perpendicular heating drives the growth of helium cyclotron waves, which in turn reduce the anisotropy of the helium ions. Far downstream of the shock, obliquely‐propagating helium cyclotron waves dominate the sheath turbulence.

Matrix‐assisted Laser/Desorption Ionization Time‐of‐flight Mass Spectrometer with Orthogonal Acceleration Geometry: Preliminary Results

An orthogonal acceleration time-of-flight mass spectrometer has been constructed with a matrix-assisted laser desorption/ionization source. The instrument has been set up in a preliminary configuration for an assessment of the viability of the orthogonal acceleration geometry. Measurements of resolving power and statistical evaluation of the mass accuracy/precision are reported here for ions in the range m/z 614 to 1351. The preliminary results indicate that resolving powers in the range of 2800 to 3600 (full width at half maximum) are readily achievable without the use of an ion mirror. A two-point external calibration of the mass scale was found to be stable over several days. The precision of the mass measurement has been estimated statistically in an error analysis involving about 40 measurements in the mass range specified above. The distribution of errors was found to be normal (linear normal probability plot) with a mass accuracy of about 0.1 to 70 ppm (average error) depending on how the calibration was performed, while the precision is found to be 90 ppm (standard deviation) irrespective of calibration method. The observed errors are consistent with calculations based on the resolving power and the 2.5 ns digitizer frequency used in this work. Measurements are restricted in mass range with the current instrument because it has a low ion energy (3 kV) accelerator and detector configuration that is linked to the accelerator by a long narrow drift-tube (1.5 m×20 mm). A new large-detector configuration with a 20 kV accelerator and a wider drift region is briefly described.

Matrix-assisted Laser/Desorption Ionization Time-of-flight Mass Spectrometer with Orthogonal Acceleration Geometry: Preliminary Results

An orthogonal acceleration time-of-flight mass spectrometer has been constructed with a matrix-assisted laser desorption/ionization source. The instrument has been set up in a preliminary configuration for an assessment of the viability of the orthogonal acceleration geometry. Measurements of resolving power and statistical evaluation of the mass accuracy/precision are reported here for ions in the range m/z 614 to 1351. The preliminary results indicate that resolving powers in the range of 2800 to 3600 (full width at half maximum) are readily achievable without the use of an ion mirror. A two-point external calibration of the mass scale was found to be stable over several days. The precision of the mass measurement has been estimated statistically in an error analysis involving about 40 measurements in the mass range specified above. The distribution of errors was found to be normal (linear normal probability plot) with a mass accuracy of about 0.1 to 70 ppm (average error) depending on how the calibration was performed, while the precision is found to be 90 ppm (standard deviation) irrespective of calibration method. The observed errors are consistent with calculations based on the resolving power and the 2.5 ns digitizer frequency used in this work. Measurements are restricted in mass range with the current instrument because it has a low ion energy (3 kV) accelerator and detector configuration that is linked to the accelerator by a long narrow drift-tube (1.5 m×20 mm). A new large-detector configuration with a 20 kV accelerator and a wider drift region is briefly described.

Characterization of protein glycosylation by mass spectrometry

Electrospray ionization, a natural interface with microbore and capillary high-pressure liquid chromatography, has become the method of choice for the reliable structural characterization of protein glycosylation by mass spectrometry at the picomole level. Its advantages include inherent sensitivity in the femtomole range, compatibility with collisional activation methods that both permit the detection and monitoring of structurally specific ions and enable the induction of glycopeptide fragmentation that facilitates determination of glycoform sequence and branching. Developments in high-performance electrospray mass spectrometry include sample introduction at nanoliter flow rates, tandem magnetic sector/orthogonal time-of-flight instruments, Fourier transform instruments, and new ion optical strategies, including ion traps. Although a sensitive and important complementary technique, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry suffers from matrix-dependent deposition of excess internal energies, which produce extensive metastable fragmentation and (photo)adduct formation. These metastable fragments may be focused into a mass spectrum by employing an ion mirror (reflection) in time-of-flight instrumentation. In favorable cases, structural information may be obtained.

Measurement of Insulator Surface Potential Using Time‐of‐flight Secondary Ion Mass Spectrometry

In this study we demonstrate a technique which determines the absolute electrostatic potential of the surface of insulating samples analyzed by time-of-flight (TOF) secondary ion mass spectrometry (SIMS). Small changes in surface potential from the nominal sample bias of 500 V were deduced from shifts in the TOF spectra by use of a large extraction distance and modified ion mirror design. Experimental peak shapes and shifts were duplicated by modeling the ion flight times throughout the mass spectrometer to confirm the results. By monitoring peak positions we were able to adjust charge compensation parameters, thereby keeping the surface potential at the desired value. This technique offers the opportunity to investigate the fundamentals of sample charging, or can be used to evaluate the effectiveness of charge compensation during SIMS analysis of insulators.

Ion optics of a high resolution multipassage mass spectrometer with electrostatic ion mirrors

Ion trajectories in an electrostatic ion mirror are calculated. The interferences of the extended fringing fields of the mirror with finite aperture are studied. The results of the calculations are represented by three transfer matrices, which describe ion trajectories under the effects of a fringing field at the entrances, of an idealized mirror region, and of a fringing field at the exit. The focusing effects and ion-optical properties of mass spectrometers with electrostatic ion mirrors can be evaluated by using these transfer matrices. A high performance multipassage mass spectrometer is designed. The system has one magnet and four electrostatic sector analyzers and two ion mirrors. The double focusing condition and stigmatic focusing condition are achieved in any passage of the system. The mass resolution increases linearly with the number of passages in a magnet.

Preferential Fragmentation of Protonated Gas-Phase Peptide Ions Adjacent to Acidic Amino Acid Residues

Matrix-assisted laser desorptiodionization (MALDI) is an effective method for producing protonated peptide and protein ions in the gas phase.' Together with electrospray ionization, MALDI has become an ionization method of choice for analyzing peptides and proteins by mass spectrometry (MS).2 The analyses are usually carried out by measuring the molecular masses of the intact polypeptides or peptide fragments produced by proteolysis or chemical degradation of proteins in solution. An alternative method for obtaining detailed primary structural information uses fragmentation of the protonated peptides in the gas phase followed by MS analysis of the fragment ions. A powerful variation of this approach, called tandem MS, employs mass spectrometric selection of the peptide ion of interest prior to fragmentation, followed by analysis of the fragment ion^.^^^ Fragmentation of peptide ions produced by MALDI has been studied in time-of-flight (TOF) mass analyzers (fitted with ion mirrors), instrumentation well-suited to the pulsed ionization ~ource.~*~ Mass analyzers that utilize the principle of ion trapping are also well-suited to the pulsed MALDI ion so~rce.~ Precursor ion selection can be achieved with high resolution.8 The prolonged time window of the measurement in the quadrupole ion trap to =- 1 s) compared with that in the ion mirror TOF analyzer (< s) enables the observation of fragmentation processes with low rate constants. In the reflecting TOF analyzer, highly preferential peptide bond cleavage has been observed at the bond carboxy (C)-terminal to aspartic acid residues, with Asp-Pro bonds exhibiting particularly high lability.6 In the present study, we investigate the fragmentation of MALDI-produced protonated peptide ions (containing an Arg residue and at least one acidic amino acid residue) in a quadrupole ion trap mass spectrometer and also observe preferential cleavage of the peptide bond adjacent to aspartic acid residues. In addition, we report the first observation of preferential fragmentation adjacent to glutamic acid residues. The investigation was carried out on a MALDI quadrupole ion trap mass spectrometer constructed at Rockefeller University? Ions are generated external to the ion trap using 355 nm laser irradiation,I0 accelerated to 60 eV, focused with an einzel lens, and injected axially into the trap. The trapped ions are (1) Hillenkamp, F.; Karas, M.; Beavis, R. C., Chait, B. T. Anal. Chem.

Wave and ion evolution downstream of quasi‐perpendicular bow shocks

Distribution functions of ions heated in quasi‐perpendicular bow shocks have a large perpendicular temperature anisotropy that provides free energy for the growth of Alfvén ion cyclotron (AIC) waves and mirror waves. Both types of waves have been observed in the Earth's magnetosheath downstream of quasi‐perpendicular shocks. The question of whether these waves are produced at the shock and convected downstream or whether they are produced locally in the magnetosheath has not yet been answered. If the latter were true, then under most magnetosheath conditions AIC waves should dominate the wave activity, yet frequently mirror waves either dominate or are competitive with the AIC mode. We address this question by using two‐dimensional hybrid simulations to give a self‐consistent description of the evolution of the wave spectra downstream of quasi‐perpendicular shocks. Both mirror and AIC waves are identified in the simulated magnetosheath. They are generated at or near the shock front and convected away from it by the sheath plasma. Near the shock, the waves have a broad spectrum, but downstream of the shock, shorter‐wavelength modes are heavily damped and only longer‐wavelength modes persist. The characteristics of these surviving modes can be predicted with reasonable accuracy by linear kinetic theory appropriate for downstream conditions. Throughout the downstream region, the power in compressive magnetic oscillations is of the same order as the power in transverse oscillations. We also follow the evolution of the ion distribution function. The shocked ions that provide the free energy for wave growth have a two‐component distribution function: a core population of directly transmitted ions and a smaller halo of initially reflected ions that contains the bulk of the free energy. The halo is initially gyrophase‐bunched and extremely anisotropic. Within a relatively short distance downstream of the shock (of the order of 10 ion inertial lengths), wave‐particle interactions remove these features from the halo and reduce the anisotropy of the distribution to near‐threshhold levels for the mirror and AIC instabilities. A similar evolution has been observed for ions at the Earth's bow shock.

Wave and particle evolution downstream of quasi-perpendicular shocks

Distributions of ions heated in quasi-perpendicular bow shocks have large perpendicular temperature anisotropies that provide free energy for the growth of Alfvén ion cyclotron (AIC) and mirror waves. These modes are often observed in the Earth's magnetosheath. Using two-dimensional hybrid simulations, we show that these waves are produced near the shock front and convected downstream rather than being produced locally downstream. The wave activity reduces the proton anisotropy to magnetosheath levels within a few tens of gyroradii of the shock but takes significantly longer to reduce the anisotropy of He ++ ions. The waves are primarily driven by proton anisotropy and the dynamics of the helium ions is controlled by the proton waves. Downstream of high Mach number shocks, mirror waves compete effectively with AIC waves. Downstream of low Mach number shocks, AIC waves dominate.

Ion‐Optical solutions in time‐of‐flight mass spectrometry

AbstractTime‐of‐flight mass spectrometers are able to cover mass ranges to around a half a million Da. The mass resolution of these instruments has been improved drastically by incorporation of electrostatic mirrors. The ion‐optical solutions enabling these achievements and other promising features are discussed in terms of: First‐ and second‐order space focusing in time for linear drift time‐of‐flight mass spectrometers; velocity focusing by time lag; ion‐packet bunching in post‐source impulse focusing; first‐ and second‐order energy focusing in time, in single‐ and double‐stage homogeneous electrostatic mirrors with normal and oblique ion incidence; gridless designs; perfect time focusing in parabolic potential and axial symmetry; cylindrical ion mirrors and toroidal electrostatic deflectors as time‐of‐flight mass analyzers; three electrostatic sector geometry applied in secondary ion microscopy and four toroidal sector systems for tandem mass spectrometry; poloidal electrostatic sectors. Estimation of time‐of‐flight mass spectrometer performance limits is discussed and the stable as well as the metastable peak shapes found in time‐of‐flight mass spectra.

Improved energy compensation for time-of-flight mass spectrometry

A model for improved energy compensation in time-of-flight (TOF) mass spectrometry has been developed and tested. This model includes effects of both the acceleration and drift region on mass resolution for surface desorption TOF mass spectrometers that employ ion mirrors to improve mass resolution. Appropriate placement of an additional stage onto the conventional one- and two-stage mirrors provides compensation for flight time spreads, caused by initial ion kinetic energy distributions, in both regions. Experimental results that validate the model calculations are presented for a modified commercial two-stage ion mirror. For example, m/Δ m for Na + was improved from ∼ 100 to ∼ 200 using only a 200-eV drift energy and a 58-cm drift path.

Quantitative determination of kinetic energy releases from metastable decompositions of sputtered organic ions using a time-of-flight mass spectrometer with a single-stage ion mirror

A method has been developed for determining the kinetic energy released when metastable organic ions, produced by particle-induced desorption—ionization, decompose in a time-of-flight mass spectrometer having a single-stage ion mirror. To the best of our knowledge, this is the first report of a fully developed, quantitative procedure for this particular combination of ionization method and mass analysis. In order to obtain the kinetic energy released in a specific metastable decay, the rate constant for the unimolecular reaction has to be estimated, and the widths of the precursor and charged fragment ion peaks have to be measured. The rate constant for a specific decomposition reaction is determined by deflecting all ions away from the optic axis at positions of increasing distance along the flight path through the first field-free region of the spectrometer and by counting the neutral fragments that reach the detector located at the back end of the mirror. The widths of the precursor ion peak and the charged fragment ion peak are measured respectively after both types of ion species have independently followed precisely the same flight path in space and time. With a single-stage mirror, the latter condition is met by reflecting the precursor and fragment ions with mirror potentials that are in proportion to the respective masses of the ions. Theoretical, experimental, and error analyses are described and illustrated with examples.

Electronic sputtering of fullerenes and the influence of primary ion charge state

Even-numbered positive ion carbon clusters, fullerenes ( C 2 n , n = 20, 21, …), are formed and ejected as a result of the interaction of swift atomic ions with a solid film of organic polymer — poly(vinylidene difluoride), PVDF. Studies of that phenomenon including the dependence of fullerene yield on the incident MeV ion's stopping power and initial radial velocity distributions of ejected ions, have been reported earlier [G. Brinkmalm et al., Chem. Phys. Lett. 191 (1992) 345; Phys. Rev. B 47 (1993) 7560]. In this paper, the dependence of the yield of the ejected cluster ions from PVDF as a function of the charge state of the incident MeV ion is reported. Swift 72.3 MeV 127I ions in charge states ranging from + 13 to + 25 from the Uppsala EN-tandem accelerator are used in the experiments, and the secondary ion yield is measured in a time-of-flight mass spectrometer, equipped with an electrostatic ion mirror. The results are compared to data for secondary ions, ejected by the same primary ions, from samples of synthetic fullerenes (C 60 and C 70). No dependence of the yield on the charge state of the incoming ion is observed for the high mass even-numbered carbon clusters from PVDF, while yields of C 60 and C 70 ions from synthetic fullerene targets show a weak charge state dependence. This finding indicates that excited polymer material from layers deeper than the MeV ion charge equilibration length in the solid contributes to the formation of ejected carbon cluster ions ejected from PVDF.

A novel four grid ion reflector for saturation of laser multiphoton ionization yields in a time of flight mass spectrometer

A four grid electrostatic ion reflector is used for confining the acceptance volume in a multiphoton laser ionization TOF mass spectrometer. For each specific charge q/m the new TOF instrument creates two ion bunches containing the ions originating respectively inside and outside a sharply-limited acceptance volume of the spectrometer. The formation of two ion bunches is achieved by a step-like flight time dispersion function provided by a double grid in the ion mirror, and yields two well separated peaks in the TOF spectrum. By choosing the separation of these two peaks to be 0.2–0.3% of the ion's total flight time, an acceptance compromise was achieved between mass resolution, peak separation and detectivity of the system. The space confining capability of the instrument is demonstrated by the saturated multiphoton ionization (MPI) of sputtered iron and tantalum. In both cases, saturation of the ion yield is clearly accomplished. The prospective uses of the new instrument are further illustrated by measuring quantitatively the competition between Ta + and Ta 2+ MPI yields with increasing laser intensity.

Time-of-flight mass reflection with a large area of ion collection

This paper is devoted to the construction of a time-of-flight mass reflection with the following features: 1. a time resolution of several hundreds for various ion sources (providing elemental analysis with surface ionization, 252Cf or dust impact ion sources); 2. a huge area of the ion formation region (200 cm 2 with the possibility of extending further without decreasing analytical characteristics or lengthening the reflectron); 3. an extremely simplified compact design with very high transparency of the ion mirror grid (> 98%) and decreased demands for accuracy of manufacturing and voltage supply. An experimentally observed satisfactory agreement between the theoretical (both analytical and numerical) model of the reflectron and its practical realization, which is made possible by taking into account large-scale field perturbations, caused by grids on the ion mirror and the secondary electron multiplier, as well as near-emitter chromatic aberration.

Formation of fullerenes in MeV-ion sputtering from organic solids.

Even-numbered positive-ion carbon clusters (${\mathrm{C}}_{2\mathit{n}}$, n=20,21,...) are ejected from a solid film of organic polymer---poly(vinylidene difluoride) (PVDF)---when bombarded by MeV atomic ions. Carbon-cluster ions are formed as a result of a single primary-ion impact. The distribution of cluster sizes suggests that the clusters have closed carbon-cage (fullerene) structure. Measurements of the yield and initial-velocity distributions of the ejected cluster ions are performed in a time-of-flight mass spectrometer equipped with an electrostatic ion mirror. The results are compared to data for ions ejected by MeV projectiles from films of synthetic fullerene (${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$) targets. The initial-radial-velocity distribution for cluster ions ejected from the polymer differ markedly from those of synthetic fullerene targets. While ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ ions from synthetic fullerene targets have ejection-angle distributions symmetric with respect to the target-surface normal, the mean takeoff angle of carbon-cluster ions ejected from PVDF is off-normal and towards the direction of the incoming primary ion. It is also a clear indication that carbon-cluster ions ejected from PVDF originate in the high-energy-density region created by the fast primary ion. Experiments for establishing the dependence of the carbon-cluster yield Y on the energy loss of the primary ion have been performed. A stronger dependence of the yield for the ${\mathrm{C}}_{60}$ ions desorbed from PVDF targets [Y\ensuremath{\propto}(dE/dx${)}^{3.4}$] than for ions from targets with synthetic ${\mathrm{C}}_{60}$ [Y\ensuremath{\propto}(dE/dx${)}^{2.0}$] has been observed. This supports the conclusion that carbon-cluster ions from PVDF are formed in the axially expanding high-energy-density plasma region.

The influences of the extended fringing fields of an electrostatic ion mirror on ion trajectories and flight times

Ion trajectories and flight times of the oblique incidence and exit of an electrostatic ion mirror have been calculated. The effects of the extended fringing fields of a mirror with finite apertures have been estimated. The results of the calculations are represented by two transfer matrices for the entrance and exit fringing fields. This matrix yields a small shift and bend of ion trajectory and a small variation of flight time on a transformation plane. Ion-optical properties of the systems with an ion mirror can be evaluated by using these matrices. Large effects of the fringing fields are calculated in a mirror system.

Mirror and ion cyclotron anisotropy instabilities in the magnetosheath

Magnetosheath plasmas are typically characterized by a proton β &gt; 1 and a proton temperature anisotropy T⊥/T∥ &gt; 1. Such a plasma is unstable to both the ion cyclotron anisotropy and mirror instabilities, and evidence for both of these growing modes has been found in the magnetosheath. In this paper, we use one‐dimensional hybrid simulations to investigate the kinetic properties of these two instabilities. We find that, for moderate values of ion β and temperature anisotropy, the two instabilities produce similar levels of turbulence, but that the cyclotron instability apparently produces higher fluctuation levels than the mirror instability for high β or anisotropy. Although both instabilities saturate by reducing the ion temperature anisotropy, mirror waves accomplish this through the use of the wave magnetic fields, whereas cyclotron waves do so by using the wave electric fields in resonant interactions with the ions. Consequently, mirror waves preferentially affect those ions with large magnetic moment, while cyclotron waves affect a broader range of ions and thereby dominate the isotropization process even when the mirror mode is somewhat stronger. We also investigate the utility of transport ratios as mode identifiers, and find that the Alfvén ratio in particular can be useful in distinguishing between the two modes.

A plasma desorption time-of-flight mass spectrometer with a single-stage ion mirror: improved resolution and calibration procedure

A first order electrostatic ion mirror has been constructed and mounted on a time-of-flight spectrometer coupled to the Uppsala EN-tandem accelerator at The Svedberg Laboratory. In electronic sputtering of CsBr with MeV 127I ions a highest mass resolution around 10500 at a full-width at half maximum for the cluster ions (CsBr) 2Cs + has been obtained. The resolution for organic molecules is somewhat lower than this value but still significantly better than in a linear system. The mirror also provides great simplification of the calibration procedure and an improvement in mass accuracy compared with a linear spectrometer. For stable ions, and with the help of an einzel lens, the transmission approaches 100% in the mirror. The mirror is equipped with two stop detectors which make it possible to study the decay of metastable ions.