Nier-type Ion Source Research Articles

Optimizing 40Ar/39Ar analyses using an Isotopx NGX-600 mass spectrometer

40Ar/39Ar geochronology relies on magnetic sector mass spectrometers to determine relative isotopic abundances. Ongoing technological developments within noble gas mass spectrometers over the last decade have led to analysis of increasingly smaller samples and higher precision, but also result in more complex data correction and interpretation. We describe a new multi-collector noble gas spectrometer, the Isotopx NGX-600, that is configured to optimize 40Ar/39Ar measurements. The NGX-600 is equipped with 9 Faraday collectors and one ion counting electron multiplier. Each Faraday is equipped with Isotopx ATONA® amplifier technology, enabling measurements spanning a dynamic range of amplified beam current from below 10−16 A to above 10−9 A. The performance of the NGX-600 is evaluated using both a conventional Nier-type ion source, and a next generation low temperature ion source, which allows for trap current variation from 200 μA to 1000 μA. We have performed over 3000 analyses of atmospheric argon to: (1) assess optimal measurement and integration times for blanks, baselines, and air aliquots of various ion intensities, (2) quantify the sensitivity via measurements of first principles 40Ar/39Ar standards, (3) compare the sensitivity between the conventional and new low temperature Nier-type ion sources, and (4) evaluate corrections associated with inter-Faraday biases, instrumental mass bias, and Faraday-multiplier gain. In addition to optimization experiments, we report a comparative analysis of both single crystal fusion and incremental heating data from Quaternary volcanic rocks obtained using both the 5-collector Nu Instruments Noblesse and the NGX-600 spectrometers.

Numerical simulations of magnetic electron-impact ion source

An electron impact ion source equipped with a magnet has been designed for enhancing the ionization efficiency of such ion sources with potential applications in Knudsen effusion mass spectrometry or electron impact sources used in gas mass spectrometry. This modernized device utilizes the trapping of electrons in parallel electrical and magnetic fields with the goal of maximizing the probability of impact between an electron and an atom or molecule to be ionized. The influence of magnetic and electrical fields on ions and electron trajectories inside the ion source was studied with numerical simulations. Based on the electron and ion trajectories we have modified the classical Nier-type ion source and found the optimal geometrical and potential parameters to provide better electron ionization and higher ion extraction efficiency. This design has the potential to enhance ionization efficiency by a factor of ∼15–20 compared with existing designs.

A newly developed prototype magnetic sector mass spectrometer with high resolving power

In recent years, high-resolution mass spectrometers have been applied and made tunable to various fields. In this study, a prototype magnetic sector mass spectrometer with high resolving power has been developed. Its design is based on employing a 90̂ magnetic sector-field analyzer with double trajectory, a miniature Nier-type ion source and two channel-type electron multipliers for ion detection. The ion accelerating voltage is controllable from 200 to 2500 V and the mass range demonstrated herein is (1–73) amu. The resolution of the instrument for the lower mass channel helium and higher mass channel nitrogen is about 0.008 amu and 0.05 amu (full width at half maximum), respectively, or m/Δ m ≈550 in the whole mass range of (1–40) amu. The present sensitivity is measured to be approximately 8.4 × 10−5 A/Pa.

Electron interactions with the focused electron beam induced processing (FEBID) precursor tungsten hexachloride.

Secondary electrons with an energy distribution below 100 eV are formed when high-energy particles interact with matter. In the focused electron beam induced deposition, high-energy beams are used to decompose organometallic compounds on surfaces. We investigated the electron ionisation of WCl6 and dissociative electron attachment to WCl6 in the gas phase in order to better understand the decomposition mechanism driven by secondary electrons. A double-focusing mass spectrometer coupled with a Nier-type ion source was used to perform the present studies. The electron ionisation studies were performed with an electron energy of 70 eV and dissociative electron attachment studies in the energy range of ~0-14 eV. Tungsten hexachloride rapidly oxidises, leading to the formation of a mixture of pure WCl6 and WCl4 O together with WCl2 O2 species. The fragmentation of the three chlorinated compounds is effective, although electron ionisation to WCl6 leads to W(+) in contrast with WCl2 O2 and WCl4 O leading to WO2 (+) and WO(+) , respectively, as lighter fragments. With regard to electron attachment, decomposition of the precursor molecules is observed; however, W(-) was not detected within the detection limit of the instrument. Electron ionisation and dissociative electron attachment (DEA) to WCl6 , WCl4 O and WCl2 O2 lead to strong fragmentation. In electron ionisation, the fragmentation by loss of chlorine atoms was observed for both WCl6 and the oxidised species. Additionally, the loss of all chlorine ligands is observable for WCl6 as well as the oxidised species. The DEA results have shown dissociation by the scission of chlorine atoms as well as by the scission of an oxygen atom. The formation of chlorine and oxygen anions was observed, indicating the formation of a neutral counterpart containing the metal atom, free to be attacked by the next electron.

Mapping changes in helium sensitivity and peak shape for varying parameters of a Nier-type noble gas ion source

Tuning a Nier-type ion source involves adjusting many different parameters which affect the resulting signal in complicated ways. We have mapped the sensitivity of 4He and the peak shape while varying the total extraction voltage, the half-plate bias, the repeller voltage, and the electron energy. With the particular source settings that we used, we see an asymmetric rise and fall in the sensitivity as the extraction is raised, but a symmetric rise and fall as the half-plate bias voltage is varied. The best peak shape is found generally at the same extraction value of the maximum sensitivity, but at a distinctly different half-plate bias than the maximum sensitivity; thus it is necessary to monitor the peak shape while tuning the half-plate bias. The extraction and the repeller values of the maximum sensitivity and the best peak shape are strongly correlated, and therefore these two parameters must be tuned together. And finally, we see a double-peak in the sensitivity as the electron voltage is increased, so it may be worthwhile to check a wide range of electron voltages when tuning.

Measurement of Ion Kinetic-energy Distributions in Electron-impact Ion Sources of a Quadrupole Mass Spectrometer

An ideal ion source should create stable ion beams of sufficient intensity with an ion energy spread as low as possible. Most electron-impact ion sources used in mass spectrometers have been based on the design by Nier. Calculation of the potential distribution in the ion source revealed that the ion extraction efficiency could be improved by applying a positive repeller potential to the ionization chamber, which is normally at ground potential in a Nier-type ion source. Ion kinetic-energy distributions were measured in a Nier-type ion source and in a modified ion source by applying a retarding pole bias potential to the quadrupole at various emission currents and chamber pressures. As the emission current or the chamber pressure was increased, the most probable energy was shifted to a higher energy in both ion sources. However, the ion kinetic-energy spread was smaller and varied less in the modified ion source while it was larger in the conventional Nier-type ion source.

Computer simulation of electron and ion trajectories in electron-impact ion sources of a quadrupole mass spectrometer

Electron and ion trajectories have been simulated in electron-impact ion sources of a quadrupole mass spectrometer. The ion trajectories in the conventional Nier-type ion source show poor extraction efficiency of the generated ions into the quadrupole. Based on the electron and ion trajectory simulation results, the Nier-type ion source has been modified to improve the ion extraction efficiency. The modified ion source exhibits much better ion extraction efficiency and smaller kinetic energy spread than the Nier-type ion source.

Discrimination effects for ions with initial kinetic energy produced by electron ionization of C2H2 in a Nier-type ion source

Fully three-dimensional (3-D) ion trajectory simulations of a Nier-type ion source were performed for mono-energetic ions with a spatial resolution of the potential array of 0.1mm. Experimentally determined ion beam profiles were fitted with a superposition of simulated ion beam profiles in an effort to determine the initial ion kinetic energy distribution and the extraction efficiency from the ion source for a given product ion. This method allows the analysis of ion beam profiles of highly energetic fragment ions that are affected by a reduced ion extraction efficiency. As an example, ion beam profiles of product ions formed by electron impact ionization of acetylene (C2H2) at an electron energy of 100eV were analyzed with the present method as well as with a method developed by Poll et al. [Int. J. Mass Spectrom. Ion Process. 112 (1992) 1], which is based essentially on a simplified two-dimensional (2-D) ion trajectory analysis. For highly energetic fragment ions, the results obtained by the two methods differ by as much as 84%.

Surface-Induced Dissociation of Polyatomic Hydrocarbon Projectile Ions with Different Initial Internal Energy Content

The specific effect of initial internal energy of CH 5 +, C 2 H 4 +, C 2 H 5 +, and C 2 H 6 + ions on the extent of surface-induced fragmentation was investigated. The ions were prepared either in a gas discharge Colutron type (using a hydrogen-methane mixture at 0.2-0.5 Torr) or in a low-pressure Nier-type electron impact ion source using various target gases. Whereas projectile ions from the Colutron source are thermalized due to collisions in the high pressure environment and thus contained a negligible amount of internal energy, projectile ions from the Nier-type ion source resulting from direct electron impact ionization reactions have internal energies up to several eV. Their internal energy content was estimated using break-down curves and photoelectron spectra from literature. Results obtained here show that their different initial internal energy content had a considerable effect on the extent of fragmentation of the surface-excited projectile ions: ions with initial internal energy fragmented at much lower collision energies than internally relaxed projectile ions. It appears that the initial internal energy content of the projectile ions is entirely preserved in the projectile ion during the ion/surface collision, and thus is available in the subsequent dissociative processes as additional energy to the internal energy acquired by the projectile ion in the surface-excitation process.

四重極型質量分析計のパターン係数のイオン源による違い

A new method is proposed for the derivation of mass spectrum cracking pattern which is supposed to be possessed by ions in a variety of electron bombardment type ion sources. The cracking pattern of Argon in grid type, axial type and Nier type ion source is obtaind. The pattern coefficient was derived from partial ionization cross section of the sample gas with aid of l-V distribution data of ionizing electron, which represents the specific ionizing electron path length li of an electron having an energy of eVi.

Absolute partial and total electron impact ionization cross sections for C 2H 6 from threshold up to 950 eV

Electron impact ionization of ethane (C 2H 6) was studied with a modified Nier type ion source and a double focussing mass spectrometer (reversed geometry). Using the ion-beam-deflection-technique and a SIMION based correction procedure it is possible to determine accurately parent and fragment ion currents produced. This allows the determination of relative partial ionization cross section functions from threshold up to 950 eV for the production of C 2H + 6, C 2H + 5, C 2H + 4, C 2H + 3 C 2H + 2, C 2H +, C + 2; Ch + 3, CH + 2, CH +, C +; C 2H 2+ 5 and C 2H 2+ 3, respectively. Absolute partial ionization cross section functions for these ions were obtained by charge-weighted summing of all the observed partial ionization cross sections and by normalizing to be absolute total ionization cross section at 100 eV reported recently by Djuric. [N. Djuric, I. Cadez and M. Kurepa, Int. J. Mass Spectrom. Ion Processes 108 (1991) R1]. Comparisons are made to earlier measurements.

Ion attachment to Van der Waals clusters

Mixed ionized clusters have been produced in a supersonic nozzle beam experiment by attachment of stagnant cations (i.e. NO+ and Xe+) to neutral van der Waals clusters (i.e.Ar n ) within a Nier type ion source. This new ionization technique leads to less fragmentation than electron impact ionization and the measured cluster distributions exhibit icosahedral shell and subshell closures which have not been detected in the case of electron impact of Ar n -clusters ionization so far. Additionally, the obtained appearance energies and metastable fractions give insight into the production mechanism and the stability of the resulting ions.

Ion attachment to van der Waals clusters: reactions of NO + with Ar m

A new method is described of rendering ionized neutral van der Waals clusters. This involves the association of NO + ions to a neutral argon cluster beam in a Nier-type ion source. This new (and alternative to electron and photon) ionization mode appears to be softer than electron impact ionization (i.e. leading to less fragmentation) and measured cluster ion distributions exhibit the typical icosahedral shell and subshell closures (i.e. in the range from n = 13 to n = 55) which are not present in the case of electron impact ionization.

Discrimination effects for ions with high initial kinetic energy in a Nier-type ion source and partial and total electron ionization cross-sections of CF 4

Fragment ions produced by dissociative electron impact ionization of molecules often have large amounts of kinetic energy (up to several electronvolts). Presently performed computer simulations of the ion trajectories in a Nier-type ion source show strong discrimination effects in the extraction characteristics of such ions, leading to huge errors in measured fragmentation patterns and absolute partial ionization cross-section functions. Using the simulation results a correction function (extraction efficiency vs. fragment kinetic energy) is claculated here for a given potential distribution of a Nier-type ion source extraction system. This correction procedure is applied to previously measured partial ionization cross-sections of CF 4. The corrected partial and total CF 4 cross-sections are in excellent agreement with other recent measurements using advanced methods.

Electron impact ionization cross sections of molecules: Part I. Experimental determination of partial ionization cross sections of SF 6: a case study

Electron impact ionization of SF 6 has been studied as a function of electron energy from threshold up to 180 eV using a Nier type ion source in combination with a double focusing sector field mass spectrometer (reversed geometry). Absolute partial ionization cross section functions for the production of SF + 5, SF + 4, SF + 3, SF + 2, SF +, S + , F +, SF 2+ 4 and SF 2+ 2 have been determined using an improved experimental method based on ion trajectory calculations. In addition, the total ionization cross section function of SF 6 has been obtained. The present data are compared with previous experimental and theoretical results in order to evaluate the reliability and accuracy of the method used.

N 2O: A case study for acceleration region kinetics in a double-focusing mass spectrometer with a conventional nier-type ion source

A method is presented for the investigation of unimolecular reactions of short-lived ionic species and ion/molecule reactions occurring in the acceleration region of a double-focusing mass spectrometer equipped with a standard Nier-type ion source. This method, which is described as acceleration region kinetics (ARK), has been applied initially to an investigation of ionic species formed from nitrous oxide. N 2O, 15NNO, N 15NO, N 2 18O and 15NH 4NO 3 have been employed in this study. The temporal range of dissociative lifetimes accessible within the acceleration region is estimated as 10 −9–10 −7 s. The determination of half-lives of N 2O +·* in non-competing reactions to yield NO + and N +· 2 has been carried out using ARK and has been compared with the results of linked-scans at constant B 2/ E, where B and E represent magnetic and electric fields, respectively. Releases of kinetic energy have been measured in the second field-free region of the mass spectrometer for reactions of N 2O +·, N 2O 2+ and isotopically substituted analogues. The kinetic energy release associated with extrusion of the central N atom in isotopically labelled N 2O +· is virtually indistinguishable from that accompanying loss of the terminal N atom. Collision induced dissociation has been employed to demonstrate the contribution of N 2O +· in excited states to the dissociative channel leading to N +· 2 and O, a reaction having a greater energy requirement than that leading to NO + and N ·. The release of kinetic energy associated with charge separation of the dication, N 2O 2+, to form NO + and N + was observed to be greater than published values, and the existence of the competing channel to form N +· 2 and O +· is disputed. Isotopic labelling has been used to support the claim that ion signals ascribed previously to N +· 2 and O +· are due to CO +· and O +· from charge separation of CO 2+ 2 dications isobaric with N 2O 2+ dications.

A study on mass discrimination in a magnetic sector mass spectrometer

The effect of voltage scanning on the extraction of ions from a simplified Nier type ion source, the function of the repeller and the effect of space charge have been studied. It is also shown that, due to the choice of small dimensions for the ion source and the proper choice of electric and magnetic fields in the ion source, the influence of the source magnetic field on the ion trajectories can be neglected. A modified ion source is described in which the mass discrimination occurring under the condition of voltage scanning has been reduced. The conditions under which the initial thermal velocity may be neglected are discussed. Mass discrimination has been studied theoretically and experimentally, based on a study of computer-calculated ion trajectories.