Application of a hexapole collision and reaction cell in ICP-MS Part I: Instrumental aspects and operational optimization

The application of an ion-guiding buffer gas-filled hexapole collision and reaction cell in ICP-MS has been studied in order to give a preliminary performance characterization of a new instrument providing this feature for increasing the ion yield and decreasing contributions from Ar induced interfering molecular ions. As buffer gas He was used while H₂ served as reaction gas. Addition of the latter can be an effective means for reduction of typical argon induced polyatomic ions (Ar⁺, ArO⁺, Ar₂ ⁺) by orders of magnitude owing to gas phase reactions. Molecular interferences generated in the cell can be suppressed by a retarding electric field established by a dc hexapole bias potential of –2 V. Received: 10 May 1999 / Revised: 4 June 1999 / Accepted: 12 June 1999     

Probing trapped ion energies via ion-molecule reaction kinetics: Quadrupole ion trap mass spectrometry

We present a detailed study of the energies of the ions stored in a quadrupole ion trap mass spectrometer (QITMS). Previous studies have shown that the rate constant, k, for the charge exchange reaction Ar⁺ N⁺ ₂ →, N⁺ ₂+Ar increases with increasing ion-molecule center-of-mass kinetic energy (K.E._cm). Thus, we have determined k for this chemical “thermometer” reaction at a variety of Ar and N₂ pressures and have assigned K.E._cm values as a function of the q₂ of the Ar⁺ ion both with and without He buffer gas present in the trap. The K.E._cm energies are found to lie within the range 0.11–0.34 eV over the variety of experimental conditions investigated. Quantitative “cooling” effects due to the presence of He buffer gas are reported, as are increases in K.E._cm due to an increase in the q₂ of the Ar⁺ ion. “Effective” temperatures of the Ar⁺ ions in He buffer are determined based on a Maxwell-Boltzmann distribution of ion energies. The resulting temperatures are found to lie within the range ≈ 1700–3300 K. We have also examined the K.E._cm, values arising from the chemical thermometer reaction of O⁺ ₂ with CH₄, as previous assignments of effective ion temperatures based on this reaction have been called into question.     

Probing trapped ion energies via ion-molecule reaction kinetics: Fourier transform ion cyclotron resonance mass spectrometry

The kinetic energy-dependent Ar⁺+ N₂ ion-molecule reaction has been used as a chemical “thermometer” to determine the kinetic energy of ions produced by electron ionization and trapped by using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The rate constant for this reaction obtained on the FTICR mass spectrometer was compared to previous work, which allowed a kinetic energy estimate to be made. In addition, the effects of varying parameters such as trapping voltage and pressure on ion kinetic energy were investigated. No evidence of the differing reactivity of higher energy electronic states of Ar⁺, such as ²P_1/2, was observed and the results of a model of this system are presented that support this observation. Pressure studies revealed that with an average of as few as 13 ion-molecule collisions, Ar⁺ ions are collisionally relaxed to an extent unaffected by additional collisions. Based on recent variable temperature selected ion flow drift tube measurements, FTICR ion energies are estimated to be slightly above thermal.     

ICP–MS with hexapole collision cell for isotope ratio measurements of Ca, Fe, and Se

To avoid mass interferences on analyte ions caused by argon ions and argon molecular ions via reactions with collision gases, an rf hexapole filled with helium and hydrogen has been used in inductively coupled plasma mass spectrometry (ICP–MS), and its performance has been studied. Up to tenfold improvement in sensitivity was observed for heavy elements (m > 100 u), because of better ion transmission through the hexapole ion guide. A reduction of argon ions Ar⁺ and the molecular ions of argon ArX⁺ (X = O, Ar) by up to three orders of magnitude was achieved in a hexapole collision cell of an ICP–MS (“Platform ICP”, Micromass, Manchester, UK) as a result of gas-phase reactions with hydrogen when the hexapole bias (HB) was set to 0 V; at an HB of 1.6 V argon, and argon-based ions of masses 40 u, 56 u, and 80 u, were reduced by approximately four, two, and five orders of magnitude, respectively. The signal-to-noise ratio ⁸⁰Se/ ⁴⁰Ar₂ ⁺ was improved by more than five orders of magnitude under optimized experimental conditions. Dependence of mass discrimination on collision-cell properties was studied in the mass range 10 u (boron) to 238 u (uranium). Isotopic analysis of the elements affected by mass-spectrometric interference, Ca, Fe, and Se, was performed using a Meinhard nebulizer and an ultrasonic nebulizer (USN). The measured isotope ratios were comparable with tabulated values from IUPAC. Precision of 0.26%, 0.19%, and 0.12%, respectively, and accuracy of 0.13% 0.25%, and 0.92%, respectively, was achieved for isotope ratios ⁴⁴Ca/ ⁴⁰Ca and ⁵⁶Fe/⁵⁷Fe in 10 μg L^–1 solution nebulized by means of a USN and for ⁷⁸Se/⁸⁰Se in 100 μg L^–1 solution nebulized by means of a Meinhard nebulizer. Received: 15 December 2000 / Revised: 26 March 2001 / Accepted: 27 March 2001     

Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂N⁺. Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂NH^.+, some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂NH₂⁺. In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅C₆, leading to a novel iminium radical cation, Ar=N^.+Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.     

Coordinative saturation of cationic niobium– and rhodium–argon complexes

Mass spectra of Nb⁺ and Rh⁺ complexes with argon ligands exhibit `magic' peaks Nb⁺Ar₄ and Rh⁺Ar₆, similar to observations for V⁺Ar₄ and Co⁺Ar₆, indicating coordinative saturation. A consistent explanation is obtained by assuming that the rare gas ligands seek out electron density minima in the valence shell of the ion, which permit a closer approach to the metal core and a stronger charge-induced dipole bond. Ab initio density functional calculations, which predict stable square planar complexes for the d⁴ ions and octahedral for the d⁸ species, support this interpretation and show that rare gas complexes of d⁴ metal ions fit perfectly well into the coordination chemical framework based on the Jahn–Teller effect.     

Direct Determination of Gold in Rock Samples Using Collision Cell Quadrupole ICP-MS

This study investigated the determination of Au in rock samples using collision cell quadrupole inductively coupled plasma mass spectrometry (ICP-MS). It is essential to remove various interferents using a collision cell because polyatomic ions such as ¹⁸¹Ta¹⁶O⁺ and ¹⁸⁰Hf¹⁶O¹H⁺ can interfere with the direct determination of monoisotopic ¹⁹⁷Au when using ICP-MS. The addition of oxygen as a reaction gas removed isobaric interferents by transforming TaO⁺ and HfOH⁺ to TaO₂⁺, TaO₃⁺, and HfO₂H⁺, HfO₃H⁺, respectively, in the cell without significant Au⁺ loss. The ion kinetic energy effect (IKEE) due to the potential difference between the plasma and the hexapole affected the reactions in the cell. Au and interfering ions were very sensitive to cell bias voltage (Vc) at constant plasma potential (Vp) and quadrupole bias voltage (Vq). Under the condition of hot plasma, the transmission of ions was promoted, and the maximum Au signal intensity was 50% greater than under normal conditions. At Vc > 7 V, TaO⁺ ions were removed to background level. Optimized conditions for real sample analysis were obtained by introducing He as an additional collision gas in hot plasma. TaO⁺ ions were removed to background level at He flow rates above 0.6 mL min⁻¹, and the Au signal remained high. The detection limit (three times the standard deviation of the blank) of this method was 3.06 pg g⁻¹. The results for reference materials (STM-1 and DGPM-1) and spiked samples showed good agreement between specified and measured concentrations.     

Multiphoton ionization and photodissociation of (NO)mArn clusters

Picosecond multiphoton ionization of (NO)_mAr_n clusters produced in a supersonic expansion of NO/Ar gas mixtures has been studied using time-of-flight mass spectrometry. Two-photon ionization with 266 nm photons show that dilute gas mixtures (1% NO/Ar) yield clusters limited to m≤7, but with as many as 37 argon atoms. Magic numbers are observed for NO⁺Ar₁₂, NO⁺Ar₁₈, (NO) ₂ ⁺ Ar₁₇, NO⁺Ar₂₂, and (NO) ₂ ⁺ Ar₂₁ and are understood in terms of solvation of the NO⁺ and (NO) ₂ ⁺ by argon in icosahedral arrangements. Four-photon ionization with 532 nm light produces dissociation of the clusters to yield only NO⁺Ar_n with n up to 54. This distribution exhibits an additional magic number at n=54, consistent with the completion of a second solvation sphere about the NO⁺. The known wavelength dependence for photodissociation of (NO) ₂ ⁺ and (NO) ₃ ⁺ and comparison of MPI spectra obtained with 266, 355, and 532 nm light indicate that the dissociation is occurring in the cluster ions.     

Bestimmung neutraler Reaktionsprodukte in Ionen-Molekül-Reaktionen bei Drücken bis 1,3 mbar

An apparatus has been designed and built up to investigate reactions of accelerated ions with a neutral gas target. It was of great importance to analyze the neutral reaction products. Such reactions and these products played a significant role in the identification of the elements 104 and 105 by ZVARA et al. Preliminary results are reported for the systems Ar⁺+H₂ and Kr⁺+CO₂, in which neutral reaction products could be identified.     

High resolution studies of the origins of polyatomic ions in inductively coupled plasma-mass spectrometry,Part I. Identification methods and effects of neutral gas density assumptions,extraction voltage,and cone material

Common polyatomic ions (ArO⁺, NO⁺, H₂O⁺, H₃O⁺, Ar₂⁺, ArN⁺, OH⁺, ArH⁺, O₂⁺) in inductively coupled plasma-mass spectrometry (ICP-MS) are identified using high mass resolution and studied using kinetic gas temperatures (T_gas) determined from a dissociation reaction approach. Methods for making accurate mass measurements, confirming ion identifications, and correcting for mass bias are discussed. The effects of sampler and skimmer cone composition and extraction voltage on polyatomic ion formation are also explored. Neutral species densities at several locations in the extraction interface are estimated and the corresponding effects of the T_gas value are calculated. The results provide information about the origins of background ions and indicate possible locations for their formation or removal.     

Copper determination using ICP-MS with hexapole collision cell

Determination of copper using inductively coupled plasma mass spectrometry (ICP-MS) suffers from polyatomic overlays originating from Na⁺ and Mg²⁺ matrix elements due to the formation of ⁴⁰Ar²³Na⁺ and ⁴⁰Ar²⁵Mg⁺ in the mass-to-charge ratios of 63 and 65, respectively. The collision/reaction cell technology belongs to the most modern methods used to overcome polyatomic interferences. Gas-filled collision/reaction cell can cause an additional mass bias effect influencing analytical precision of the method. In this study, the additional mass bias effect of the hexapole collision/reaction cell ICP-MS was studied on an example of n(⁶⁵Cu)/n(⁶³Cu) isotope ratio. As a result, a method for suppressing polyatomic interference on the mass-to-charge ratio of 63 and 65 was introduced and additional mass bias of the collision/reaction cell was lowered to an acceptable level.     

Dissociation of polyatomic ions in the inductively coupled plasma

A general method for identifying the origin of a particular polyatomic ion is described. Based on a postulated dissociation reaction, measured ion signal ratios (e.g. Ar₂⁺/Ar⁺) are combined with mass bias corrections and estimates of the density of the neutral product (usually Ar, O or H atoms) to determine a gas kinetic temperature T_gas. The temperature can also be measured by the reduction in pressure when the ICP is sampled (compared to room temperature argon), or by other means. Dissociation energies and spectroscopic constants for the ions are necessary. For the particular instrument used, some of the findings of this study are: (a) ArO⁺ and ArN⁺ can be either dissociated (if the plasma potential is high) or created (if the plasma potential is low) by collisions between the sampler and skimmer; (b) the strongly-bound oxide ions O₂⁺ and MO⁺ for the rare earths are observed at levels consistent with T_gas ∼5300 K in a ‘hot’ plasma, but ClO⁺ is formed in excess; and (c) the abundances of most other polyatomic ions like H₂O⁺ and ArH⁺ correspond to higher densities than would be expected in the ICP itself.     

Methane Activation by Diatomic Molybdenum Carbide Cations

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas‐phase and condensed‐phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass‐selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time‐of‐flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C?C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C?H bond activation and C?C coupling involved in methane transformation over molybdenum carbide‐based catalysts.     

Investigation of the afterglow time regime in pulsed radiofrequency glow discharge time‐of‐flight mass spectrometry

The pulsed power operation mode of a radiofrequency (rf) glow discharge time‐of‐flight mass spectrometer was investigated, for several ions, in terms of intensity profiles along each pulse period. Particular attention was paid to the plateau and transient afterglow regions. An rf pulse period of 4 ms and a duty cycle of 50% was selected to evaluate the influence of discharge parameters in the afterglow delay and shape of Ar⁺, Ar₂⁺ and several analytes (Br, Cl, Cu) contained in polymeric layers. Pulse shapes of Ar⁺ and Ar₂⁺ ions vary with pressure and power. At low pressures the highest intensity is observed in the plateau while at higher pressures (>600 Pa) the afterpeak is the dominant region. Although the influence of the applied power is less noticeable, a widening of the afterglow time regime occurs for Ar⁺ when increasing the power. Maximum intensity of the argon signal is measured in the afterglow at 30 W, while the area of such afterpeak increases with power. The maximum intensity of Ar₂⁺ is obtained at the highest power employed (60 W) and the ratio maximum intensity/afterglow area remains approximately constant with power. Analytes with ionization potentials below (Cu) or just above (Br) the argon metastable energy show maxima intensities after argon ions decay, indicating they could be ionized by collisions with metastable Ar atoms. Chlorine signals are observed in the afterglow despite their ionization potential is well above the energy of argon metastable levels. Moreover, they follow a similar pattern to that observed for Ar₂⁺, indicating that charge‐transfer process with Ar₂⁺ could play a significant role. Copyright © 2011 John Wiley & Sons, Ltd.     

On the role of dissociative ionization in the formation of argon dimer ions

The formation of Ar ₂ ⁺ ions has been investigated by means of the threshold photoelectron photoion coincidence (TPEPICO) technique. Two pathways for the formation of Ar ₂ ⁺ ions are important. One is a direct path via excitation of Rydberg states of Ar₂ with consecutive autoionization. The other path is dissociative ionization of larger argon clusters, in this case argon trimers. These two pathways lead to Ar ₂ ⁺ ions with different internal energy. The pathways are easily distinguished in the TPEPICO-TOF spectra by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar ₃ ⁺ → Ar ₂ ⁺ + Ar was measured as a function of the photon energy and compared to the KER expected from statistical theory. The agreement is satisfying and confirms that Ar ₃ ⁺ ions do indeed dissociate at the thermochemical threshold. At higher photon energy the excited²Π(3/2)g state of Ar ₃ ⁺ is also detected from a second component in the KER. By applying a kinetic energy discrimination it is possible to measure cluster ion spectra in the presence of larger clusters but essentially without interference from the latter.     

Inductively coupled plasma mass spectrometry with an enlarged sampling orifice and offset ion lens. II. Polyatomic Ion interferences and matrix effects

A new inductively coupled plasma mass spectrometer with an enlarged sampling orifice (1.31-mm dia.) and an offset ion lens yields very low levels of many troublesome polyatomic ions such as ArO⁺, ArN⁺, Ar₂ ⁺, ClO⁺, and ArCl⁺. The signals from refractory metal oxide ions are ≈ 1% of the corresponding metal ion signals, which is typical of most ICP-MS devices. Grounding the first electrode of the ion lens greatly reduces the severity of matrix effects to <- 20% loss in signal for Co⁺, Y⁺, or Cs⁺ in the presence of 10 mM Sr, Tm, or Pb. This latter lens setting causes only a modest loss (30%) in sensitivity for analyte elements compared to the best sensitivity obtainable by biasing the first lens. Alternatively, matrix effects can also be mitigated by readjusting the voltage applied to the first lens with the matrix present.     

Metastable decay of rare gas cluster ions: mechanism and kinetics

Metastable decay of cluster ions has been discovered only recently. It was noted that one has to take this metastable decay into account when using mass spectrometry to probe neutral clusters, because ion abundance anomalies in mass spectra of rare gas and molecular clusters are caused by delayed metastable evaporation of monomers following ion production. Moreover, it was found that(i) the individual metastable reaction rates k depend strongly on cluster size and cluster ion production pathways and that(ii) there exists experimental evidence (k=k(t)) and a theoretical prediction that a given mass selected cluster ion generated by electron impact ionization of a nozzle expansion beam will comprise a range of metastable decay rates. In addition, it was discovered that metastable Ar cluster ions which lose two monomers in the μs time regime decay via sequential decay series Ar _n ⁺ *→Ar _n?1 ⁺ *→Ar _n?2 ⁺ * with cluster sizes 7≤n≤10 andn=3 (similar results were obtained recently in case of N₂ cluster ions). Conversely, the dominant metastable decay channel of Ar ₄ ⁺ * into Ar ₂ ⁺ was found to proceed predominantly via a single step fissioning process.     

Application of a hexapole collision and reaction cell in ICP-MS Part II: Analytical figures of merit and first applications

The application of an ion-guiding gas-filled hexapole collision/reaction cell in ICP-MS has been studied to characterize the analytical figures of merit that can be achieved with this approach. For the elements investigated, application of a buffer and a reaction gas resulted in improved sensitivities which are lowest for Be with about 7 · 10⁷ cps per μg mL^–1 and highest for Ba with about ¶6 · 10⁸ cps per μg mL^–1. Relative standard deviations (RSD) < 0.1% were obtained. Application of the reaction gas H₂ was used to suppress polyatomic ions caused by argon. The reduction amounted up to four orders of magnitude so that elements such as Ca, K, Cr, Fe, As and Se could be analyzed in nitric and hydrochloric acid or in methanol. Detection limits of 6 pg mL^–1 for Cr in 2% methanol, 23 pg mL^–1 for As and 9 pg mL^–1 for Se in 0.28 M HCl were achieved. For other elements detection limits ¶< 1 pg mL^–1 were realized in the medium and high mass range. Accuracy was proved using the NIST 1643d standard reference material.     

The adduction behavior of water reactant ions with mobility shift reagents in ion mobility spectrometry is determined by the number of locations for adduction,interaction energies,proton affinities,and steric hindrance of these species

The introduction of mobility shift reagents (SRs) into the buffer gas of mobility spectrometers yields SR-ion clusters that decrease ion mobilities and allow the separation of overlapping ions. With a large amount of papers on the introduction of SRs in ion mobility spectrometry (IMS) few investigations explain the behavior of the adducts of reactant ions with SRs and it is not clear what type of peaks to expect which obscures the interpretation of spectra. Electrospray-ionization IMS was coupled to quadrupole mass spectrometry, and 2-butanol (B), ethyl lactate (L), and methanol were introduced as SRs into the buffer gas. The hybrid functional X3LYP/6–311++(d,p) with Gaussian 09 was used for theoretical calculations of SR-ion interaction energies. Adducts of the reactant ions with B and L presented different behaviors; even at low flow rates, L consumed all sodium, reactant ions, and water by adduction, because a) in the experimental conditions, SRs were more concentrated in the buffer gas than reactant ions, b) L’s high proton affinity and c) L’s three electron-donor oxygens, increases adduction. Therefore, chemical equilibria in the buffer gas were only between L and L_nH⁺, L_mH₃O⁺, or L_xNa⁺ adducts and, consequently, these sets of adducts had different mobilities. The lower mobility of L_mH₃O⁺ compared to L_nH⁺ was explained on the base of the lower steric hindrance in LH₃O⁺ for attachment of L molecules. The behavior of reactant ions with B was different: B_nH₃O⁺ and B_nH⁺ overlapped because the relatively low proton affinity and the single and weaker interaction site in B allowed protons and water to be exchanged between species. Finally, L₄H⁺, L₄H₃O⁺, B₄H⁺ and B₅H⁺ ions, not reported before, were seen for large SR concentrations. This study explains two different behaviors of the adducts of SRs with reactant ions using interaction energies, proton affinities, steric hindrance, and the number of locations for adduction.