Quantification of water and plasma diagnosis for electrothermal vaporization–inductively coupled plasma–mass spectrometry: the use of argon and argide polyatomics as probing species

The water content of the carrier flow originating from an electrothermal vaporization unit (ETV) attached to an inductively coupled plasma mass spectrometer was monitored by following the argon hydride ion (ArH⁺) at m/z=37. The goal was to measure the water expelled by the ETV at sample vaporization and evaluate the influence of this parameter on the ion-generation efficiency. Linear responses from the argon hydride were obtained when the water loading in the plasma injector flow was increased from 0 to 3.3 mg/min. Other argides and water-derived species (Ar⁺, Ar⁺₂ and O⁺₂) were also monitored simultaneously and the effects from operating parameters have been calculated for each species. The magnitude of these effects can eventually be used as diagnosis tools. It was also found that signals for zinc, copper, lead, antimony and arsenic were greatly influenced by slight variations in water loading at low water levels. These signal fluctuations are greatly attenuated and transients' shapes restored by convoluting each element transient with the ArH⁺ or Ar⁺₂ curves that were recorded simultaneously. Envisioned applications that would benefit from a water-enhanced signal include spray electrothermal vaporization, direct sample insertion and laser ablation for inductively coupled plasma–mass spectrometry. The argon dimer Ar⁺₂ seems more appropriate for making the correction since it provides a direct insight on the plasma temperature and provides a robust signal.     

Analytical atomic spectroscopy using ion trap devices

Investigations using ion trap devices for analytical atomic spectroscopy purposes have focused on the use of an inductively coupled plasma (ICP) ion source with ion trap mass spectrometric (ITMS) detection. Initial studies were conducted with an instrument assembled by simply appending an ion trap as the detector to a fairly conventional ICP/MS instrument, i.e. leaving an intermediate linear quadrupole between the plasma source and the ion trap. The principal advantages found with this system include the destruction of nearly all problematic and typical ICP/MS polyatomic ions (e.g., ArH⁺, ArO⁺, ArCl⁺, Ar₂⁺, etc) and a dramatic reduction of the primary plasma source ion, Ar⁺. These results prompted the development of a second-generation plasma source ion trap instrument in which direct coupling of the ICP and ion trap has been effected (i.e. no intermediate linear quadrupole); the same performance benefits have been largely preserved. Initial operation of this instrument is described, characterized, and compared to the originally described ICP/ITMS and conventional ICP/MS systems. In addition, experiments aimed at improving ICP/ITMS sensitivity and selectivity using broadband resonance excitation techniques are described. Finally, the potential for laser optical detection of trapped ions for analytical purposes is speculated upon.     

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.     

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.     

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     

Identification of Fe‐polycarboxylic complexes by electrospray ionization mass spectrometry and reduction of interferences by ion chromatography/inductively coupled plasma mass spectrometry with an octopole reaction system

Stable complexes are required during the ion chromatographic (IC) separation of Fe‐polycarboxylic acid complexes. Electrospray ionization mass spectrometry (ESI‐MS) was used to identify 1:1 stoichiometric complexes of Fe[HEDTA], Fe[EDTA]^1? and Fe[DTPA]^2?, and the spectra showed that these Fe complexes were stable in solution. Furthermore, inductively coupled plasma mass spectrometry (ICP‐MS) using an octopole reaction system (ORS) reduced polyatomic ion ⁴⁰Ar¹⁶O⁺ interference in the detection of ⁵⁶Fe via the addition of either H₂ or He to the ORS, with He at a flow rate 3.5 mL min^?1 being the optimum collision gas. Finally, IC/ICP‐MS was used for the separation and detection of Fe complexes with an eluent containing 30 mM (NH₄)₂HPO₄ at pH 8.0, but only Fe[HEDTA], Fe[EDTA]^1? and Fe[DTPA]^2? were observed within 10 min with reasonable resolution. Detection limits in the range of 10–13 µg L^?1 were achieved using He as the collision gas. The proposed method was used for the determination of Fe species in soil solutions. Copyright © 2010 John Wiley & Sons, Ltd.     

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

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.     

Charge transfer reactions between xenon ions and iron atoms in an argon-xenon inductively coupled plasma

Xenon is added to the axial channel of an argon inductively coupled plasma (ICP) at doses up to 1.5% of the aerosol gas flow. Emission is collected from the gas flowing into the sampling orifice of a mass spectrometer (MS). These Xe doses have little effect on the electron density n_e or on the intensities of Fe (I) emission lines. Certain Fe (II) lines are enhanced when Xe is added, particularly those from Fe⁺ states that can be populated by near-resonant charge transfer between Xe⁻ and neutral Fe. Calculations based on measured values of n_e indicate that Xe⁺ should be present at densities of up to 7 × 10¹⁴ cm⁻¹, which should be sufficient Xe⁺ to drive the proposed charge transfer reactions.     

Alleviation of polyatomic ion interferences for determination of chlorine isotope ratios by inductively coupled plasma mass spectrometry

A simple variation in sample preparation and introduction allows the measurement of chlorine isotope ratios by inductively coupled plasma mass spectrometry (ICP/MS). Dissolution of the sample in D₂O rather than H₂O attenuates the major polyatomic ion ³⁶ArH⁺ and frees m/z 37 for determination of ³⁷Cl⁺. The isotope ratio ³⁵Cl/³⁷Cl in a 50 mg/L solution of Cl as LiCl is determined with a relative standard deviation of 0.21%. Sample memory is low, as the ³⁵Cl signal decays to less than 1% of its original value after ～2 min of cleanout with D_2O . The detection limit for Cl using this procedure is approximately 20 μg/L.     

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.     

Comparison of temperature measurements in ICP and MIP with Ar and He as plasma gas

Various temperature measurements have been carried out in microwave induced plasmas (MIP) generated with a surfatron and inductivcly coupled plasmas (ICP) both with argon and helium as plasma gas. Iron has been used for the determination of excitation temperature, and OH and N⁺₂ for rotational temperatures. In the case of the Ar ICP, equilibrium is attained between the various temperatures (4500 K), as previously described. On the other hand, in the He ICP and the MIPs, iron provides the highest temperature (4500 K) but discrepancies are obtained with results from N⁺₂ and OH. These two species show lower values, especially OH (2000 K).     

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.     

Determination of hafnium at the 10% level (relative to zirconium content) using neutron activation analysis,inductively coupled plasma mass spectrometry and inductively coupled plasma atomic emission spectrometry

Hafnium at the very low level of 1–8 ppm (in relation to zirconium) was determined in zirconium sulfate solutions (originating from investigations of the separation of ca. 44 ppm Hf from zirconium by means of the ion exchange method) by using three independent methods: inductively coupled plasma mass spectrometry (ICP MS), neutron activation analysis (NAA) and inductively coupled plasma atomic emission spectrometry (ICP-AES). The results of NAA and ICP MS determinations were consistent with each other across the entire investigated range (the RSD of both methods did not exceed 38%). The results of ICP-AES determination were more diverse, particularly at less than 5 ppm Hf (RSD was significantly higher: 29–253%). The ion exchange method exploiting Diphonix^® resin proved sufficient efficiency in Zr–Hf separation when the initial concentration ratio of the elements ([Zr]₀/[Hf]₀) ranged from 1200 to ca. 143,000.     

Real-time analysis of CuO by inductively coupled plasma emission without external calibration

The study of a method, devoted to real-time detection of metallic pollutants present in stack gas, is investigated. This method is based on spectroanalysis using an inductively coupled plasma (ICP) emission system without external calibration. The fluidized bed technology is employed to inject metallic species into the ICP emission. The mass fluxes of copper oxide (CuO) are then determined by using the intensity ratios of the metallic element spectral lines with those of the plasma gas element (argon or dry air). These ratios and the plasma characteristics (atomic excitation temperature, degree of thermal disequilibrium θ=T_e/T_h) are inserted into a calculation code of plasma composition to determine the mass flux. The results are in good agreement using either argon plasma or dry air plasma. A study of the fluidized bed properties is made to compare our values with those resulting from the elutriation calculation of the copper oxide.     

Combination of cool plasma and collision-reaction interface for correction of polyatomic interferences on copper signals in inductively coupled plasma quadrupole mass spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) is an important instrumental technique for elemental analysis. However, some elements suffer from spectral interferences caused by ions derived from argon plasma gas and matrix components. The determination of copper isotopes is affected by ⁴⁰Ar²³Na⁺ and ⁴⁰Ar²⁵Mg⁺. The performance of an ICP-MS with a collision reaction interface (CRI) and cool plasma conditions for correction of spectral interferences was evaluated here. The efficiency of the CRI was studied introducing H₂ or He through sampler and skimmer cones. Gas introduction through the sampler cone was ineffective. Complete elimination of spectral interferences was reached when introducing 60 or 80 mL min⁻¹ of H₂ in the skimmer cone, but sensitivity losses were as large as 99%. Further, the effect of interferences was checked when the argon plasma was operated under cool plasma conditions. The effects of the applied radiofrequency (0.6, 0.8, 0.9, and 1.0 kW), sampling depth (5.5, 8.5 and 11.5 mm), and dwell time (25 and 50 ms) were studied considering interference reduction and sensitivities. Best conditions were reached at 0.8 kW. Subsequently, both CRI and cool plasma conditions were combined to evaluate their performance on reduction of polyatomic Na and Mg argide interferences. Spectral interferences were eliminated using a CRI with 20 mL min⁻¹ H₂ introduced through the skimmer cone, cool plasma conditions at 0.8 kW and sampling depth of 8.5 mm. This work demonstrated the feasibility of combining CRI and cool plasma for circumventing some spectral interferences on Cu determination by ICP-QMS.     

Determination of dissociation temperature for ArO in inductively coupled plasma-mass spectrometry: Effects of excited electronic states and dissociation pathways

The method of comparing experimental and calculated ion ratios to determine a gas kinetic temperature (T_gas) characteristic of the origin of a polyatomic ion in inductively coupled plasma-mass spectrometry (ICP-MS) is applied to ArO⁺. Repeated measurements of ion ratios involving this species yield erratic T_gas values. Complications arise from the predicted presence of a low-lying excited electronic state (²Π) above the ⁴Σ ground state. Omission of this excited state yields unreasonably high temperatures (> 10,000 K) for nine out of nineteen trials. Inclusion of the excited electronic state in the partition function of ArO⁺ causes temperatures to increase further. The problem appears to be related to the prediction that ArO⁺ in the ²Π excited state dissociates into Ar⁺ and O, different products than ArO^{+ 4}Σ which dissociates into Ar and O⁺. Adjustments to the calculations to account for these different products yield reasonable temperatures (2100 to 3500 K) that are consistent from day-to-day and similar to those seen for other weakly-bound polyatomic ions.     

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.     

High yield synthesis and characterization of isotopically enriched monoethylmercury chloride (C2H5201HgCl)

An important but commercially unavailable compound isotopically enriched monoethylmercury chloride (C₂H₅²⁰¹HgCl), has been synthesized from commercially available ²⁰¹HgO (98.11% enriched isotopic purity) and tetraethyltin. The required synthesis time is 1 h at 90 °C, and the product is the single product of monoethylmercury chloride, yielding more than 95% as ²⁰¹Hg in C₂H₅²⁰¹Hg⁺ (98.19 ± 0.22% enriched isotopic purity). The synthesized product was analyzed with high‐performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC‐ICP‐MS) to determine its concentration, isotopic composition and purity. The synthetic isotopically enriched monoethylmercury synthesized can be used in speciated isotope dilution mass spectrometry (SIDMS) and isotope dilution mass spectrometry (IDMS) analyses as a standard. Copyright © 2005 John Wiley & Sons, Ltd.