Spatially resolved measurements of ion density behind the skimmer of an inductively coupled plasma mass spectrometer

Ions are extracted from the inductively coupled plasma through a conventional sampler and skimmer and then deposited on an array of graphite targets at the exit of a set of electrostatic ion lenses. The Sc⁺ signal is enhanced by choosing appropriate potentials on the ion lenses. The Sc⁺ signal is suppressed by the presence of concomitant Cs ions at high concentrations. Comparisons of grounded ion lenses and two different ion lens potential settings are made. The signal is enhanced more extensively by the ion lenses when there are no concentrated concomitant ions. This study indicates that matrix effects in inductively coupled plasma mass spectrometry could possibly be alleviated by choosing ion lens potentials such that the ions enter the ion optics with a relatively broad beam cross section, the beam then being focused to a smaller size. A photon stop inside the ion lens stack reduces ion transmission and changes the shape of the beam profile from conical to bimodal.     

Changes in plasma composition during the expansion into the first vacuum stage of an inductively coupled plasma mass spectrometer

The effects of collisions on the composition of the plasma passing through the first vacuum stage of an inductively coupled plasma mass spectrometer were monitored in three sets of experiments. Rates of collisional quenching of an excited state in the neutral calcium atom were estimated from changes in experimental fluorescence lifetimes. Intensities from collisionally-assisted fluorescence provided evidence of energy transfer between excited states. Changes in analyte number density along the axis of the supersonic expansion in the first vacuum stage provided evidence that ion-electron recombination occurs to a significant extent during the expansion. Together, the experiments create a picture of the first vacuum stage in which collisions play an important role in shaping the composition of the plasma that is ultimately delivered to the mass analyzer.     

Optically determined velocity distributions of metastable argon in the second stage of an inductively coupled plasma mass spectrometer

Velocity profiles of argon metastable atoms have been measured in the second stage of an inductively coupled plasma mass spectrometer (ICP-MS). These profiles were obtained from laser-induced fluorescence excitation spectra of the Ar I 4s[3/2]° to 4p[5/2] transition. Velocities were determined from the Doppler shift of the exciting radiation. An argon-filled hollow cathode lamp served as a stationary wavelength reference. The velocity distribution at the skimmer orifice is bimodal, indicative of a flow disturbance at or slightly upstream of the skimmer orifice.     

A comparison of ion and atom behavior in the first stage of an inductively coupled plasma mass spectrometer vacuum interface: Evidence of the effect of an ambipolar electric field

Velocities of argon atoms and calcium ions were measured in the first vacuum stage of an inductively coupled plasma mass spectrometer using high-resolution laser-excited fluorescence spectroscopy. The calcium ions reached terminal velocities in the supersonic expansion that were consistently 5–6% higher than those of argon atoms, despite minimal differences in the masses of the two species. A computational model of the expansion was developed that shows the development of an ambipolar electric field along the expansion axis. With reasonable assumptions about electron temperatures in the expansion, the model accounts for the differences between the terminal velocities of the neutral argon atoms and the singly-charged calcium ions.     

Progress in analytical application of an inductively coupled plasma/mass spectrometer system

Summary A method for the determination of elements in liquids using inductively coupled plasma mass spectrometry (ICP-MS) is described. The different possibilities and problems together with instrumental parameters are reviewed. After the dissolution step samples are analyzed directly without any preconcentration. Detection limits for 53 elements have been tested and are found to be less than 0.1 ng/ml. Monoisotopic elements can be detected at levels less than 0.05 ng/ml. Interference problems have been studied and hints are given to predict molecular species using algorithm implemented in software packages. The application to trace element analysis in the geological field is demonstrated with a basalt reference sample to verify figures of merit for this method. Accuracy is checked with a NBS SRM steel sample.

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Erfahrungen und Möglichkeiten mit der Anwendung eines induktiv gekoppelten Plasma/Massenspektrometer Systems

     

Isotope measurements in uranium using a quadrupole inductively coupled plasma mass spectrometer (ICPMS)

A series of measurements were carried out to establish the reliability associated with isotope ratio (235/238) measurements on uranium samples using a quadrupole inductively coupled plasma mass spectrometer (ICPMS). Figures of merit related to the isotopic measurements were determined using non certified as well as certified materials provided by the New Brunswick Laboratory (NBL). The experimental results showed that repeatability is around 0.5% while reproducibility was calculated as 0.27%. Mass discrimination was determined as 0.03% per mass unit and the system linearity check over five orders of isotope ratios yielded a mass discrimination factor (K factor) of 1.0002±0.0081 (0.81%, 2s). The mean error of measurement obtained from six different certified reference materials was 0.77%.     

Characterization of the supersonic expansion in the vacuum interface of an inductively coupled plasma mass spectrometer by high-resolution diode laser spectroscopy

The supersonic expansion in the first vacuum stage of an inductively coupled plasma mass spectrometer has been characterized by laser-induced fluorescence of metastable argon atoms in the expansion. Atom velocities and temperatures were determined from Doppler shifts and linewidths, respectively, in the excitation spectra of the argon atoms. Shock structures characteristic of a supersonic expansion, the barrel shock and the Mach disk, were manifest as bimodal velocity distributions. The terminal velocities reached by the atoms were characteristic of conditions in the plasma source upstream from the entrance to the vacuum interface.     

Application of an orthogonal method for optimizing an inductively coupled plasma mass spectrometer.

The orthogonal method can be used to predict the optimum instrumental conditions and to guide the optimizing operations. This simplifies the ICP-MS optimization. In general, the final optimum parameter combination used in an analysis can be obtained after fine adjustments of the preferred parameters derived from the orthogonal experiment. This method can also be used to study the relationship between the parameter settings and the signal intensity deflection to improve the analysis of elements of a limited mass range or of isotopes.     

Use of a parallel path nebulizer for capillary-based microseparation techniques coupled with an inductively coupled plasma mass spectrometer for speciation measurements

A low flow, parallel path Mira Mist CE nebulizer designed for capillary electrophoresis (CE) was evaluated as a function of make-up solution flow rate, composition, and concentration, as well as the nebulizer gas flow rate. This research was conducted in support of a project related to the separation and quantification of cobalamin (vitamin B-12) species using microseparation techniques combined with inductively coupled plasma mass spectrometry (ICP-MS) detection. As such, Co signals were monitored during the nebulizer characterization process. Transient effects in the ICP were studied to evaluate the suitability of using gradients for microseparations and the benefit of using methanol for the make-up solution was demonstrated. Co signal response changed significantly as a function of changing methanol concentrations of the make-up solution and maximum signal enhancement was seen at 20% methanol with a 15 μl/min flow rate. Evaluation of the effect of changing the nebulizer gas flow rates showed that argon flows from 0.8 to 1.2 l/min were equally effective. The Mira Mist CE parallel path nebulizer was then evaluated for interfacing capillary microseparation techniques including capillary electrophoresis (CE) and micro high performance liquid chromatography (μHPLC) to inductively coupled plasma mass spectrometry (ICP-MS). A mixture of four cobalamin species standards (cyanocobalamin, hydroxocobalamin, methylcobalamin, and 5′ deoxyadenosylcobalamin) and the corrinoid analogue cobinamide dicyanide were successfully separated using both CE-ICP-MS and μHPLC-ICP-MS using the parallel path nebulizer with a make-up solution containing 20% methanol with a flow rate of 15 μl/min.     

Factors affecting analyte transport through the sampling orifice of an inductively coupled plasma mass spectrometer

Laser-excited ionic fluorescence has been used to study the effects of sample matrix, operating conditions, and load coil shielding on analyte ion transport efficiency through the sampling orifice of an inductively coupled plasma mass spectrometer. Significant changes in ion transport efficiency result from changes in sample composition, RF forward power, nebulizer flow and torch shield configuration. The changes in ion transport efficiency correlate well with changes in the potential recorded on a single floating probe placed 1 mm upstream from the sampling orifice.     

Fundamental aspects of ion extraction in inductively coupled plasma mass spectrometry

The present understanding of the ion extraction process in inductively coupled plasma mass spectrometry (ICP-MS) is reviewed critically. Topics include ion production in the ICP, origins of polyatomic ions, causes of and remedies for the secondary discharge, properties of the supersonic jet and of the beam leaving the skimmer, space charge effects, and matrix interferences. Areas of recent interest are also described from the perspective of the ion extraction process. These recent topics include “cool” plasmas, the three-aperture interface, ion extraction from helium plasmas, and ion sampling considerations unique to magnetic sector, time-of-flight, and ion trap mass spectrometers.     

More efficient ion transport for inductively coupled plasma mass spectrometry

A modification of the ion-lens system in an inductively coupled plasma mass spectrometer has been studied in an attempt to improve the ion transport efficiency. The modification consists of the placement of a metal rod down the axis of the lens-stack. The rod is suspended in place via a ceramic insulator inserted through the photon stop. A negative potential is applied to the rod to help focus the positively charged ions into the quadrupole. Prior to the implementation of this modification, a simple model was developed to determine if the voltages required would be practical. The resulting simulated ion trajectories showed that focusing occurs with applied voltages on the order of −10 V, which was approximately the optimal voltage found experimentally. The experimental data presented show that, with the modification, the limits of detection for 15 elements (m/z ranging from 24 through 238) improved by factors ranging from two to ten times over those of the unmodified instrument     

Determination of bromide, bromate and other anions with ion chromatography and an inductively coupled plasma mass spectrometer as element-specific detector

An implementation of the Dionex IonPac AS12A analytical column with an element-specific ICP-MS detection is described for the simultaneous determination of halogen and oxyhalogen anions, sulfate, phosphate, selenite, selenate and arsenate. The chromatographic separation was achieved in less than 4 min with an aqueous 11 mM (NH4)2CO3 (pH 11.2, adjusted with aqueous ammonia) as eluent. Special emphasis was given to optimize the ICP-MS detection conditions for the reliable detection (RSD<5%) of bromate and bromide at a bromine concentration level of 1.0 microg l(-1) with 50 microl sample injection volume. In order to achieve the highest detector response for bromine species an ultrasonic nebulizer equipped with a membrane desolvator had to be employed. The detection limits (S/N=3, sample injection volume 50 microl) obtained with the IC-ICP-MS after the optimization were 0.67 microg l(-1) for BrO3-, 0.47 microg l(-1) for Br-, 69 microg l(-1) for ClO2-, 4 microg l(-1) for Cl-, 47 microg l(-1) for ClO3-, 13 microg l(-1) for SO4(2-), 36 microg l(-1) for PO4(3-), 0.4 microg l(-1) for SeO3(2-), 0.3 microg l(-1) for SeO4(2-), and 0.4 microg l(-1) for AsO4(3-).     

Characterization of the ion beam inside the skimmer cone of an inductively coupled plasma mass spectrometer by laser excited atomic and ionic fluorescence

Laser-induced atomic and ionic fluorescence have been used to characterize the material extracted from an inductively coupled plasma through a differentially pumped interface of the type used in inductively coupled plasma mass spectrometry. Measurements were made in the 8 mm downstream from the tip of the skimmer cone. Reference measurements were also made outside the interface at the tip of the sampling cone. Sc and Ba ions and Pb atoms were used as test analytes. Sc⁺ densities drop more rapidly than either Ba⁺ or Pb densities. The transmission efficiencies of both Sc⁺ and Ba⁺ are suppressed by the addition of either Mg or Pb to the analyte solution. The effects of the two matrix additions are approximately the same. Based on the magnitude of the fluorescence signal for Ba⁺, a lower limit of 0.3% for the transmission of Ba⁺ from the plasma into the second vacuum stage has been calculated.     

Ion trajectories through the input ion optics of an inductively coupled plasma-mass spectrometer

A commercial program (MacSimion) has been used to model the einzel lens and bessel box input lens system of an inductively coupled plasma-mass spectrometer (ICP-MS). A number of plots are presented illustrating the effect of various lens voltages on ion trajectories. The trajectories are dependent on ion kinetic energy and since ion kinetic energy is mass dependent in ICP-MS, these plots clarify the mass dependence of ion lens voltage settings. Plots are also presented illustrating the interdependence of some lens voltage settings and how, in fact, different lens voltage settings can result in similar ion throughput. The model can be used to predict relative signal intensities for a range of ion masses as a function of lens potentials and these are shown to agree with experimentally measured data.     

Comparison of mass spectrometric and optical measurements of temperature and electron density in the inductively coupled plasma during mass spectrometric sampling

Electron density (n_e) and ionization temperature (T_ion) are measured using atomic emission spectrometry (AES) from the small funnel of gas just outside the sampling orifice of an inductively coupled plasma-mass spectrometer (ICP-MS). Rotational temperature (T_rot) is measured using an OH emission band. T_ion is also determined for the same elements (Zn and Cd) by using M⁺ ion signal ratios by MS. For matrix-free solutions, typical values are n_e=1.6×10¹⁵ cm⁻³, T_rot=3340 K, T_ion (MS)≈T_ion (AES)≈7000 K. This agreement between the T_ion values supports other observations that, for atomic analyte ions M⁺ of similar m/z values in matrix-free solutions, the relative signals in the mass spectrum reflect the corresponding relative abundances in the ICP region being drawn into the sampler. Using either MS or AES, T_ion for Cd is 300–400 K higher than that for Zn, which indicates that T_ion can vary for different elements in the ICP. Sodium nitrate matrix at levels up to 1000 ppm Na does not cause a measurable change in n_e; 2000 ppm Na causes n_e to increase to 2.1×10¹⁵ cm⁻³. Sodium matrix has a large effect on the MS signal levels but does not greatly change the resulting T_ion values measured optically.     

Molecular hydrogen emission in the vacuum ultraviolet from an inductively coupled plasma

Vacuum ultraviolet radiation is observed through a sampling orifice in a metal cone inserted directly into an Ar inductively coupled plasma (ICP). The many-lined spectrum of the H₂ Lyman bands is observed from an argon ICP with either H₂ gas or nebulized water added. The most prominent bands are from the ν = 3, 4, 5 or 6 vibrational levels of the B ¹Σ⁺_u electronic state of H₂. The energies of the ν = 3, 4 and 5 levels are close to those of the Ar metastable levels and of photons from the Ar resonance lines (11.5 to 11.8 eV). These H₂ levels are probably populated at least in part by selective energy transfer reactions between Ar metastable atoms and H₂ and/or by absorption of Ar resonance photons by H₂. The H₂ emission emanates only from the upstream reaches of the axial channel, i.e. the part of the axial channel that is inside the load coil and induction region.     

Antimony speciation in environmental samples by interfacing capillary electrophoresis on-line to an inductively coupled plasma mass spectrometer

Antimony is a widely distributed trace element of ecotoxicological interest. A pathway via bioalkylation of inorganic Sb species is discussed in the literature, resulting in organically bound Sb species. Therefore, Sb speciation becomes increasingly a matter of interest for risk assessment in the environment. This contribution investigates the possibilities of CE on-line hyphenated to ICP-MS for Sb speciation. Two methods are employed, both highly resolving the species but only one preserving the species stability. The latter used Na2HPO4/NaH2PO4, 20 mM, pH 5.6 as the background electrolyte and NaOH or acetic acid as stacking electrolyte 1 or stacking electrolyte 2, respectively. Detection limits of 0.1 microgram/1-0.7 microgram/1, depending on species, were achieved. When analysing liquid phases from fouling and sewage sludge up to eight antimony species were detected. Sb (V) as well as methylated Sb species were found.     

Uranium isotopic analyses by using a sector field inductively coupled plasma mass spectrometer

A sector field mass spectrometer using an inductively coupled plasma as an ion source was used in order to determine the uranium isotopic ratios in reference materials supplied by the National Institute of Standards and Technology (NIST) (NBS 950, 010, 030, 200, 500, 750, and 970). The accuracy obtained for the major isotopes was better than 0.2%.     

Optical characteristics of an afterglow extracted from an inductively coupled plasma

An inductively coupled plasma is extracted into a quartz vacuum chamber through an orifice to observe optical characteristics of the extracted afterglow. The Mach disk, the barrel shock and the zone of silence, which are familiar in supersonic molecular beam experiments with neutral gases, are clearly observed. Axial profiles of the emission of various lines are measured with a photodiode array spectrometer. Intensities of ionic lines as well as atomic lines are stronger at the Mach disk than in the zone of silence. The location of the Mach disk with varying chamber presure is in conformity with an experimental equation obtained in supersonic molecular beam experiments.