The effect of the sampling interface on spatial distributions of barium ions and atoms in an inductively coupled plasma ion source

Planar laser-induced fluorescence was used to examine the effect of the sampling cone on analyte atom and ion distributions in the inductively coupled plasma used as an ion source for elemental mass spectrometry. Comparisons of planar laser-induced fluorescence images in the presence and absence of the sampling interface reveal that the insertion of the sampling cone into the plasma dramatically lowers singly-charged ion densities in the 1–2 mm region immediately upstream from the sampling cone, but increases densities in the region between 2 mm and 10 mm upstream from the sampling cone. Some of the drops in densities near the sampling cone can be attributed to acceleration of the plasma through the pumped sampling orifice. A shift in equilibrium between doubly and singly charged barium ions caused by cooling of the plasma is proposed to account for the increases in densities of Ba⁺ in the upstream region.     

Modeling the gas flow upstream and in the sampling nozzle of the inductively coupled plasma mass spectrometer via the Direct Simulation Monte Carlo algorithm

The Direct Simulation Monte Carlo algorithm has been applied to the flow of neutral argon gas through the first vacuum stage of the Inductively Coupled Plasma Mass Spectrometer. Good agreement is found between the simulation results and the equations of fluid dynamics, including the approximate hemispherical sink model of Douglas and French. The simulation reveals details of boundary layer formation in the nozzle, including a reduction in the total flow through the nozzle of about 15% from the ideal value calculated by Douglas and French.     

Toward a fuller understanding of analytical atomic spectrometry.

Although inductively coupled plasma emission and mass spectrometry have been in widespread use for a long time, there is still a gap in our knowledge concerning how atoms are formed in the ICP, by which mechanisms they are excited, how ionization occurs, and how these factors are influenced by operating parameters of the ICP such as gas flow rates, radiofrequency power, and viewing or sampling position. Furthermore it is still not clear how or whether the sampling cone of the mass-spectrometer interface affects the ICP itself. Although many measurements have been made downstream from the sampling orifice, less is clear about the influence of the interface upstream of the sampling process. In our laboratory is available a suite of measurement tools and techniques that are being exploited to address these questions. These techniques permit us to map, on a spatially resolved basis, such critical features as electron-energy distributions, gas-kinetic temperatures, ground-state analyte atom and ion number densities, electron concentrations, and densities of excited-state argon species. In turn, these parameters can be measured in the presence and absence of suspected interferents and in the presence and absence of an ICP-MS sampling cone. Here, the basis of these measurements will be described. Also, based on recent results, mechanisms of excitation of analyte atoms and ions will be proposed and the influence of the ICP-MS sampling cone will be examined.     

High resolution imaging of barium ions and atoms near the sampling cone of an inductively coupled plasma mass spectrometer

Planar laser-induced fluorescence was used to map density distributions of ground state barium atoms, ground state barium ions, and excited-state barium ions in the region between the load coil and the sampling cone of an inductively coupled plasma mass spectrometer. The effects of power, nebulizer gas flow rate, and the addition of lithium to the sample on the distributions were studied. The maps reveal that the radial distributions of atomic species across the diameter of the plasma are compressed as the plasma is drawn into the sampling orifice, and that as a result of that compression, the distribution of ions across the 1-mm diameter of the sampling orifice is non-uniform. The distribution changes as conditions in the plasma change. The skimmer cone that separates the first and second stages of the differentially-pumped vacuum interface transmits ions exclusively from the center of the distribution that exists at the sampling cone. As a result, the overall efficiency with which ions are transmitted through the vacuum interface varies as conditions in the plasma change.     

Comparison between Two Turbulence Models and Analysis of the Effect of the Substrate Movement on the Flow Field of a Plasma Jet

In the first part of the present study, an appropriate inflow turbulent boundary condition is chosen. Then, a comparison is made between two turbulence models for a plasma jet discharged into air atmosphere. The plasma jet gas phase flow is predicted with the standard k–ɛ model and the RNG model of turbulence. Particles behavior is modeled using stochastic particles trajectories. A validation of the plasma jet model is made by comparison with experimental data. This part of the study shows that the flow features are better predicted with the RNG model. The choice of appropriate boundary conditions seems to be crucial for a better simulation of plasma thermal spraying. Afterwards, computations are performed for projection of Ni particles. It is found that the computed particles velocities and temperatures are also better predicted with the RNG model compared with the k–ɛ model. The second part of this study is concerned with the effect of the substrate movement on the gas flow field. This is performed in order to simulate a realistic coatings process where a relative movement between the torch and the substrate always exists. Three substrate velocities have been used and it is found that the flow fields are affected only very near the substrate wall.     

Enhanced heat transfer in pin fin heat sink working with nitrogen gas–water two-phase flow: variable pin length and longitudinal pitch

In the current study, thermal–hydraulic characteristics of nitrogen gas–water two-phase flow through a plate–pin fin heat sink are investigated experimentally. Water flow through the smooth case, i.e., heat sink without pin fin, is considered as baseline. Four new models of the pin fin with variable longitudinal pitch and pin length having low-to-high and high-to-low arrangements are proposed. They are named for short as LP–LH, LP–HL, PL–LH, and PL–HL. The results indicate that in all heat sinks, the Nusselt number values of the two-phase flow in comparison with those of the single-phase flow are higher, but the friction factor values of the two-phase flow in comparison with those of the single-phase flow are lower. Also, the friction factor and the Nusselt number of both the single-phase flow and the two-phase flow in the heat sinks with pin fin are greater than those in the heat sink without pin fin. The highest values of the Nusselt number are recorded for PL–HL at water mass flow rate of 0.0093 kg s^?1 and gas volume flow rate of 0.8 L min^?1. At these flow rates in PL–HL, the Nusselt number of the two-phase flow is increased about 45.6% relative to the single-phase flow.

     

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.     

Electrokinetic diffusioosmotic flow of Ostwald-de Waele fluids near a charged flat plate in the thin double layer limit

Electrokinetic diffusioosmotic flow of Ostwald-de Waele, or power-law, fluids near a large charged flat plate is theoretically investigated for very thin double layers. Solutions to the flow velocity both up-close and far from the flat plate as well as the effective viscosity are presented for general values of the flow behavior index. Results show that given a wall zeta potential, ζ, diffusivity difference parameter, β, and constant imposed solute concentration gradient, both the near and far field diffusioosmotic flow velocities obtained for the respective dilatant and pseudoplastic liquids considerably deviate from those obtained for Newtonian liquids as found in previous literature. This likely suggests that the electrokinetic diffusioosmosis and its complementary effect of diffusiophoresis depend sensitively not only on the ζ-β parametric pair, but also on the possible non-Newtonian characteristics of the electrolytic liquid phase of the system. The theory presented herein can also be readily modified to model or describe electrodiffusioosmosis in power-law fluids, which is likely found in flow situations where the fluid non-Newtonian response, imposed solute concentration gradient, and an additional externally applied electric current density (or electric field) are of equal importance.     

Influence of velocity and surface temperature of alumina particles on the properties of plasma sprayed coatings

In this paper are described the main characteristics of the plasma spraying process of alumina deposits, i.e., the temperature and flow field of the plasma jets obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation. The measurements on the alumina particles are compared with the predictions of a mathematical model. The experimental and computed particle velocities are in rather good agreement. However, this is not the case for the particle surface temperature. Possible reasons for the discrepancy are proposed (influence of the carrier gas, thermophoretic forces, and poor penetration of the particles into the plasma core even for an injection velocity twice that of the optimal calculated one, as shown by recent measurements). Finally the correlations between the particle velocities and surface temperature, and the properties of the alumina coating (porosity, crystal structure, mechanical properties) are studied.     

In vitro study of experimental factors affecting the microdialysis results

The aim of the present study was to determine the influence of a series of experimental conditions (probe, perfusate flow rate and the method used to ascertain recovery) as well as the pharmacokinetic variables (concentration and time) on the estimation of the recovery coefficient of microdialysis probes. Two in vitro pharmacokinetic assays were also carried out to compare the results provided by microdialysis and those obtained with traditional sampling. The probes used were made in our laboratory and ciprofloxacin was used as a model compound. The results revealed that in all cases recovery was dependent on the probe and independent of time for a 80 min sampling time period. The effects of concentration on recovery depend on the flow rate; this was not statistically significant for a flow rate of 2 μl/min but an increase in flow rate to 6 μl/min transformed this parameter into a concentration-dependent variable. A decrease in recovery parallel to the increase in flow rate was found, with an exponential relationship between the two variables. Statistically significant differences were also found between the recovery values obtained for direct dialysis (18.44±1.61) and retrodialysis by loss of the analyte of interest (16.79±3.42). The values of the protein binding of ciprofloxacin as calculated by equilibrium dialysis and by microdialysis were similar. Characterization of the in vitro kinetic profile revealed no statistically significant differences for coefficients and exponents obtained by traditional sampling and microdialysis, although the confidence intervals of the curves were wider for microdialysis.     

Low flow,externally air cooled torch for inductively coupled plasma atomic emission spectrometry with axial viewing

The torch wall is cooled largely by air passing through a cooling jacket added to the outside of a Fassel torch. The plasma is viewed axially through a cooled cone interface centered on the axial channel. The outer argon gas flow can be reduced to 7 l min⁻¹ with no compromise in performance or torch lifetime. The plasma exhibits the same ‘robustness index’ and interference effects from Na as the conventional, high-flow ICP supplied with the particular spectrometer used. Detection limits (DL) for lines at ∼200 nm are poorer by approximately a factor of two, while those for lines at ∼400 nm are actually better than values typically seen for the same lines by axial viewing of a conventional, high-flow ICP.     

Study of deposition offset in plasma spray of zirconia

Numerical modeling and experimental measurements have been performed to study the effects of powder carrier gas flow rates and powder sizes on the deposition offset in a plasma spray of yttria-stablized zirconia. The mathematical model involved simultaneous solution of the continuity, momentum and energy equations of the plasma gas, the dynamics and heat transfer of powder particles in the plasma, and the coupling effects between the plasma and panicles. Experiments included measurement of particle velocities by laser strobe technique and measurement of deposition offset. Calculated plasma temperatures and velocities are greater than 13,000 K and 2,000 m/s, respectively, in the vicinity of nozzle exit. For the plasma-particle momentum transfer, the drag coefficient was computed in two ways- with corrections accounting for the strongly varying plasma properties, and without these corrections. Calculated and experimental results, in respect to deposition offset, are in agreement to within 25% when calculated without varying properties corrections, and within about 40% with corrections; agreement in respect to average particle velocities is within 20% when calculated without varying properties corrections, and within the range 30–50% with corrections.     

Hydrodynamic voltammetry at channel electrodes: Part VII. Current transients at double channel electrodes

Expressions for the transient current at the downstream electrode in response to galvanostatic electrolysis at the upstream electrode in the channel flow cell were derived by applying double Laplace transformation when the electrode reaction at the upstream electrode is kinetically controlled. The ratio of the transient current to the steady state current or the transient collection efficiency was calculated as a function of electrode geometry and θ

, where U_m is the mean flow velocity in the channel cell, D the diffusion coefficient of the electroactive species, b the half height of the channel, x₁ the length of the upstream electrode and t the time elapsed since the beginning of the galvanostatic electrolysis at the upstream electrode. Curves for the transient collection efficiency can be applied to evaluating the amount of adsorption at the upstream electrode when metal at the electrode is anodically dissolved in solution. Digital simulation was carried out. Transient curves, obtained analytically, were in good agreement with those evaluated from the digital simulation. In order to allow one to draw transient curves readily, we derived a simple approximate equation.     

Improvement of plasma spraying efficiency and coating quality

A commercial torch has been modified to introduce an additional anti-vortex and shroud gas flow to counter the detrimental effects brought about by the vortex plasma gas flow which is used to stabilize the cathode arc attachment and to increase the anode life. Deposition efficiency and coating quality are used as criteria to judge the modified versus the nonmodified torch. High-speed videography and computerized image analysis systems are used to determine the particle trajectories, velocities, and the plasma jet geometry. The results show that the additional anti-vortex and shroud gas flow to the torch can keep the particles closer to the torch axis and reduce the amount of entrainment of cold air into the plasma jet. The consequence is that deposition efficiency and coating quality are substantially improved.     

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.     

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.     

Advantages of using a modified orthogonal sampling configuration originally designed for LC-ESI-MS to couple CE and MS for the determination of antioxidant phenolic compounds found in virgin olive oil

A ThermoFinnigan sheath liquid flow capillary electrophoresis-mass spectrometry system designed for coupling via a co-axial interface was coupled through an adapted via an alternative, commercially available interface for orthogonal sampling. The affordable, reversible structural alterations made in the commercial LC-MS interface resulted in improved analytical performance.The results of a conventional capillary electrophoresis (CE) method using a commercial co-axial source to determine antioxidant phenolic acids present in virgin olive oil, were compared with those obtained by using a modified orthogonal sampling position. In both cases, separations were done using a 10 mM ammonium acetate/ammonium hydroxide buffer solution at pH 10.0 and a constant applied voltage of 25 kV. The operating variables for the mass spectrometry interface were re-optimized for the modified orthogonal orientation. This allowed the sheath liquid, sheath gas flow rates and capillary voltage to be lowered with respect to the co-axial coupling configuration. In addition, the orthogonal sampling position provided a higher selectivity by effect of ion sampling excluding larger droplets—with an increased momentum along the axis—which were drained through the sink at the bottom of the ion source. Also, the new configuration facilitated sample ionization, improved electrospray stability and led to stronger signals as a result.The new system was validated in terms of precision (repeatability), linearity, and limits of detection and quantification. A comparison of the validation data with the results previously obtained by using a commercial co-axial configuration revealed the adapted orthogonal sampling position to provide better repeatability in both migration times and relative peak areas (<1% and 7% respectively with n = 15 replicates), a good linear range (with levels in the microgram-per-litre region) and lower limits of detection—especially for the compounds detected with the lowest sensitivity when co-axial ESI was used, as HFA, GEN, FER and VAN finding LOD among 24-3.0 μg L⁻¹ respectively.     

Computational fluid dynamics modeling of multicomponent thermal plasmas

A comprehensive computational model has been developed Jbr flowing thermal plasmas in the absence of electromagnetic fields, with particular emphasis on plasma jets. The plasma is represented as a rnulticomponent chemicalh, reacting ideal gas with temperature-dependent thermodynamic and transport properties. The plasma flow is governed by the transient compressible Navier-Stokes equations in two or three space dimensions. Turbulence is represented by subgrid-scale and k- models. Species diffusion is calculated by an effective binary diffusion approximation, generalized to allow /or ambipolar diffusion of charged species. Ionization, dissociation, recombination, and other chemical reactions are computed by general kinetic and equilibrium chemistry algorithms. Radiation heat loss is currently modeled as a temperature-dependent energy sink. Finite-difference approximations to the governing equations are solved on a rectangular spatial mesh using explicit temporal differencing. Computational inefficiency at low Mach number is avoided br reducing the effective sound speed. The overall computational model is embodied in a new computer code called LAVA. Computational results and comparisons with experimental data are presented Jbr LAVA simulations of a steady-stare axisymmetric argon plasma jet flowing into cold argon.     

A comparison of two techniques for inducing fibrous crystallization from solution

Experimental results are presented and discussed comparing two techniques for inducing fibrous growth of polyethylene crystals from flowing solutions: a plunging cone device and a standard rotating cylinder (Couette) device. Comparison of crystal “overgrowth” diameter to spacing ratios for the several experiments with predictions based on an earlier postulated model lead to conclusions regarding the model inadequacy. A detailed comparison is made of the melting behavior of crystals produced by both flow fields and those results are discussed in light of a modified model for the growth process presented elsewhere. The conclusion is reached that differences in melting behavior may be attributed to differences in extension of the high molecular weight chains involved in the growth processes occurring in the two different flow fields.     

NMR imaging of flow in hollow fiber hemodialyzers

In this paper, we report the use of NMR (nuclear magnetic resonance) imaging for the investigation of flow profiles and flow-velocity distributions in hemodialyzers containing hollow-fiber membranes. For this purpose, we calculated a velocity-encoding and velocity-compensated spin echo NMR imaging sequence. The performance of the sequence was tested on a simple flow phantom and the results were compared with simulated data showing good agreement regarding the geometry of the velocity profiles as well as the maximum flow velocities. Two different small-scale model hemodialyzers were investigated by means of NMR imaging to gather information about flow-velocity distributions in the two compartments of hemodialyzers. The results show that the incorporation of textile fibers as spacer yarn in one of these model hemodialyzers strongly narrowed the flow-velocity distribution in the dialysate compartment. Additionally, the spatial flow-velocity distribution was measured in a clinical-scale hemodialyzer to compare the results from model and clinical systems. Flow velocities were measured in the potting region of one of the model hemodialyzers and at a short distance downstream in the dialysate compartment. It was shown that swelling of the membranes in the potting region leads to higher flow velocities of the inner membrane flow.