Analyte ionization in the inductively coupled plasma

Spatially resolved ion-atom emission intensity ratios for Sr, Ca, Mg, Cd and Zn have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.0 kW at a vertical height of 16 mm above the load coil. Measured values of electron density have been used to construct a theoretical local thermal equilibrium (LTE) framework, and ion-atom emission intensity ratios calculated from this framework have been compared to experimentally measured values. The measured ion-atom emission intensity ratios were found to be within an order of magnitude of these calculated LTE ratios.The experimental degree of ionization for these five elements was determined for the various rf input powers. These values have been compared to the analagous LTE values. Both degree of ionization and departure from LTE were found to be strongly correlated with the ionization potential of the element.The radial spatial dependence of the degree of ionization for Cd at an rf power of 1.25 kW has been measured for aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ for vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. The spatial distribution of electron number density was measured at an rf power of 1.25 kW and at aerosol flow rates of 0.6, 0.8 and 1.21 m⁻¹ and a correlation between degree of ionization and electron density identified. Finally the relative concentration of Cd ions has been calculated from ion spatial emission profiles and plasma operating conditions which produce a maximum in the ion density identified.     

Spatial profile measurement of ionization and excitation temperatures in an inductively coupled plasma

The developed instrument for spatial profile measurement [1] has been applied to the measurement of ionization and excitation temperatures in an inductively coupled plasma (ICP). The silicon intensified target (SIT) detector allowed it to measure a large number of emission spectra in a short period. The ease of acquisition enabled building up complete contour maps of ionization and excitation temperatures. The contour maps of various temperatures reveal that local thermal equilibrium does not exist in the whole ICP. The comparison between ionization temperature profiles for Ar and Ca indicates that in the normal analytical zone of the ICP, Ca is ionized as expected from the Ar ionization temperature. Excitation temperatures derived from low-level Fe I lines are lower than those derived from high-level Fe I lines over a large part of the plasma. The result confirms that for Fe I lines the ICP is characterized as an ionizing plasma in the whole ICP and the low atomic levels are overpopulated with respect to the high atomic levels.     

Some considerations regarding temperature,electron density,and ionization in the argon inductively coupled plasma

The measured density of electrons in the ICP cannot be explained on the basis of a pure LTE calculation. A mechanism which involves radiation trapping and the transfer of excitation energy from the annular regions of the ICP to the aerosol channel is offered. This mechanism called “assisted ionization” leads to a more accurate prediction of electron density at a particular temperature. Assisted ionization is the result of the coupling of high energy resonance radiation from Ar(I) in the annular regions of the ICP into the analyte channel. The response of analyte atoms and ions to temperature and electron density in the channel can be estimated by inclusion of the analyte ionization equilibrium in an overall equilibrium which includes argon atoms and excited state argon species.     

Spatial profile measurement of electron number densities and analyte line intensities in an inductively coupled plasma

A versatile instrument for spatial profile measurement has been developed and applied to the measurement of electron number densities and analyte emission intensities in an inductively coupled plasma (ICP). A precise Y-Z stage on which the ICP source was mounted was set on a rail-based optical bench. By translating the ICP source with a precision of ± 0.01 mm, the H_β Stark broadening and analyte line intensities were measured with the use of a silicon intensified target (SIT) and a photomultiplier (PMT). Micro-computer assisted data acquisition allowed it to measure a large number of emission profiles in a short period. The ease of acquisition enabled to build up complete contour maps of electron number densities, Ca neutral atom (Ca I) and Ca ion (Ca II) line intensities, and intensity ratios of the Ca II and Ca I lines. The maximum electron number density was 4 × 10¹⁵ cm^?3 occurring low in the plasma and 5 mm off axis. In a contour map of the electron number densities a hollow region was found low in the plasma, and the distribution pattern looked like a deep “trench”. Along the central channel of the ICP, the peak position of Ca II emission occurred higher than that of Ca I emission, and the spatial distribution of Ca II emission was wider and taller than that of Ca I emission. It has been verified that Ca I is emitted mainly at the region where the electron number density is less than 1 × 10¹⁴ cm^?3.     

Characteristic temperatures and electron number densities in an R.F. capacitively coupled plasma

The excitation temperatures of Ar and Fe, the ionization temperatures of Ar and Ca and the electron number densities have been determined for a radiofrequency capacitively coupled plasma in the tip-ring electrode geometry. The temperatures and the electron number densities possess their maximum value close to the electrodes.     

Characteristic temperatures and electron number densities in an R.F. capacitively coupled plasma

The excitation temperatures of Ar and Fe, the ionization temperatures of Ar and Ca and the electron number densities have been determined for a radiofrequency capacitively coupled plasma in the tip-ring electrode geometry. The temperatures and the electron number densities possess their maximum value close to the electrodes.     

Spatially resolved,intracavity absorption for inductively coupled plasma diagnostics

Continuous wave intracavity absorption provides a sensitive method for probing the distribution of absorbing species in a plasma flame. There exists considerable potential for plasma mechanistic studies using this method both as a spatial diagnostic for selected species, and as a spectral diagnostic for the detection of low concentration or low oscillator strength species.     

Spatial profiles of electron density in the inductively coupled plasma

Spatially resolved, radial electron number density (n_e) profiles have been measured at rf power settings of 1.00, 1.25, 1.50, 1.75 and 2.00 kW, and vertical heights of 4, 8, 12, 16 and 20 mm above the load coil. These profiles have been condensed and presented as electron density contour plots for each input power. The precision of the method was evaluated by doing ten replicate measurements of electron density. The relative standard deviation varied between 2 and 10 % with the maximum at the plasma centre. Electron density was measured with and without the presence of the easily ionizable element—Cs, and no significant difference was observed.     

Evaluation of argon metastable number densities in the inductively coupled plasma by continuum source absorption spectrometry

Absolute number densities for the metastable and radiative 4s argon levels in an inductively coupled plasma have been determined for a variety of plasma conditions by the technique of continuum source absorption. As primary source, a 300 W xenon arc was used and much care was taken in screening the optical detection system from the intense background emission of the plasma. A 1.29-m focal length grating monochromator provided a variable bandwidth so that absorption measurements could be carried out, with varying degree of sensitivity, on 19 different argon lines. The number densities were derived from the corresponding curves of growth, calculated for each line. Concentrations ranging from 2.3 × 10¹⁰ to 7.4 × 10¹¹ cm⁻³ were obtained for the different levels, depending upon the presence or absence of nebulizing gas and water in the plasma. At observation heights greater than 20 mm above the coil, the number density approaches the value predicted by Boltzman equilibrium for a temperature of 6500 K. The detection sensitivity of the present apparatus is about 5 × 10⁹ cm⁻³. For seven lines, damping parameter values were also estimated and found to vary from 0.4 to 1.1.     

Axial profiles of excitation and gas temperatures in an inductively coupled plasma

A vertically movable horizontal slit driven by a computer-controlled stepping motor was placed close to the front of the entrance slit of a monochromator. The axial channel of the plasma being imaged onto the entrance slit, the observation height was scanned by slicing the image of the plasma with the horizontal slit. Excitation and gas temperature profiles were calculated under various operating conditions from emission profiles of Fe I and OH lines, respectively. From the axial emission and temperature profiles, two excitational regions governed by different excitation mechanisms were postulated along the axis of the plasma. In the first region from 0 to 8 mm above the load coil, low-energy lines were predominantly excited and their emission profiles were controlled mainly by the dissociation rate of molecules. In the second region from 10 to 20mm above the load coil, high-energy lines were predominant and volatilization interferences were small.     

Optical detection of laser-induced ionization in the inductively coupled plasma for the study of ion-electron recombination and ionization equilibrium

The recombination kinetics between the strontium ions and the electrons have been studied in an inductively coupled plasma. Two excimer lasers have been used to pump two dye lasers, which were made spatially and temporally coincident in the plasma region investigated. With the first laser system the strontium atoms were photoionized and the second laser was then used to probe the ions formed by measuring the resulting ionic fluorescence. Since the second laser was delayed by external triggering with respect to the ionizing laser, the temporal fate of the ions could be continuously monitored. The recombination time constant was found to be 15.5 μs, indicating the absence of fast ion chemistry (in the tens of ns range) which was observed in an air-acetylene flame. Moreover, it was found that: (i) the addition of an easily ionizable element (K, Li) increased the rate of recombination; and (ii) the recombination time constant was directly proportional to the ion/atom ratio of strontium. It was concluded that a change in the electron energy distribution is more relevant to the rate of recombination than changes in the absolute number density of electrons.     

Langmuir probe measurements of electron temperature in an inductively coupled plasma

The feasibility of using double Langmuir probes to measure electron temperature (T_e) in an Ar inductively coupled plasma (ICP) was evaluated. Experimental methods for probing the plasma and for reducing rf interference were devised. Despite these measures, the probe signal was noisy and erratic if the ICP had the normal analytical configuration with a hole through its center, so measurements were restricted to an ICP without an axial channel. Theoretical criteria indicated that Langmuir probe measurements in an atmospheric pressure ICP were in a borderline regime in which the measured T_e values may have been depressed somewhat (relative to the actual T_e values in the ICP) due to cooling of electrons as they approached the probe. The T_e values obtained from the center of the ICP were 7500 K at a forward power of 1.0 kW and 10 000 K at 1.25 kW for a measurement position 8 mm above the load coil. Electron density (n_e) measurements by the Langmuir probe method were comparable to or higher than n_e values calculated from the Saha equation at the measured T_es. The T_e and n_e values were high enough to indicate that, if electron cooling and ion-electron recombination occurred near the probes, these effects were not extreme and/or the use of two probes compensated for them in some fashion. The probe measurements also indicated that T_e increased with the potential difference between the probes. This latter observation provided tentative evidence that the electron kinetic energy distribution was non-Maxwellian with an excess of higher energy electrons relative to lower energy electrons.     

Kinetic temperatures and electron densities in the plasma of a side view Grimm-type glow discharge

A special source in which the Grimm-type plasma is viewed side-on for spectroscopic measurements was constructed. The kinetic gas temperatures and electron densities were derived from the line profiles of Ar I 415.8 nm and He I 447.1 nm respectively.     

Energy transport in the inductively coupled plasma

Time- and space-resolved electron density measurements, made both above the load coil and in the load coil region of a pulsed inductively coupled plasma, are presented. These data, coupled with argon and calcium emission data, give values for the rates of both radial and vertical transport in the plasma. The data indicate that analyte emission behavior is governed primarily by the rate at which the central channel can be heated through radial transport processes. The electron densities measured in the load coil region agree well with electron densities calculated by models assuming local thermodynamic equilibrium, but agree poorly with non-equilibrium models. Some of the timedependent emission behavior observed in previous work with modulated plasmas is explained by non-uniform heating of argon in the load coil region.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

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.     

Fission yield measurements by inductively coupled plasma mass-spectrometry

Correct prediction of the fission products inventory in irradiated nuclear fuels is essential for accurate estimation of fuel burnup, establishing proper requirements for spent fuel transportation and storage, materials accountability and nuclear forensics. Such prediction is impossible without accurate knowledge of neutron induced fission yields. The uncertainty of the fission yields reported in the ENDF/B-VII.0 library is not uniform across all of the data and much of the improvement is desired for certain fissioning isotopes and fission products. We discuss our measurements of cumulative fission yields in nuclear fuels irradiated in thermal and fast reactor spectra using Inductively Coupled Plasma Mass Spectrometry.     

A new continuous calibration method for inductively coupled plasma spectrometry

A new calibration method was developed and applied to inductively coupled plasma atomic emission spectrometry. External calibration was performed as follows. A container was filled with a given volume of deionized (V _p) water. Then a concentrated standard was introduced at a controlled rate (Q _e) into the tank by means of a peristaltic pump. The resulting solution was stirred throughout the experiment. Simultaneously, the solution inside the tank was pumped from the vessel to the plasma at a given rate (Q _s). The signal was continuously recorded. The variation of the concentration of the solution leaving the tank with time was determined by applying a basic equation of stirred tanks. The representation of the emission intensity versus the time and the further conversion of the time scale into a concentration scale gave rise to the calibration line. The best results in terms of linearity were achieved for V _p=15 cm³, Q _e=0.6–0.75 ml min⁻¹ and Q _s=1–1.2 ml min⁻¹. Graphs with more than 40 standards were obtained within about 10 min. The results found were not statistically different from those afforded by the conventional calibration method. In addition, the new method was faster and supplied better linearity and precision than the conventional one. Another advantage of the stirred tank was that procedures such as dynamic calibration and standard additions could be easily and quickly applied, thus shortening the analysis time. A complete analysis following these procedures based on the measurement of 30 standards took about 5 min. Several synthetic as well as certified samples (i.e., bovine liver, mussel tissue and powdered milk) were analyzed with the stirred tank by applying four different calibration methodologies (i.e., external calibration, internal calibration, standard additions and a combination of internal standardization and standard additions), with the combination of internal standardization and standard additions being the method that provided the best results. The element concentrations obtained were not significantly different from the actual or certified values.     

Applications of inductively coupled plasma mass spectrometry and laser ablation inductively coupled plasma mass spectrometry in materials science

Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) have been applied as the most important inorganic mass spectrometric techniques having multielemental capability for the characterization of solid samples in materials science. ICP-MS is used for the sensitive determination of trace and ultratrace elements in digested solutions of solid samples or of process chemicals (ultrapure water, acids and organic solutions) for the semiconductor industry with detection limits down to sub-picogram per liter levels. Whereas ICP-MS on solid samples (e.g. high-purity ceramics) sometimes requires time-consuming sample preparation for its application in materials science, and the risk of contamination is a serious drawback, a fast, direct determination of trace elements in solid materials without any sample preparation by LA-ICP-MS is possible. The detection limits for the direct analysis of solid samples by LA-ICP-MS have been determined for many elements down to the nanogram per gram range. A deterioration of detection limits was observed for elements where interferences with polyatomic ions occur. The inherent interference problem can often be solved by applying a double-focusing sector field mass spectrometer at higher mass resolution or by collision-induced reactions of polyatomic ions with a collision gas using an ICP-MS fitted with collision cell. The main problem of LA-ICP-MS is quantification if no suitable standard reference materials with a similar matrix composition are available. The calibration problem in LA-ICP-MS can be solved using on-line solution-based calibration, and different procedures, such as external calibration and standard addition, have been discussed with respect to their application in materials science. The application of isotope dilution in solution-based calibration for trace metal determination in small amounts of noble metals has been developed as a new calibration strategy. This review discusses new analytical developments and possible applications of ICP-MS and LA-ICP-MS for the quantitative determination of trace elements and in surface analysis for materials science.     

Excitation temperature and electron density in the inductively coupled plasma—aqueous vs organic solvent introduction

Using a photodiode array spectrometer, spatially resolved Fe I, excitation temperatures and electron densities have been measured for an ICP with both water and xylene solution introduction. The excitation temperature is lower in the ICP with organic aerosol than in the ICP with aqueous aerosol at a fixed power and height. For an ICP with organic aerosol the input power must be increased by approx. 0.5 kW to simulate the temperatures reached in an aqueous ICP.