Matrix interactions in an inductively coupled argon plasma optimised for simultaneous multi-element analysis by atomic emission spectrometry

Matrix interactions in an inductively coupled argon plasma, optimised in the normal analytical zone (NAZ) for simultaneous multi-element analyses, have been studied using a factorial designed experiment to measure observed concentration changes of 16 analytes in the presence of K, Ca, NaSiO₃, polymaleic acid (PMA) and diethylenetriaminepentaacetic acid (DTPA). The main effects of the matrix compounds on the analyte wavelengths investigated all resulted in a suppression of the observed concentration means and follow the order PMA ⪢ Ca > K > NaSiO₃ and DTPA. The large PMA effect was mainly due to changes in transport efficiency, whereas the effects of Ca, K, NaSiO₃ and DTPA originate in the plasma. Calcium depressed the ion emission lines more than the neutral atom lines. A similar but much less marked effect was observed for K and NaSiO₃.     

Rapid multi-element analysis of groundwater by high-resolution inductively coupled plasma mass spectrometry

A rapid and sensitive method was developed to determine, with a single dilution, the concentration of 33 major and trace elements (Na, Mg, Si, K, Ca, Li, Al, P, S, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Cd, In, Sn, Sb, Cs, Ba, Re, Hg, Pb, Bi, U) in groundwater. The method relies on high-resolution inductively coupled plasma mass spectrometry (HR ICP-MS) and works across nine orders of magnitude of concentrations. For most elements, detection limits for this method are considerably lower than methods based on quadrupole ICP-MS. Precision was within or close to ±3% (1) for all elements analyzed, with the exception of Se (±10%) and Al (±6%). The usefulness of the method is demonstrated with a set of 629 groundwater samples collected from tube wells in Bangladesh (Northeast Araiharzar). The results show that a majority of tube well samples in this area exceed the WHO guideline for As of 10 g L^–1, and that those As-safe wells frequently do not meet the guideline for Mn of 500 µg L^–1 and U of 2 µg L^–1.     

Studies of a radio frequency inductively coupled argon plasma for optical emission spectrometry—III. Interference effects under compromise conditions for simultaneous multi-element analysis

This work concerns interference effects in a 0.7-kW, 50-MHz inductively coupled plasma (ICP) provided with an ultrasonic nebulizer (USN) and desolvation apparatus (DA). The observations were made under (ICP) conditions adopted previously as “compromise conditions for simultaneous multi-element analysis.” Various matrices and analytes were considered.An arrangement of two identical USN's with separate DA's was used to distinguish between interferences due to processes in the plasma (“plasma effects”) and the nebulizer—desolvation apparatus (“nebulizer—desolvation effects”). The latter were identified as “desolvation effects” and attributed to a variation in the loss of analyte in the DA. This desolvation effect, whose magnitude varies between ±10%, is related to the difference in volatility between matrix and analyte. The experiments revealed plasma effects that cannot be reconciled with the common pictures of ionization interference and are not due to incomplete volatilization or dissociation either. Possible explanations are considered. The overall interference level in the ICP studied is discussed and practical conclusions regarding the use of desolvation, “pure” aqueous solutions as standards, and spectroscopic buffers are drawn.     

An inductively coupled plasma using 1 lmin of argon

The development and preliminary evaluation of an ICAP consuming only 1 $\text{l}\text{min}$ of argon is described. External cooling of the torch with pressurized air and a two tube plasma torch arrangement are essential. Detection limits and matrix effects are comparable to those of a conventional ICAP.     

Entrainment of ambient air into a spectrochemical inductively coupled argon plasma

A spectrochemical inductively coupled argon plasma (ICP) is normally operated in the open air. Therefore, it is suggested in the literature that entrainment of air molecules into such an ICP may cause loss of electrons, especially so at the plasma's edge. The present study discusses the significance of this effect. The density and temperature of electrons and nitrogen molecules around the edge of the plasma were measured by Thomson and rotational Raman scattering. A region where both electrons and nitrogen were present in detectable amounts (10¹⁹ and 10²⁴ m⁻³, respectively) could not be observed. Above the torch inner wall the nitrogen concentration drops rapidly towards the plasma. Measurements suggest that the nitrogen concentration at 1 mm from the plasma is only a few percent, and in the active zones of the plasma (far) below 0.1%. This is not enough to affect the plasma significantly. Moreover, electron loss due to diffusion of nitrogen into the plasma is calculated to be much slower than the loss observed in earlier studies. Hence, air entrainment is unlikely to play a significant role in the ICP. A possible alternative is the formation and destruction of molecular rare gas ions.     

Axial distribution of analyte emission in inductively coupled argon plasma

Summary Using a spectrometer, ICP axial profiles of analyte emission for a large number of atom and ion lines with different excitation and ionization potential have been studied. Obtained experimental results for axial distribution of atom and ion lines can be correlated with some of the proposed models for excitation mechanisms of analytes in argon ICP. On the basis of experimental data it can be concluded that in the case of simultaneous multielement analysis the observation heights between 14 and 17 mm above the load coil can be recommended. Experimental data show that there is no simple correlation between maximum intensity of lines and their excitation and ionization potential. Experimental results for axial distribution for ionic lines show that there is agreement between measured axial profile and calculated Boltzmann distribution of excited ionic state population.

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Axialverteilung der Analytemission im induktiv gekoppelten Argonplasma

Zusammenfassung Die ICP-Axialprofile der Analytemission wurden mit Hilfe eines Spektrometers für eine große Anzahl von Atom- und Ionenlinien mit verschiedenem Anregungs- und Ionisierungspotential untersucht. Die erhaltenen Ergebnisse können zu einigen Modellen für den Anregungsmechanismus im Argon-ICP in bezug gebracht werden. Aus den experimentellen Daten wird geschlossen, daß bei der simultanen Multielementanalyse Beobachtungshöhen zwischen 14 und 17 mm über der geladenen Wendel zu empfehlen sind. Es ergab sich, daß zwischen der Maximalintensität der Linien und ihrem Anregungs- bzw. Ionisierungspotential keine einfache Beziehung besteht. Die Untersuchung der Axialverteilung bei den Ionenlinien zeigte, daß Übereinstimmung besteht zwischen den gemessenen Axialprofilen und der berechneten Boltzmann-Verteilung.

     

Appreciation of a collisional-radiative model for the description of an inductively coupled argon plasma

The collisional-radiative model has been applied to the argon ICP discharge in order to elucidate the excitation mechanism in the plasma. The population density distributions of 25 argon energy levels were calculated under a steady-state approximation by using the literature values of electron number density, 5 × 10 ¹⁴cm^?3 and electron temperature, 9000 K. In the case of an optically thin plasma, in which the induced absorption can be neglected, the calculated population densities showed an overpopulation for low lying states, and were very close to LTE values for the upper levels. These results suggest the following excitation mechanisms in the argon ICP; corona model for lower levels and ladder-like excitation and ionization by electron impact for upper levels. According to the present calculation, the non-overpopulation of argon metastable can be interpreted by the interconversion between metastable and radiative states. It has been found that the induced absorption of resonance lines in an optically thick plasma and the motion of species in an inhomogeneous plasma have significant effects on the population densities. The non-linear processes by collision between heavy particles were not predominant compared to electron impact processes.     

Surface modification of low-density polyethylene by inductively coupled argon plasma

The surface chemistry and nanotopography of low-density polyethylene (LDPE) were modified by downstream, inductively coupled, radio frequency (rf) Ar plasma without inducing surface damage. The extent of surface modification was controlled by the applied ion energy fluence, determined from the plasma ion density measured with a Langmuir probe. The treated LDPE surfaces were characterized by atomic force microscope (AFM) imaging, contact angle measurements, and X-ray photoelectron spectroscopy (XPS). Analysis of AFM surface images confirmed that topography changes occurred at the nanoscale and that surface damage was insignificant. Contact angle measurements demonstrated an enhancement of the surface hydrophilicity with the increase of the plasma power. XPS results showed surface chemistry changes involving the development of different carbon-oxygen functionalities that increased the surface hydrophilicity. Physical and chemical surface modification was achieved under conditions conducive to high-density inductively coupled rf plasma.     

A rate model of inductively coupled argon plasma analyte spectra

A model which explicitly considers the various rates of excitation, deexcitation, ionization and recombination for analyte species in an ICAP of defined electron density and temperature is presented. The model reveals that radiative decay, radiative recombination and radiative absorption affect the level populations of a fundamentally collisionally dominated plasma. In addition. Penning ionization is shown to have a negligible effect on spectrally derived temperatures except for elements of high second ionization potential.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

On the charge transfer in an inductively coupled argon plasma

Absolute population densities for several excited states of magnesium are obtained for several locations in an inductively coupled plasma (ICP). They were used to construct Boltzmann-Saha plots for these positions and show that magnesium is close-to-LTE. The deviations from LTE are mainly limited to the levels sensitive to charge exchange with argon ions. These measured deviations can be explained by a simple model which shows that, although charge transfer is a dominant excitation and ionization mechanism in an ICP, the associated LTE deviations are limited in magnitude.     

Simplex optimisation of inductively coupled plasmas

Meaningful comparisons of the analytical performances of different inductively coupled plasmas necessitate preliminary optimisation. The variable step-size simplex procedure is applied to optimise signal-to-background ratios for the five continuously variable operating parameters of a plasma, i.e. the power in the plasma, the observation height, and the injector, plasma and coolant gas flow rates. A series of univariate searches confirmed the results and also illustrated the importance of the various parameters. Results are presented for the manganese 257.6-nm ion line in both argon- and nitrogen-cooled plasmas and for the arsenic 228.8-nm atom line with argon coolant. Optimal power levels in these three cases were identified as 0.59, >1.2 and 0.57 kW, respectively.     

Influence of water on conditions in the inductively coupled argon plasma

Aqueous media are used almost universally for sample introduction in both inductively coupled plasma atomic emission spectrometry (ICPAES) and in inductively coupled plasma/mass spectrometry (ICP/MS). In the process of aqueous sample introduction a substantial mass of water is introduced into the plasma as a combined aerosol/vapor mixture. In the present studies, the masses of water present as aerosol and vapor were controlled, in order to examine their separate influence on the key plasma properties of electron density n_e and ionization temperature T_ion. Water loading in the plasma was indeed found to have a major influence on n_e and T_ion, and plots of these parameters as a function of water loading are presented. Plasma viewing height and operating power were also found to be important variables in influencing the way in which water interacts with the plasma. The implications of water loading on background emission and noise level are also considered.     

Modification and optimization of a 50 MHz inductively coupled argon plasma with special reference to analyses using organic solvents

The torch and nebulizer of an existing argon ICP system were modified and the system was (re-) optimized for aqueous and organic liquids. The paper describes the design considerations and construction of(1) a new, streamlined torch including a torch base used in this study, where a demountable rather than a prealigned version of the torch was preferred;(2) a cross-flow pneumatic nebulizer with adjustable teflon capillaries including a spray chamber with flow spoiler, concentric aerosol pick-up tube, and “U” tube with unequal legs to smooth the flow of wasted liquid to the drain.The (re)-optimization of the ICP system for analysis of aqueous solutions with inorganic matter or with both inorganic and organic matter is discussed in the light of earlier work in this laboratory regarding the selection of “compromise conditions” and the choice of representative spectral lines and measurement criteria for establishing such compromise conditions. In this context the authors consider the concepts of norm temperature and “hard” and “soft” lines, as well as recent results of measurements of spatial distributions in ICPs. The authors further describe experiments aimed at the optimization of the operating conditions of an “organic ICP” using methyl isobutyl ketone (MIBK) as organic solvent. Trends of net line and background signals and signal-to-background ratios with the ICP parameters (power; outer, intermediate and carrier gas flow; observation height; liquid feed rate) are reported, and a rational choice of compromise conditions for the ICP is argued.Performance characteristics of the modified ICP system, such as detection limits, precision and interference level, achieved under compromise conditions, have been communicated in a previous report [Spectrochim. Acta36B, 1031 (1981)] to demonstrate the capabilities of the system for analysis of aqueous solutions. Detection limits in MIBK and oil diluted in MIBK are reported in the present work as an illustration of the performance of the system when used for organic liquid analysis.     

Laser ablation–inductively coupled plasma–dynamic reaction cell–mass spectrometry for the multi-element analysis of polymers

In this work, the potential of laser ablation–inductively coupled plasma–mass spectrometry for the fast analysis of polymers has been explored. Different real-life samples (polyethylene shopping bags, an acrylonitrile butadiene styrene material and various plastic bricks) as well as several reference materials (VDA 001 to 004, Cd in polyethylene) have been selected for the study. Two polyethylene reference materials (ERM-EC 680 and 681), for which a reference or indicative value for the most relevant metals is available, have proved their suitability as standards for calibration.Special attention has been paid to the difficulties expected for the determination of Cr at the μg g^− 1 level in this kind of materials, due to the interference of ArC⁺ ions on the most abundant isotopes of Cr. The use of ammonia as a reaction gas in a dynamic reaction cell is shown to alleviate this problem, resulting in a limit of detection of 0.15 μg g^− 1 for this element, while limiting only modestly the possibilities of the technique for simultaneous multi-element analysis. In this regard, As is the analyte most seriously affected by the use of ammonia, and its determination has to be carried out in vented mode, at the expense of measuring time.In all cases studied, accurate results could be obtained for elements ranging in content from the sub-μg g^− 1 level to tens of thousands of μg g^− 1. However, the use of an element of known concentration as internal standard may be needed for materials with a matrix significantly different from that of the standard (polyethylene in this work).Precision ranged between 5% and 10% RSD for elements found at the 10 μg g^− 1 level or higher, while this value could deteriorate to 20% for analytes found at the sub-μg g^− 1 level. Overall, the technique evaluated presents many advantages for the fast and accurate multi-element analysis of these materials, avoiding laborious digestion procedures and minimizing the risk of analyte losses due to the formation of volatile compounds.     

A three-electrode direct current plasma as compared to an inductively coupled argon plasma

Summary A direct comparison of some analytical properties of a three-electrode direct current plasma and an inductively coupled argon plasma in the case of pneumatic nebulization of aqueous solutions was performed. The measurements were carried out under similar conditions using a 3.4-m spectrograph. The spectra to be compared were recorded on photographic plates in the spectral range from 250 nm to 430 nm. Strong molecular band systems of OH, NH, and N ₂ ⁺ were observed in the case of the direct current plasma. Detection limits for 27 spectral lines of 20 elements were determined for both sources yielding a slight advantage in favour of the inductively coupled plasma. The effect of sodium upon line and background intensities was investigated and found to be generally higher in the direct current plasma.

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Ein Drei-Elektroden-Gleichstromplasma im Vergleich zu einem induktiv gekoppelten Argonplasma

     

Absolute spectral radiance of 27 MHz inductively coupled argon plasma background emission

The spectral radiance of the plasma continuum of a 27 MHz argon ICP has been measured for two sets of operating conditions in the wavelength range from 192 to 600 nm. At an rf forward power of 1250 W and a viewing height of 15 mm above the load coil, a maximum spectral radiance of (1.5±0.3) × 10¹² photons s⁻¹ mm⁻² sr⁻¹ nm⁻¹ was observed, at a wavelength of approximately 450 nm. At 200 nm, the spectral radiance is 60 times lower. For wavelengths below 400 nm, the absolute plasma emission spectrum is well represented by a graybody function with a temperature of 5480 K and an emissivity of 4 × 10⁻⁵.