Influence of water on the spatial excitation behavior of selected elements in the inductively coupled plasma

A photodiode array spectrometer has been used to generate vertical spatial emission profiles of elements introduced into an inductively coupled plasma. Atomic (soft) and ionic (hard) lines have been studied in both the presence and absence of water. The data obtained suggest that the amount of water present has a marked influence on the vertical spatial profiles. Both atomic and ionic emission lines are similarly affected. The predominant role of water appears to be kinetic control of analyte desolvation which occurs prior to excitation.     

Some spatial effects observed in the axially viewed filament argon microwave induced plasma with solution nebulization

Spatial profiles of analyte emission in an axially viewed argon filament microwave induced plasma sustained in the TE₁₀₁ rectangular cavity have been measured along a discharge tube cross-section for neutral atoms as well as ion lines of several elements. The filament diameter was approximately 1 mm. The analyte solution was introduced by means of an ultrasonic nebulizer without desolvation. The radial emission distribution depends on the operating parameters and is different for each of the analytes examined. Spatial distributions of excitation temperature (4000–6000 K) measured with Ar I lines by the Boltzmann plot method as well as electron temperature (6000–8000 K) by line to continuum emission ratio measurements at Ar I 430 nm and electron number density (1–1.5×10¹⁵ cm⁻³) by the Stark broadening method of the H_β line were determined to support the evidence of plasma processes. In the presence of excess sodium the enhancement of emission intensity and its shift to the plasma center appears to be the result of increased analyte penetration to the plasma. Changes in spatial emission profiles for Ca atoms and ions suggest that for this element ambipolar diffusion may be important as an additional interference mechanism. A possibility of minimizing spectral interferences from argon emission lines by choosing an off-axis plasma region for emission intensity measurements is indicated.     

Spatial characterization of analyte emission and excitation temperature in an inductively coupled plasma

Vertical, lateral and radial profiles of analyte emission in an inductively coupled plasma have been measured using photodiode array spatial profiling spectrometers. These profiles have been measured for both neutral atom and ionic lines of several elements. Neutral atom lines can be sub-divided into two basic groups on the basis of their vertical spatial emission characteristics. One group in which the peak vertical position of emission correlates positively with normal temperature and a second group in which the peak vertical position of emission correlates negatively with normal temperature. Utilizing radially resolved emission intensities and vertical and radial profiles of neutral atom excitation temperature, the caracteristic emission patterns of both groups of neutral atom lines can be explained. Analyte ionic line spatial emission characteristics, both vertically and radially, are shown to be relatively species independent and radial emission intensity ratio maps of ionic and neutral atom lines of the same element are presented that indicate the potential importance of plasma boundary regions as regions of major non-LTE behavior.     

Interference from easily ionizable element matrices in inductively coupled plasma emission spectrometry—a spatial study

Utilizing a photodiode array based spatial profiling spectrometer an extensive spatial characterization of the effect of an excess of an easily ionizabie element (EIE) on analyte emission in the inductively coupled plasma has been carried out. Significant spatial shifts, enhancements and depressions are induced on analyte emission by the presence of an EIE and it is shown that limited fixed height measurements may be seriously misleading with respect to interpretation of the effects of ElEs. In general, the addition of excess EIE enhances emission in the lower regions of the analyte channel for both atom and ion lines and in the upper regions of the analyte channel, emission intensity is depressed for both atom and ion species. Radial emission maps (for CaI and CaII) reveal that the enhancements low in the discharge occur along the off axis boundary of the analyte channel and the plasma discharge. Higher in the discharge CaI emission is generally depressed but CaII emission undergoes a central axis depression coupled with an off axis enhancement. Overall the data suggests that low in the discharge enhancement appears to be the result of increased collisional excitation and that higher in the discharge ambipolar diffusion may exert an influence on the spatial distribution of analyte species.     

The vertical spatial characteristics of analyte emission in the inductively coupled plasma

Utilizing a photodiode array based spatial profiling spectrometer vertical spatial profiles of analyte emission have been measured for a large number of neutral atom and ion lines in the inductively coupled plasma. The lines can be subdivided into two basic categories. One group (called “ soft ” lines after Boumans) have spatial emission behaviour that is very dependent on power, aerosol flow and analyte excitation and ionization characteristics. The second group (called “ hard ” lines after Boumans) have spatial emission behaviour that is relatively insensitive to all of the above parameters. In all cases ion lines have hard line behaviour as do the more energetic atom lines. Under fixed ICP conditions all hard lines have their peak emission at essentially the same position in the discharge which is always higher in the discharge than that for soft lines. It is also shown that the spatial behaviour of soft lines can be directly correlated with normal temperature.     

The role of electrons in the emission enhancement by easily ionized elements low in the inductively coupled plasma

The enhancement of emission intensity in the presence of easily ionizable matrix elements low in the plasma is related to the electron excitation cross-section for the transition in question. The matrix also causes a local increase in the electron density in the atom zone of the plasma. The observations can be explained qualitatively by an ambipolar diffusion model.     

The effect of easily ionizable concomitant elements on non-spectroscopic interferences in inductively coupled plasma-mass spectrometry

Reported are the effects of easily ionizable concomitant elements on non-spectroscopic interferences in ICP-MS. Analyte ion suppression was studied for ⁷Li, ¹¹B, ⁵⁸Ni, ⁴⁵Sc, ⁸⁹Y and ²⁰⁵Tl in the presence of concomitant elements spanning a mass range from 23 (Na) to 207 (Pb) dallons.For the analytes studied, it was found that the greater the atomic mass of the concomitant element, the greater was the analyte ion count rate suppression. For a given set of experimental conditions, the greater the atomic mass of the analyte, the lower was its susceptibility to ion count rate suppression by any concomitant element.The severity of non-spectroscopic interferences decreased as the sampler orifice was positioned further away from the center of the plasma and also as the sampling depth was increased. Dilution of a solution containing a given molar ratio of concomitant to analyte reduced the extent of analyte ion suppression.Non-spectroscopic interferences in ICP-MS can be attributed to ambipolar diffusion effects in the plasma that result from the presence of easily ionizable concomitant elements.     

Interferences due to easily ionised elements in a microwave-induced plasma system with graphite-furnace sample introduction

Several aspects of both enhancement and suppression of the analyte emission intensity caused by an easily ionised element (EIE) have been studied in an atmospheric pressure He microwave-induced plasma (MIP). A sequence of experiments, designed to elucidate possible mechanisms of this EIE effect, examines the following aspects: the concentration dependence of the effect for various EIEs; spatially separated vaporisation of EIE and analyte into the plasma; the effect of operating parameters upon the EIE-induced enhancement; the influence of the EIE on the excitation temperature and on the efficiency of coupling of microwave energy to the cavity. The EIE-induced suppression of emission intensity is consistent with reduced power dissipation in the plasma, due to decoupling of the plasma from the microwave power source, whereas the EIE-induced enhancement of emission intensity is best explained by a radiative energy transfer mechanism.     

Ionization interferences under various operating conditions in a 9, 27 and 50 MHz ICP,and a study of shifts in level populations of calcium through simultaneous absorption-emission measurements in a 9 MHz ICP

Radially resolved absorption and emission measurements were employed for a better understanding of the excitation mechanism of nebulized species operating under conditions favourable for the occurrence of ionization interferences in an atmospheric pressure 9 MHz ICP. Three monitored spectral lines of calcium were used to observe changes in ground and excited level populations of atoms and ions, in ion excitation temperatures using the two-line method. Observations were made at a fixed height, namely 25 mm above the rf coil and varying carrier gasflows from 2 to 51 min^?1 and were correlated with the position of the “initial radiation zone” (IRZ) in the plasma. Ionization interferences occurring only inside the IRZ indicate an excitation mechanism depleting ion ground level population and populating excited atom and ion levels. No changes in atom absorbances or excitation temperatures were observed ruling out ionization suppression as dominating mechanism. Indications are that increased collisional excitation for Ca ions and ambipolar diffusion may be the dominant excitation mechanism operating in the analyte channel. Recombination reactions (three body or radiative) or charge transfer reactions may be responsible for an increase of excited atom level populations. It is obvious that non-thermal processes are operating under conditions favourable for ionization interferences occurring in the ICP.     

The effect of multi-component aerosol particles on quantitative laser-induced breakdown spectroscopy: Consideration of localized matrix effects

Spectral measurements were performed in a laser-induced plasma to assess the changes in sodium or magnesium analyte emission response from particle-derived sources with the addition of concomitant mass to the aerosol particles. Temporally resolved measurements revealed up to a 50% enhancement in analyte emission with the addition of the elements copper, zinc or tungsten at mass ratios from 1:9 to 1:19, although the enhancement generally diminished by delay times of 60 μs. Additional measurements in magnesium–cadmium aerosol particles were performed to assess the temporal profile of plasma temperature in the spatial vicinity of the aerosol particles using the ion-to-neutral emission ratios. These measurements revealed a general increase in localized plasma temperature with increasing delay time, which is attributed with an initial suppression of plasma temperature about the aerosol particles as plasma energy is required to vaporize and ionize the aerosol particle mass. These measurements provide direct evidence of a matrix effect for aerosol particles, which is attributed primarily to perturbations in the localized plasma properties. These perturbations are minimized at longer plasma delay times; hence quantitative LIBS analysis of aerosol particles should be performed with careful attention given to the temporal plasma evolution. The data further elucidate the complex interactions between the plasma gas and the aerosol particles, during which the finite time-scales of particle dissociation, and heat and mass transfer are equally important.     

Time-resolved spectroscopy and imaging diagnostics of single pulse and collinear double pulse laser induced plasma from a glass sample

The characterization of laser-induced plasma from a glass sample was performed in the single- and double-pulse excitation regimes. The detailed information about density distributions of excited atoms and ions in the expanding plasma was obtained by using the imaging detection system providing measurements of the spatial, temporal, and spectral plasma emission characteristics. The expansion dynamics was shown to differ strongly between two excitation regimes. The enhancement factors of the line emissions in the double-pulse mode were found to be spatial dependent and to differ for the different elements in the plasma plume. The obtained results are useful for a better understanding of the main physical processes leading to the analytical improvement achieved by the use of double-pulse laser-induced breakdown spectroscopy (LIBS).     

Matrix effects during trace element analysis in plant samples by inductively coupled plasma atomic emission spectrometry with axial view configuration and pneumatic nebulizer

Plant sample matrix effects have been investigated during trace element analysis by inductively coupled plasma atomic emission spectrometry with axial view and pneumatic nebulizer. Eight elements often analyzed in environmental samples were studied: As, Cd, Co, Cr, Cu, Ni, Pb and Se. To simulate the effects caused by digested plant samples, the spectral line profiles of analytes were measured on solutions containing various concentrations of elements encountered in plant matrix such as K, Ca and Mg. Depressions in the emission intensities occurred and were more dramatic with high concentrations of the concomitant species. The inter-element effects were found to be caused by the matrix amount entering into the central channel of the plasma. In fact, energy consumed for droplets desolvation and particles of salts vaporization leads to lower plasma temperatures and produce changes in excitation mechanisms. Ion emissions were especially affected. The matrix effects can be removed using a high generator power (1.4 kW) and a moderate uptake flow rate of solution into the plasma (pump speed: 15 rev./min). The use of the natural plant sample led to the same conclusions. These operating conditions decreased the sensitivity but routine analyses of the plant material could be carried out.     

Combined effects of inorganic acids in inductively coupled plasma optical emission spectrometry

The acid interferences in inductively coupled plasma atomic emission spectrometry (ICP OES) were studied in a multivariate way, considering the simultaneous presence of four mineral acids (hydrochloric, nitric, sulfuric and perchloric acid) with their concentrations ranging from 5 to 80% (w/w). A low power ICP OES was used, after optimization of operating parameters in order to achieve both plasma robustness and maximum signal to background ratios, favorable for trace element determinations in real samples. In order to investigate the interference mechanism, the combined effects of mineral acids on several parameters (solvent transport rate, analyte transport rate, excitation temperature, electron number density and magnesium ionic to atomic line intensity ratio) were evaluated and discussed. It was found that the combined effects of inorganic acids in ICP OES are, in general terms, more complex than the simple addition of the single effects. The more relevant interactions are between hydrochloric and nitric acids and those with sulfuric acid. Owing to these interactions, the resulting effects on the analytical signals are lower than the expected ones. The extent of any interaction depend on the nature of the interference. For the physical effects which are related with a change in the viscosity of the solution, an attenuation of the acid effect due to the presence of another acid at high concentration was evident. On the contrary, for the interferences related with a change in the plasma excitation conditions, the combined effects are higher than the addition of the single ones.     

Matrix interferences in the determination of trace elements in waste waters by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization

The use of inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization (USN-ICP-AES) for determining Ag, Al, As, Ba, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Ni, Pb, Sb, Sr, V and Zn in complex matrices of Ca, Na, K and P in waste waters is described. Generally, depressions in the analyte emission intensity occur in the presence of concomitants. Matrix interferences can be minimized by increasing the operating power and lowering the carrier gas flow rate. However, the enhancement of the signal-to-background ratios (SBRs) shows an opposite trend. Therefore, routine analyses were performed at a compromise power setting of 1,350 W, a carrier gas flow rate of 0.8 L min(-1) and an observation height of 14 mm above the load coil and using a matrix matched calibration procedure. Limits of detection (LODs) at chosen operating conditions were at microg L(-1) levels for most of the elements studied, including mercury when KBr is added to the analyte solution to enhance sensitivity. LODs were not significantly changed in the presence of matrix elements. Recoveries for the majority of added elements from spiked waste water samples are between 93 and 105% using a matrix matched calibration.     

Experimental studies of charge transfer reactions between argon and the third row metals calcium through copper in the inductively coupled plasma

The effect of charge transfer reactions on analyte excitation and ionization in the inductively coupled plasma was studied by two independent techniques. In one technique, pulsed lasers were used to either deplete the ground state of neutral analyte atoms or enhance the population of selected states of the singly charged ion. In both cases the perturbed species were collision partners with argon in potential charge transfer reactions. The effects of charge transfer collisions could be detected in the form of changes in emission from product species. In the second technique, a simple correlation method was used to detect the link via charge transfer of neutral atom ground states and highly excited ionic levels. In the presence of charge transfer collisions, the populations of such linked levels show strong positive correlations. The two techniques were used to study the effects of charge transfer reactions on the third row elements Ca–Cu. With the exception of Cr and Mn, all of the elements studied showed positive evidence of excitation and ionization by charge transfer collision with argon.     

从基体干扰角度进行电感耦合等离子体原子发射光谱分析条件的优化

本文探讨了不同分析条件下基体干扰效应的分布规律,从消除基体干扰效应角度对ICP-AES操作条件的优化进行了讨论。结果表明,在一般分析条件下,典型基体元素的零干扰点主要出现于10～15mm的观察区域。在此区间仔细选择观察高度,同时结合入射功率、载气流量的调整及加入基体缓冲剂,可以将基体的影响减至最小。     

General calculations of vertically-resolved emission profiles for analyte elements in inductively coupled plasmas

A simple model is suggested to approximately describe the vertical spatial behavior of analyte emission in inductively coupled plasmas (ICPs). The degree of analyte ionization determines the relative numbers of neutral atoms and ions at each observation height. The excitation temperature (T_exc) determines the fraction of ions in each ionic excited state and the fraction of atoms in each atomic excited state. The degree of ionization varies with observation height in a manner that is independent of T_exc. Calculated vertical intensity profiles show the same general shape and element-to-element trends as the analogous experimental data.     

Laser excited atomic fluorescence for studies of enhancement effects in the direct current plasma

This paper reports a preliminary use of laser excited atomic fluorescence spectrometry (LEAFS) to study analyte population enhancement caused by easily ionized elements (EIEs) in the direct current plasma (DCP). Spatial atom density profiles in the DCP were obtained using resonance fluorescence at the calcium atom line at 422.7 nm, with and without the addition of an EIE. Variations in atom density caused by an EIE were found to be far too small to account for the marked enhancements of atomic emission signals which are caused by EIEs.Direct line fluorescence of the barium ion, excited at 614 nm and detected at 455 nm, was used to probe the effect of an EIE on excited state populations. Measurements in the analytical region of the plasma close to the core revealed that enhancements of fluorescence signals at low laser powers disappeared at laser powers which were sufficient to saturate the atomic transitions. While this result does not clarify any of the mechanisms of excitation in the DCP, it does lend support to two of the fundamental postulates of a recent model of the spectrochemical excitation processes in the DCP. These are first, that the analytical region of the DCP is not in local thermodynamic equilibrium (LTE) and second, that EIE enhancement proceeds by modulating the rates of power distribution among various plasma zones.In the outer zone of the analytical region of the DCP, depressive interferences occurred. These did not disappear upon saturation which indicates that they were not rate effects but effects that resulted from atom density changes.     

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s₅ (³P₂) and 1s₃ (³P₀) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the ³P₂ (11.55 eV) and the ³P₀ (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.     

Spatially-resolved observation of glow discharge plasma for atomic emission spectrometry.

An imaging spectrograph equipped with a CCD detector was employed to measure two-dimensional emission images from a glow discharge plasma in atomic emission spectrometry. The emission images at Zn I 334.50 nm for a zinc sample and at Cu I 324.75 nm for a copper sample could be obtained. Their emission intensities were not uniform in the radial direction of the plasma region but became weaker at larger distance from the central zone. The two-dimensional distribution would result from a spatial variation in the excitation efficiency of the plasma and thus provide useful information for understanding the excitation processes occurring in the plasma.