Noise power spectra of the inductively coupled plasma

Noise power spectra for the ICP were determined under various conditions, by Fast Fourier Transform (FFT) digital techniques. The major noise types observed were white noise, low frequency noise, and high frequency proportional noise. The high frequency proportional noise increased with concentration of analyte and radio frequency input power. The high frequency proportional noise decreased with increasing nebulizer flow rate and coolant gas flow rate.. The low frequency noise components extended to higher frequencies as the resonance wavelengths of the measured transitions increased. Changing the observation height in the plasma determined which noise types were present in the noise power spectrum. At observation heights near the power coils, all three noise types were present, whereas, low frequency noise predominated at significantly greater heights. Changing the torch design changed the relative amplitudes of the different high frequency proportional noise components, but did not greatly change their respective peak frequencies.     

Relative spatial profiles of barium ion and atom in the argon inductively coupled plasma as obtained by laser excited fluorescence

Relative ionic and atomic fluorescence profiles for barium have been obtained in an argon inductively coupled plasma by exciting different transitions with a nitrogen-laser pumped tunable dye laser and measuring the resulting fluorescence pulses with a boxcar averager. Spatially resolved profiles are directly obtained without the need of an Abel inversion procedure, with a volume resolution of approximately 0.2 mm³. The profiles are given along the excitation axis as well as along the observation axis, for different heights above the coil and different input powers. At low heights, the ion profile resembles a hollow pencil with a typical double-peaked, asymmetric distribution, while the atom profile seems to be complementary to the ion profile. Some scatter from water is also evident at low heights.     

Non-linear optical behavior in atomic fluorescence flame spectrometry

The non-linear dependence of the atomic fluorescence radiance upon the irradiance of the excitation source is discussed. Theoretical equations based on the assumption of steady state conditions are derived for a two-level atomic system and for a continuum source of excitation both on a relative as well as on an absolute basis. The theoretical results show that the approach of saturation sets a limit to the fluorescence radiance. The experimental results obtained with the use of a pulsed, tunable dye laser by nebulizing different elements in analytical flames are shown to be in satisfactory agreement with these theoretical results. The theory also predicts that, for a two-level system, the proportional dependence of the fluorescence signal upon the quantum efficiency observed at low irradiances is removed under saturation conditions.     

Optimization of thermostated electrodeless discharge lamps for analytical atomic spectrometry

Thermostated and unthermostated electrodeless discharge lamps (EDL's) operated at 2450 MHz with either an A-antenna or a $\text{3}\text{4}$-wavelength Broida cavity have been critically studied with respect to the effect of type and pressure of fill gas, lamp temperature, microwave power and the form of element and/or compound within the lamp, upon the source radiant output, atomic absorption and atomic fluorescence signals of Zn-213·8 nm, Pb-283·3 nm, Mn-279·5 nm, Hg-253·7 nm and Tl-377·6 nm. Temperature control of electrodeless discharge lamps eliminates most of the problems which have previously plagued their operation. Also as a result of these studies, certain misconcepts in the operation and characteristics of electrodeless discharge lamps have been clarified; e.g., the ‘skin effect’ is present in many electrodeless discharge lamps, but all lamps also exhibit a homogeneous glow discharge; at high microwave powers and/or at high lamp temperatures, spectral lines apparently exhibit little self-absorption and self-reversal; and most important, control of lamp temperature is the most critical parameter controlling spectral output because atomization within the lamps is predominantly thermal in origin. Also, contrary to the conclusions reached by some workers evaluating the analytical usefulness of atomic fluorescence flame spectrometry, it is shown that detection limits one-to-three orders of magnitude lower result when thermostated EDL's are used in atomic fluorescence spectrometry with C₂H₂-air flames.     

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.     

Evaluation of the analytical capabilities of frequency modulated sources in multielement non-dispersive flame atomic fluorescence spectrometry

Frequency modulated sources of Cd and Zn are used to produce modulated atomic fluorescence signals (at two different frequencies) in a non-dispersive flame atomic fluorescence spectrometer. To reduce the flame background level, a chlorine filter, a separated air—acetylene flame, and a solar blind photomultiplier are used. Even so, there is shown experimentally and theoretically to be a multiplex disadvantage, as compared to the conventional single slit scanning dispersive spectrometer, as a result of the flame background photon noise and an additional multiplex disadvantage at high concentrations of an interference, e.g. in the measurement of Zn (213.9 nm), Cd (228.8nm) results in a reduced S/N for Zn when the Cd signal level becomes comparable with the flame background signal level. Little future for multiplexed techniques in atomic flame spectrometry in the u.v.- Visible is predicted.     

A comparison of signal-to-noise ratios for single channel methods (sequential and multiplex) vs multichannel methods in optical spectroscopy

Four major types of spectroscopic systems for quantitative analysis of one or more spectral components we compared, with regard to signal-to-noise ratio for constant analysis time. These four methods are based on sequential-linear scan, sequential-slew scan, multichannel, and multiplex approaches. The multiplex methods can generally be classified into two types, namely Fourier transform spectroscopy and Hadamard transform spectroscopy. It is shown that for the same spectral source, for the same resolving power luminosity product of the optical system, and the same detector, the multichannel approach is the best and the sequential slew scan system is nearly as good for relatively simple spectra in the u.v.- Visible region. Multiplex methods have little to offer in the u.v.- Visible region, where the detector noise limitation seldom applies and where background shot and/or fluctuation noise are dominant but could find considerable use in the u.v.- Visible region for atomic fluorescence or emission spectroscopy especially if the density of spectral lines in the measurement region is not too great and the background intensity is low.     

Observation of spectral line profiles emitted by an inductively coupled plasma—I.: On the wavelength shift of spectral lines

Profiles of 16 spectral lines stemming from 8 elements (Ar, Na, Cu, Sr, Cd, Ba, Mg and Li) emitted by an inductively coupled plasma (ICP) have been observed and measured with a pressure-scanning Fabry-Perot interferometer. In the process of profile observations, we have found wavelength shifts of spectral lines in an ICP and for the first time studied this phenomenon quantitatively and systematically in a spectrochemical source. The profiles of spectral lines emitted by the ICP have been compared with those emitted by hollow cathode lamps. The magnitude of the wavelength shift to the red or the blue varied more or less with the plasma conditions, observation position and the concentration of a concomitant, cesium. In the present work the observed line profiles were not deconvoluted for the apparatus profiles. Typically the order of magnitude of the wavelength shift measured for spectral lines that show large shifts at an observation height of about 4 mm in an “analytical” ICP is n × 10^?3 nm, where n is about 4 for Ar I 427.2 nm and about 1 for Cu I 521.8 nm and Sr II 430.5 nm. With regard to the wavelength shift, several trends and/or regularities were found. The Stark effect is considered as the main cause of the phenomena.     

Characterization of a circular thin film plasma source for atomic emission spectroscopy

A plasma source for analytical atomic emission spectroscopy is described based on the electrical vaporization by capacitive discharge of a thin Ag film deposited on a polycarbonate membrane filter. The source is designed for the rapid, direct analysis of solid powder samples collected by filtration from fluid media. A concentric electrode system consisting of a ring-shaped graphite electrode placed on the thin film surface and a pointed graphite rod located under the membrane substrate results in a plasma with cylindrical symmetry and a radial current path. Discharge current vs time and intensity vs time profiles are compared for the concentric electrode geometry and the linear geometry used in previous studies. Two values of tank circuit inductance also are compared. Both neutral-atom and ion line radiation from an Mn sample are more intense when the center electrode is initially cathodic. The inside diameter of the ring-shaped electrode and thus the surface area of the Ag film exposed to the plasma have relatively little effect on the intensity of continuum background and line radiation from a sample deposited near the center of the film. Particle size effects, while significant, are smaller than with the linear electrode geometry. Analytical curves are presented for several lines using both a low-inductance and a high-inductance discharge. Log-log slopes range from about 0.85 to 1.05 for ion lines. Detection limits are somewhat poorer than with the linear geometry.     

Graphite furnace atomic emission spectrometry with platform atomization

The effect of inserting a pyrolytic graphite platform into a heated graphite atomizer HGA-2100 on the atomic emission of Mo, V, Al, Ni, Cu, Cr, Mn, Zn and Cd has been studied. A comparison of calibration curves obtained with and without the use of the platform showed that although the linear dynamic range is diminished by the use of the platform, atomic emission intensities are increased by as much as 23.5 times in the most favourable case (Al) studied.Whereas no detectable net emission signal could be obtained from 5.0 × 10^?9 kg of Zn and Cd atomized from the graphite tube wall, atomization and emission from the platform yielded limits of detection: 2.7 × 10^?12 kg for Zn and 4.0 × 10^?12 kg for Cd.     

Electron density spatial profiles of the DCP source

Electron densities are measured in the high current, analytical and intervening zones of a DCP whose operating parameters are systematically varied. Detailed N_e distribution profiles are obtained for various sleeve flow, nebulizer flow, arc current and matrix concentration regimes. Flowing argon is found to establish a thermal pinch in the high current zone and to steepen gradients in plasmas employed for spectrochemical analysis. The distinctive electron density distributions in the DCP are more sensitive to modulation of gas flow variables than to changes in arc current. Magnetic pressure has no discernible role in pinch formation. Electron densities in spectroscopic regions are minimally affected by easily ionized or other matrix constituents at usual analytical concentrations.     

A phosphorimetric investigation of the external heavy-atom effect in aqueous solution and its correlation with phosphorescence intensity and quantum yield

Excitation and emission phosphorescence spectra, lifetimes, and relative quantum yields of benzene and ten monosubstituted benzenes were determined at 77 K in methanol/water 10/90 v/v and in sodium iodide/methanol/water solutions. Substituents CHO, COCH(3), COC(2)H(5), OH, OCH(3), H(3), C(2)H(5), CH(2)OH and N(CH(3))(2) were investigated. Aqueous sodium iodide solutions were found to be a suitable heavy-atom solvent for phosophorimetry. Heavy-atom enhancement factors ranged from 1.2 to 8.3 according to the molecular structure and a marked decrease of the lifetime was observed for nine compounds in aqueous sodium iodide solution. A linear log-log correlation was obtained between the relative corrected phosphorescence signal and the heavy-atom enhancement factor of the monosubstituted benzenes. A similar correlation was found between the quantum yields and the heavy-atom enhancement factors. The analytical usefulness of these correlations is evident for the prediction of the magnitude of the heavy-atom effect.     

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.     

On the significance of radiation trapping in the inductively coupled plasma

On the basis of an intuitive argument and experimental evidence, the authors suggest that the populations of radiating states (³P₁,¹p_t) and metastable states (³P₂,³P₀) of atomic argon are collisionally equilibrated in the ICP discharge. A mechanism for allowing suprathermal populations to exist in the analytical region of the ICP is proposed based on the “trappin” of resonance radiation within the discharge boundaries. The possible consequences of this model are discussed.     

Two phytoalexins from sugarbeet (Beta vulgaris) leaves

Two phytoalexins have been isolated from the leaves of sugarbeet infected with Cercospora beticola and their structures have been shown to be 2′,5-dimethoxy-6,7-methylenedioxyflavanone (1) and 2′-hydroxy-5-methoxy-6,7-methylenedioxyisoflavone (2).     

An assessment of an atmospheric pressure helium microwave plasma produced by a surfatron as an excitation source in atomic emission spectroscopy

A new microwave plasma at atmospheric pressure is described. The plasma is supported by a surface wave excitation structure called “surfatron”. Both argon and helium can be used to sustain the plasma. In this study, helium has been selected. The sample introduction system consists of an ultrasonic nebulizer associated with a dcsolvation system. The analytical performance in terms of detection limits, calibration curves and some interferences have been assessed. Because of the relatively low power and the short residence time the main problem seems to be the atomization process.     

Spectrochemical analysis of mediaeval glasses by means of inductively coupled plasma and glow discharge sources

For the analysis of mediaeval glass fragments, the inductively coupled plasma (ICP) and the Grimm glow discharge source (GDS) were used.For ICP measurement, the glass samples were digested with a mixture of sulphuric, nitric and hydrofluoric acids. For GDS investigations the glass powder, mixed with Cu or Zn powder was pressed to pellets. The sputtering rate was investigated in the case of standard and glass samples. Analytical curves were found to be linear over a wide concentration range for the investigated elements Fe, Mn, Al, Zn, Cr, Ni, Cu, Sn, Ti, V, Pb and Ag. The limits of detection and relative standard deviations met the requirements for archaeological glass analysis.