Velmer A. Fassel

Inductively Coupled Plasma-Atomic Emission Spectroscopy: Prominent Lines

The prominent lines of 70 elements as emitted in an inductively coupled plasma excitation source have been identified. The lines are listed alphabetically by element and numerically by wavelength. Detection limit capabilities are estimated for each spectral line.

Excitation temperatures and electron number densities experienced by analyte species in inductively coupled plasmas with and without the presence of an easily ionized element

Spatially resolved radial excitation temperatures and radial electron density distributions experienced by analyte species in the observation zone of 15 to 25 mm above the load coil of a toroidally shaped, inductively coupled argon plasma are presented and related to the analytical performance of these plasmas. A comparison of radial temperatures measured with support gas (Ar I) lines and with a typical analyte thermometric species (Pe I) at 15 mm above the load coil is given. Radial (Fe I) excitation temperatures obtained at three observation heights are compared for aerosol carrier flows of 1.0 and 1.3 liters/min. The addition of a large amount of an easily ionized element (6900 µg of Na per ml) did not significantly change Fe I excitation temperature distributions at the respective aerosol carrier flows and observation heights. A comparison of radial electron density distributions measured with Saha-Eggert ionization and with Stark broadening methods is given for an observation height of 15 mm above the load coil. The differences between electron density values obtained by these methods is discussed. The effect of addition of 6900 µg of Na per ml on Saha-Eggert electron density distributions at three observation heights is also discussed.

Quantitative Elemental Analyses by Plasma Emission Spectroscopy

Argon-supported inductively coupled plasmas operated at atmospheric pressures are excellent vaporization-atomization-excitation-ionization sources for analytical atomic emission spectroscopy. When a polychromator is used for observing the emitted spectra, the metals and metalloids can be determined simultaneously at the ultratrace, trace, minor, and major concentration levels under one set of experimental parameters. Alternatively, programmable scanning spectrometers may be utilized for sequential determinations. The atomization-excitation process is remarkably free of interelement interactions, the powers of detection are in the part per billion range for most elements, and sample manipulation requirements prior to analyses are often minimal. The technique meets the requirements of an analytical system for the simultaneous or sequential determination of the elements at all concentration levels to an unusual high degree.

Ultrasonic nebulization of liquid samples for analytical inductively coupled plasma-atomic spectroscopy: an update

Abstract An improved, continuous-flow ultrasonic nebulizer equipped with a desolvation system for generating dehydrated aerosol particles prior to their injection into analytical inductively coupled plasmas is described. Results of a critical evaluation of the performance of the nebulizer-desolvation system are also presented. Compared to the commonly used pneumatic nebulizers studied in this work, the ultrasonic version described in this paper provided superior powers of detection, ranging from factors of 5–50, and yielded comparable to superior short and long term reproducibility for dilute acid and high salt content solutions. Clean out times for the ultrasonic nebulizer were marginally longer, by 15–20 s, than those observed for pneumatic nebulizers. “Memory effects” and “desolvation interferences” were generally reducible to negligible proportions through the application of various expediencies discussed in this paper. When substantial changes in concomitant concentrations caused measurable interelement effects, the magnitude of these effects tended to be slightly higher for the ultrasonic system. However, when the samples destined for ultrasonic nebulization were diluted by factors of ~ 10, which corresponds approximately to the superiority of the nebulization efficiency of the ultrasonic nebulizer, the magnitudes of the interelement effects were comparable.

Line Broadening and Radiative Recombination Background Interferences in Inductively Coupled Plasma-Atomic Emission Spectroscopy

Spectral line broadening and radiative electron-ion recombination processes may make significant contributions to the total spectral background level when inductively coupled plasma excitation sources are observed with spectrometers having low stray light levels. These background contributions are more easily identified in inductively coupled plasma discharges because of their stable background level and by the fact that net analyte line intensities are affected to such a small degree by changes in the concentration of concomitants. The wings of collisionally broadened lines may produce significant background changes at wavelengths removed 10 nm from the parent line center. For some elements such as Mg, linear Stark-broadened lines produced spectral background at unexpectedly large displacements from the line center. The radiative Al ion-electron recombination continuum produces a greater than tenfold increase in the background from ∼210 down to 193 nm at Al solution concentration of 2500 μg/ml. Thus, a solution containing only 250 μg/ml of Al will cause an ∼100% increase in the background level below 210 nm over that measured when deionized water is nebulized into the plasma.

Ultratrace Analyses by Optical Emission Spectroscopy: The Stray Light Problem

When atomic emission spectroscopy is employed for the determination of trace elements at concentrations near the detection limit, the spectral background will normally be a large fraction of the total measured signal. Precise background corrections are therefore required if accurate analyses are to be achieved. Experimental evidence is presented to document the fact that stray light may produce substantial shifts in the background signal measured with some spectrometers when the total composition of the sample changes. The data presented have been obtained with the inductively coupled plasma as an excitation source; similar effects can be expected with other excitation sources as well. Examples of various forms of stray light originating from grating defects (ghosts, near and far scatter) and defects in the design of spectrometers are presented. Various methods for the reduction, elimination or correction of stray light effects are also discussed.

Inductively coupled plasma-optical emission spectroscopy: Excitation temperatures experienced by analyte species

Abstract Spatially resolved, radical excitation temperature distributions experienced by analyte species injected into the axial channel of a toroidally shaped, inductively coupled argon plasma are presented. Typical axial temperatures experienced by the thermometric species (FeI) ranged from 6500 K to 5500 K in the analytical zone of 15–25 mm above the load coil. A comparison of temperatures calculated with different sets of transition probabilities is also given.

Evaluation of spectral continua as primary sources in atomic absorption spectroscopy

Abstract The results of a critical and systematic evaluation of the feasibility of employing spectral continua as primary sources in atomic absorption spectroscopy are presented. Useful sensitivities were observed for most of the metallic elements in the periodic system. For 32 elements, the sensitivities observed for a spectral continuum primary source were either comparable to or exceeded those presently obtained with sharp-line primary sources. Recordings of typical spectra are presented as well as several analytical curves illustrating the quantitative analytical applications.

Mass spectrometric evidence for suprathermal ionization in an inductively coupled argon plasma

Mass spectra have been obtained of species in the axial channel of an inductively coupled argon plasma by extracting ions from the inductively coupled plasma into a vacuum system housing a quadrupole mass spectrometer. Ionization temperatures (Tion) are obtained from relative count rates of m/z-resolved ions according to two general types of ionization equilibrium considerations: (a) the ratio of doubly/singly charged ions of the same element, and (b) the ratio of singly charged ions from two elements of different ionization energy. The Tion values derived from measurement of Ar+2/Ar+, Ba+2/Ba+, Sr+2/Sr+, and Cd+/I+ are all greater than those expected from excitation temperatures measured by other workers. The latter three values for Tion are in reasonable agreement with values obtained by optical spectrometry for a variety of argon inductively coupled plasmas.

Theoretical principles of internal standardization in analytical emission spectroscopy

Abstract The principles which affect the choice of analysis and internal standard lines in analytical atomic spectroscopy are explored theoretically using a computer-based model. The spectral source chosen for the model was the induction-coupled argon plasma fed with a water solution of metallic salts. The effect of excitation energy, ionization energy, partition functions and electron density on the analytical line pair intensity ratio is explored. Examples are presented which illustrate the contribution of each variable to the intensity ratio over the 3000–15000°K temperature range, allowing the reader to assess the importance of each based on a knowledge of the properties of his own source. Finally, some of the problems encountered in the course of this study are discussed.