R. K. Skogerboe

Aerosols, Aerodynamics, and Atomic Analysis

The suppression of atomic excitation in a microwave-induced plasma due to the presence of sodium has been studied for three different nebulization systems. Evaluation of the factors affecting this effect has shown that its occurrence depends on the nature of the analytical aerosol introduced and the aerodynamic features of the sample nebulization system. The results indicate that the sodium suppression effects observed can be largely accounted for by reductions in the analyte transport efficiency due to changes in the aerodynamic characteristics of the aerosol resulting from variations in the salt content of the nebulized solutions. It is further shown that these effects may be generally predicted on the basis of fluid mechanical principles and that these may be used to design systems to eliminate the interferences. The study underscores the essentiality of considering aerodynamic factors in formulating mechanistic explanations for interference effects in flames and plasmas.

Characterization of a dc Plasma as an Excitation Source for Multielement Analysis

A commercial dc plasma has been evaluated as an excitation source for multielement spectrometric analyses. Plasma temperatures have been measured and typical excitation profiles determined. Detection limits have been determined using a direct reading spectrometer equipped for simultaneous background correction measurements. The results indicate that precise multielement analyses can be carried out at low concentration levels comparable to those that can be obtained by flame atomic absorption methods.

Characterization of an Interelement Enhancement Effect in a dc Plasma Atomic Emission Spectrometry System

The enhancement of atomic and ionic emission in a dc plasma due to the addition of easily ionized elements has been studied. The results show that the addition of the concomitants: (1) causes enhancements in both atomic and ionic emission, (2) does not cause significant changes in the electron temperature or the electron population of the plasma excitation zone, (3) results in a downward shift in the rotational temperature of that zone, and (4) does not cause significant changes in the efficiency with which analytical species are delivered to the plasma. These observations indicate that the enhancement effect cannot be readily explained based on plasma temperature increases or the classical ionization repression mechanism. As an alternative, it is suggested that the enhancement may result from increases in the population of metastable argon with coincident increases in excitation via collision with this species. This mechanism is circumstantially supported by the present results but must be regarded as tentative.

Evaluation of the Analytical Capabilities of a dc Plasma-Echelle Spectrometer System

The analytical capabilities of a dc plasma used in combination with an echelle spectrometer have been evaluated. Measurements of the spectrometer resolution capabilities have demonstrated that they closely approximate the high values theoretically predicted but are ultimately limited by Doppler broadening effects occurring in the plasma. Investigations of possible ionization interference effects have shown that enhancements due to the addition of easily ionized elements are generally observed for elements having ionization potentials below approximately 8.5 eV; these may be compensated for by the addition of an excess of an easily ionized element. Evaluation of solute vaporization interference effects indicates that they also occur but can be negated by proper choice of operational conditions and/or the addition of releasing agents. Comparison of the detection capabilities of the system with those reported for other techniques shows that they are equivalent or superior to those characteristic of flame atomic absorption and often competitive with those obtained with the inductively coupled plasma, atomic emission system.

A Dynamic Background Correction System for Direct Reading Spectrometry

A direct reading emission spectrometer system is described which permits dynamic background correction measurements at each analytical wavelength. The system utilizes a refractor plate mounted on a tuning fork as a rapid means for square-wave shifting the spectral band pass from each analytical wavelength to the adjacent background wavelength position. A laboratory computer controls the system, receives the data, and performs the background subtraction. Evaluation of the system has shown that photon counting statistics are applicable and that reliable background corrections are obtained. Comparisons of this correction approach with others that have been used have further demonstrated its advantages for multielement analysis by emission spectrometry.

Evaluation of the Microwave-Induced Plasma for Multielement Emission Spectrometric Analyses

A microwave-induced argon plasma has been evaluated as a spectrochemical excitation source for multielement analyses of solutions. Results for 12 elements indicate that detection capabilities are typically better than 0.01 µg/ml. Analyses of food composite samples indicate that matrix effects occur but these are cancelled by the use of the internal standard method.

Evaluation of Spectral Interferences Associated with a Direct Current Plasma-Multielement Atomic Emission Spectrometer (DCP-MAES) System

Spectral interferences associated with a direct current plasma combined with a 20-element, direct reading echelle spectrometer have been characterized. The results indicate that molecular emission interferences due to entrained air are generally negligible, but the presence of high carbon concentrations may cause problems for two elements. Spectral interferences due to the presence of high concentrations (500 mg/l) of Ca, K, Mg, and Na were also studied. No problems were noted for Na or K while an interference due to atomic emission of Mg was observed. Calcium was responsible for stray light interferences due to near scatter and to reflections occurring in the secondary optics. The latter were eliminated by masking techniques and the former were compensated for via the use of a simple procedure. Overall, the spectral interferences encountered appear to be less severe than those reported for an inductively coupled plasma-direct reading spectrometer system.

Determination of Lead in Soil

The analytical determination of lead in soil is discussed with particular reference to emission spectrographic and atomic absorption spectrophotometric methods. The problems associated with the two techniques are discussed, and accuracy and precision data are presented. Data indicating that the titanium-to-lead concentration ratio can be used to differentiate between lead-contaminated and noncontaminated soils are presented.

Investigation of the Hollow Cathode Discharge as an Atomic Absorption Medium

A hollow cathode discharge has been studied as a means for vaporizing and atomizing samples for atomic absorption. The design and operational characteristics of the cell are presented with detection limits and precision data for Ag, As, Ca, Cd, Hg, Pb, Se, and Zn. In general, detection limits between 1 and 10 ng were obtained. The present limitation of the method due to the sample holder blank is discussed.

Experimental Characterization of Aerosol Production, Transport, Vaporization, and Atomization Systems. Part II: Factors Controlling Aerosol Size Distributions Produced

The effects of nebulization conditions on the size characteristics of the aqueous aerosol produced have been investigated for a cross-flow nebulizer. It is shown that the nebulizer gas flow rate does not affect the upper limit mean sizes of the aqueous droplets transported from the nebulization chamber but that the mean size of the analyte-containing aerosol itself is affected. Model equations are presented descriptive of the effects of gas flow rate and analyte concentrations on analyte aerosol size characteristics.