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Title:Analytical Implications of the Plasma Dynamics in the High-Voltage Spark Discharge
Author(s):Bye, Cheryl Ann
Doctoral Committee Chair(s):Scheeline, Alexander
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Analytical
Physics, Atomic
Physics, Fluid and Plasma
Abstract:The high voltage spark has been used many years for emission spectrochemical analysis. Yet, spark studies have produced only heuristic descriptions of spark analyte sampling and excitation. It has been dogma in the analytical spectroscopy community that quantitative models of plasma behavior would lead to understanding and control of spark analytical behavior. Both the mean discharge behavior and its fluctuations have thus been studied. To monitor plasma homogeneity, the electron density and temperature were measured at a variety of locations in the discharge. Such spark characterization studies in the past have not been definitive due to detector limitations. By exploiting charge transfer detector (CTD) technology, one can now obtain the required information bandwidth, sensitivity, dynamic range, and S/N ratios required for definitive plasma diagnostic measurements. Due to the low light levels, line breadth, and non-analytical operating conditions in the hydrogen doped spark, the possibility of using Ar(I) Stark broadening as a viable alternative to $\rm H\sb\beta$ was explored. An agreement within 10% among the methods was observed. A complete spatial map of the dispersed radiation with a S/N ratio of 200+ was obtained. These Ar(I) emission profiles were then used to calculate the electron densities. Subsequently, an Abel inversion of the dispersed spatial map was made. From these single spark radial electron density maps, it was concluded that the plasma was fairly homogeneous and reproducible. Therefore, one can rule out plasma inhomogeneity as the source of the analyte spark-to-spark emission irreproducibility.
Having concluded that plasma inhomogeneity is not the source of spark irreproducibility, analyte excitation studies were conducted to elucidate whether the irreproducibility was due to variations in the analyte excitation or uptake. To determine whether the excitation behavior is constant from spark-to-spark, single spark Boltzmann plots of spatially resolved Mo(I) emission were constructed over a limited wavelength range. The results indicate that analyte excitation is reproducible from spark-to-spark, under spatial integration conditions. To probe a larger wavelength range, a CCD detection optimized echelle spectrometer system was utilized. The echelle system has a spectral bandpass of ${\sim}200$ nm and a minimum resolution of 0.05 A. Unfortunately, echelle throughput is not sufficient for single spark analysis. However, time-integrated copper matrix studies have been conducted on a series of alloys, namely, pure copper, nickel silver and brass. Here, one has severe matrix differences, yet the analyte excitation distribution remains constant. Identical results were observed in an excitation study of pure iron and stainless steel. This suggests that analyte excitation is independent of sample matrix. Therefore, one can conclude that the major source of spark irreproducibility lies in the sampling event itself. As a consequence, further plasma studies of the spark are unwarranted. Instead, the plasma-sample interaction must be studied if further improvement in the spark spectrochemical analysis is desired.
Issue Date:1994
Description:230 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1994.
Other Identifier(s):(UMI)AAI9503151
Date Available in IDEALS:2014-12-17
Date Deposited:1994

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