Ling Bei
FW 300
Dissertation Defense
February 27, 2004
ABSTRACT
Inductively coupled plasma optical emission spectrometry (ICP-OES) is
widely used in trace-metal analysis. However, the behavior of this hot, ionized gas
and effects on analyte excitation and ionization is not fully understood. To obtain
sufficient information of the characteristics of plasma, two previous approaches, Abel
inversion and tomography, have been used. The significant drawbacks of these
approaches include complex optical and data manipulation systems, the significant
amounts of time required for data collection, and long-term plasma drift and flicker.
A much more ideal approach would be to obtain multiple instantaneous high spectral
resolution images of the entire discharge while retaining a fast speed of wavelength
access across a broad spectral range. An acousto-optic tunable filter hyperspectral
imaging (AOTF-HSI) system has the potential to meet all of these plasma imaging
requirements.
An AOTF is a digitally accessible, compact, variable wavelength optical filter
that operates based on the interaction between optical radiation and an acoustic
velocity compression or shear wave traversing an anisotropic material. Interaction
between the light and the acoustic waves allows photons of a very narrow wavelength
range to be diffracted by the AOTF.
A hyperspectral imaging (HSI) system is constructed by combining an AOTF
and a monochrome CCD camera. The initial characterization indicated that the
AOTF performed according to the theoretical equations for the frequency/wavelength
relationship and bandpass as a function of wavelength. A continuum source was used
to study the effects of AOTF power on hyperspectral image fidelity. A line emission
source was used to examine the applicability of the system for the observation of
narrow atomic emission lines. The AOTF-HSI spatial resolution was examined using
a USAF-1951 resolution target, and the system resolution achieved 8.8 m
horizontally and 12.4 m vertically with 1: 1 imaging optics. Test images varied
from a red-green-blue (RGB) target, to a stamp, to a fluorescein injected bovine eye.
For AOTF-HSI applications of ICP chemistry, nine elements, Al, Ba, Ca, Ce,
Cr, Li, Mg, Na, and Sr were used for imaging studies. Using in-house developed
software, spectral information was acquired at each pixel within the image.
Background (continuum) corrected plasma images were obtained by subtracting the
offline images from the online images. Vertical profiles of Sr atom and ion emission
are described as an example, to demonstrate the capability of AOTF-HSI on plasma
imaging.
Plasma chemistry for these elements was studied through variation of a
number of plasma parameters. The effects of ICP rf power, nebulizer sample carrier
gas flow rate, liquid sample delivery rate, and added easily ionized elements on
plasma chemistry were tested. Results indicate that correlations between elements’
excitation and ionization energies and the enhancements on emission intensities could
be clearly discerned. Lastly, a Cr and Mg mixture solution was used to demonstrate
the multielement analysis capabilities of the AOTF-HSI system.