Evaluation of echelle-spectrometer/image-dissector combinations for continuous-source atomic absorption spectroscopy
Abstract
Results of a series of studies are presented that evaluate echelle-spectrometer/image-dissector combinations for multielement quantitation by continuous-source atomic absorption spectroscopy. An initial study shows that the required levels of resolution and range are significantly beyond those available with a system that had been previously designed for line-source work. Sensitivities with this system are 100-fold lower than corresponding line-source values, and calibration curves are nonlinear throughout the concentration ranges examined. The instrument was modified to increase resolution 4-fold at the expense of spectral range; sensitivities improved 40-fold and calibration curves became linear to 0.1 absorbance unit. The results of the initial study serve as a basis for the redesign of the system to improve resolution without significantly degrading range so that multielement quantitation would still be available. The modifications include use of a 20-groove mm$\sp{-1}$ echelle grating blazed at 76$\sp\circ$, and removal of the magnetic shield from the image dissector, which substantially improves its quantitative performance. The quality of the echelle grating limits use of the instrument to wavelengths above 300nm. Use of a little-known optical phenomenon, associated with the off-Littrow mounting of blazed gratings, is also presented; positioning the 76$\sp\circ$ echelle grating at an 83.5$\sp\circ$ angle of incidence provides simultaneous improvements in bandpass, range, and intensity by factors of 2.1, 1.5 and about 2, respectively. The overall performance characteristics of the modified instrument are presented; the resolution is about 350,000 (0.0012nm at 430nm) and the spectral range is continuous from 300 to 430nm. The sequential nature of the image dissector in combination with low photoelectron rates (20kHz) limits use of the system to sequential, single-element quantitation with electrothermal atomization, and to sequential, multielement quantitation (8 s element$\sp{-1}$) with flame atomization. Calibration curves are linear to 0.2 absorbance unit, and sensitivities compare favorably with line-source values. The further development of the system to provide simultaneous multielement quantitation is also presented.
Degree
Ph.D.
Advisors
Pardue, Purdue University.
Subject Area
Analytical chemistry
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