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Effects of rf power and tube wall temperature on plasma stability and analyte emission in furnace atomization plasma excitation spectrometry Rahman, Md. Mahburbur
Furnace Atomization Plasma Excitation Spectrometry ( FAPES ) is a relatively new emission spectrochemical method. For analyte atomization and excitation, this method employs a graphite furnace and an atmospheric pressure plasma sustained inside the furnace. The main objective of this work was to characterize the plasma at high rf powers, up to 150 W, during the analyte atomization cycle. The temporal response of CO and He (I) line at different rf powers shows complex emission characteristics during the atomization step. The intensity of CO and He (I) emission decreases suddenly at higher furnace temperature and higher rf powers. This sudden decrease of intensity indicates the extinguishing of plasma at higher temperature as a result of the changing in power coupling efficiency between the load impedance and output impedance of the rf oscillator. The reflected power level also increases with increasing forward power and does not depend absolutely on the furnace wall temperature but on the temperature of rf center electrode. The spatial distribution of analyte in the plasma shows an increase in emission intensity from the center of the furnace toward the wall, reaches a maximum at 1.25 mm from the center, followed by a decrease. Both atomic absorption and emission experiments show a non - uniform temperature distribution along the length of the rf electrode. In comparison to the furnace wall , the temperature lag of the rf electrode causes analyte condensation on the rf electrode and subsequent re-vaporization, resulting in two peaks in the temporal response of the analyte. Analyte condensation on the rf electrode is severe at lower rf powers but at higher rf powers, for example 125 W, the rf electrode becomes too hot to act as a second surface and, as a result, a single peak is observed. The effect of rf power on analyte signal is a decrease in integrated intensity for both emission and absorption at rf powers higher than 30 W due to several reasons including pre-atomization loss of analyte, a change in excitation characteristics, and an increase in ionization of analyte at higher rf powers. Furthermore, the shape of the peaks shows that the residence time for excited Ag atoms is shorter than that for ground state atoms at rt power 50 W and more. This observation suggests that some of the ground state atoms do not become excited due to quenching of the plasma which is likely because of the change in power coupling efficiency between the load impedance and output impedance due to rapid change of temperature and / or rapid change in thermionic electron density in the furnace.
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