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Development and characterization of an electrothermal vaporization parallel plate capacitively coupled plasma Rahman, Md. Mahbubur

Abstract

The parallel plate capacitively coupled plasma (PP-CCP) is a fairly new and unique source for spectrochemical analysis. In this source, a radio frequency (rf) plasma is initiated and sustained in a quartz discharge tube placed between a pair of water-cooled conducting electrodes to which rf power is applied. The geometry of the discharge torch is flexible and allows the introduction of both liquid (using electrothermal vaporizer, ETV) and gaseous samples into the plasma. The primary objective of this work was to characterize the fundamental processes and properties of this CCP discharge using spectroscopic techniques. Spatially resolved emission profiles from plasma background species (e.g. He, N₂⁺, and OH) reveal that the analyte emission or emission sensitive zone is, as a result of the transverse power coupling geometry, situated adjacent to the discharge wall. The spatial distributions of plasma species remain unchanged, but the signal intensity from all three species are altered with variations of plasma power and gas flow rate. The difference in measured electronic excitation (T[sub exc]) temperatures' suggests the absence of local thermodynamic equilibrium (LTE) conditions in PP-CCP. T[sub exc] was calculated from the slope of the Boltzmann plot using Fe and He as the thermometric species and Pb excitation temperature was calculated using the two line method. Over a power range from 100-250 W, excitation temperatures are 3255-3900 K for He, 3540-4500 K for Pb, and 4300-4890 K for Fe. The rotational temperature (T[sub rot]) was also measured using both OH and N₂⁺ molecular spectra with values occurring in the 828-911 K and 845-956 K range over a power range of 75- 275 W, respectively. Results obtained from matrix interference studies show that the presence of either NaCI or NaNO₃ as a concomitant in silver analyte, causes an interference effect by both enhancing, and decreasing, the emission intensity dependent upon the amount of added easily ionizable element (EIE). The maximum signal interference, signal enhancement, was observed at 125 W plasma forward power. The degree of enhancement from an EIE decreases with increasing rf forward power switching to a signal depression at a power of 250 W. The degree of ionization for Mg and Cd vaporized into this source has also been studied. Using electrothermal vaporization and a C C P operating at 200 W, the degree of ionization is 83% and 48% for Mg and Cd, respectively. An increase in applied power increases the degree of ionization. Changes in plasma gas flow rate, up to 1.0 L min⁻¹, also slightly changes the degree of ionization, likely due to mass transport effects and minor changes in the discharge conditions. Additions of EIE (Na as NaNO₃ ) up to 10 times (w/w) of analyte increases the degree of ionization somewhat. All these results indicate the potential of the PP-CCP as an ion source for mass spectrometry. The absolute detection limit for Pb and Ag, for a 4 cm long electrode, is found to be 0.33 ng and 24 pg. The signal - to - noise ratio intensifies with increasing rf power and gas flow rate, reaching its maximum value at 250 W rf power and 0.2 L min⁻¹ gas flow rate for Pb analyte. The optimum plasma operating conditions for ultimate signal - to - noise ratio is, however, analyte dependent. The precision, another useful tool in measuring the power of an analytical method, is approximately 4-10% at a concentration of 100 times the limit of detection. The effect of electrode length (hence, the plasma volume) on some fundamental and analytical characteristics has also been studied and the results show an improvement in detection limit with increasing electrode length, reaching as low as 0.96 pg for silver and 0.34 pg for magnesium for a 6 cm long electrode. Electrode length also influences the analyte ionization in this source. With changing electrode length, two opposing factors, namely "residence time" and "power density", become active. At a plasma power of 250 W and 1.0 L min⁻¹ gas flow rate, the residence time emerged as a dominant factor when the electrode measured up to 5 cm long; with a longer electrode, the power density outweighed the residence time factor. The degree of Mg ionization reaches up to its maximum of 87% for the 5 cm long electrode.

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