PLASMA PENCIL EMISSION RESPONSE ON COMBINED MATRIX SOLUTIONS
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| Year of publication | 2018 |
| Type | Appeared in Conference without Proceedings |
| MU Faculty or unit | |
| Citation | |
| Description | In case of our plasma pencil the forwarded power is maintained at 140 W which is a good medium value. The range of operating conditions is from about 100 W to 180 W for the power and about 1.5-5 l min-1 for the plasma gas. The discharge can absorb, vaporize, atomize and excite liquid samples at the electronic excitation temperature of about 3000-4000 K which provides good stability. The discharge is apparently adapted to the water load by self-shortening of the plasma column. It was shown that intensities of atomic lines are suppressed by the presence of easily ionisable elements (EIE: 1st and 2nd group of the periodic table) in the sample solution [3]. The suppression was also found out at the presence of nitric acid. Now we launch a spectroscopic study of copper and zinc atomic lines emission in dependence on solutions composition covering various cations and anions of alkali salts as nitrates, chlorides and sulfates, also in combinations with acids. Sodium was chosen as first cation for experiments with various anions. The intensities of both copper and zinc lines always decreased at the presence of sodium and the decrease was deeper for copper lines with increasing sodium concentration. However, significant differences were found for different anions as partners of sodium in the salt. Although the experiments are yet not completed it looks like that the most intensive depression effect is brought about chlorides, less effect is observed for nitrates and sulphates and the dependence on the sulphate concentration is not monotonous. Recalculated to the concentration range the steepest intensity depression is observed for about 0.1 g l-1 of chlorides or molar equivalent of nitrates and sulphates while for the 1 g l-1 and more the line intensity changes only moderately or is nearly stable. The intensity decreases to about one half or third of the original value. The effects of EIE have been investigated many years for flames and ICP and spectral buffering is well known. However, non-monotonous dependences on EIE concentration and effects of anions of the EIE salts for CCP are still not explained. A reduction of the depression effect from sodium at the presence of nitrates or nitric acid was observed. This result goes against the effect of the sole nitric acid and must be much stronger than preferential ionization of EIE. EIE preferentially consumes energy from the plasma for its easy ionization and less energy is then available for excitation of non-EIE atoms because their electronic excitation levels are higher than EIE ionization energy. At a certain concentration of EIE only ionization of non-EIE atoms is suppressed and higher atomic emission of non-EIE can occur. This mechanism is supposed to be the result of the EIE-non-chloride anion mixture. It is difficult to propose exact reactions in the plasma to explain the more efficient excitation of copper or zinc. Nevertheless, chloride atomic lines were not detected in the spectrum (which is not a surprise owing to the temperature) but the presence of molecular anions demonstrates itself by N2 and NO vibrational bands. Energies of electronic transitions of these species are several eV which is comparable with atomic electronic transitions but they are also accompanied by a rich set of low energetic vibrational transitions. As a result, it can be supposed that reactions of e.g. Na-NO and Na-N2 in the plasma can maintain such moderate energy transfer from free electrons and argon atoms to sodium ones that the heavily ionisable analyte atoms are more excited and less ionized. Also reactions of metastable nitrogen and argon with EIE and analyte atoms can contribute to their better excitation. Consequences of these reactions in the plasma are stronger than a decrease of the nebulisation efficiency and change of the droplet size distribution at the presence of the acid. |
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