| Description |
The fact that plasma is able to dissociate number of molecules is the basis for number of plasma applications including applications in analytical chemistry, where plasma is used for the dissociation of volatile compounds, frequently hydrides, which are used as carrier molecules for number of toxic, hazardous or biogenic elements. Recently, dielectric barrier discharges (DBDs) have been systematically studied as atomizers, i.e. as a tool for the decomposition of hydrides or other volatile molecules. The situation in DBDs is relatively complicated, because the volume dissociation of hydrides combines with surface reactions on walls, which can lead to unwanted losses of analyte atoms or be used for in-situ preconcentration of analytes and to improve the detection limit of the analytical methods. Our work is a systematic (TA)LIF study of reactive species in various atomizers, including both the dominant radicals originating from the main plasma gases (i.e. atomic hydrogen, atomic oxygen, OH radicals) and various free analyte atoms (Pb, Te, Bi, Se, Sn, Ge, Sb). In order to increase the LIF signal-to-noise ratio, we developed a LIF method based on partially saturated fluorescence and verified it by comparison with absorption measurements. The laser-based diagnostics on discharge-based atomizers were supplemented by E-FISH measurements of the electric field in the discharge. Experimental results were complemented by a numerical model of the plasma chemistry and gas flow in the atomizers. Regarding the atomization mechanisms, we confirmed the theory that the decomposition of hydrides in both plasma and flame atomizers is based on reactions with atomic hydrogen, we revealed differences in spatial distribution of various analytes demonstrating the important role of the surface reactions, ex- plained the difference of the performance of various atomizers, observed the preconcentration of analyte atoms on the wall and explained why a small (i.e. significantly substoichiometric) amount of oxygen manages to initiate the preconcentration on walls. As an example of LIF measurements, the distribution of free Sb atoms in a DBD atomizer can be discussed, when stibine (SbH3) from a 1 ng/l-solution was supplied to the plasma together with Ar and H2. The results obtained with a continuous supply of SbH3 reveal a nearly homogeneous distribution of free Sb atoms through the whole active discharge (-25 to +25 mm). The homogeneous distribution of Sb atoms through the whole reactor and the relatively high Sb concentration demonstrate a good performance of the DBD for atomization of SbH3. It was possible to preconcentrate the analyte (Sb) on reactor walls, which led to an increase of the fluorescence signal by more than an order of magnitude. The release of Sb from the walls had a character of a wave travelling from the discharge center towards its edges. The strong increase of Sb concentration demonstrate the possibility to control the surface reactions and use them for analyte preconcentration and improvement of the detection limit of studied species in DBD-based atomizers.
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