In-situ generation of Ag, Cd, Hg, In, Pb, Tl and Zn volatile species by flowing liquid anode atmospheric pressure glow discharge operated in gaseous jet mode - Evaluation of excitation processes and analytical performance

The use of flowing liquid anode atmospheric pressure glow discharge (FLA-APGD) operated with the aid of a gaseous jet as an efficient and novel excitation source for optical emission spectrometry (OES) was evaluated in details. Although about 50 elements have been tested by introducing respective standard solutions into a discharge system, only emission lines of Ag, Cd, Hg, In, Pb, Tl and Zn have been identified. In this arrangement, the surface of solution was bombarded by electrons resulting in generation of volatile species of mentioned elements. After penetrating the plasma, they were excited and extraordinary atomic emission was observed for these elements. It was found that the use of the Ar or He jet provided beneficial excitation conditions, while reduction of the discharge gap led to suppression of the intensity of interfering diatomic molecular bands like NO, OH and N-2. Under optimal operating conditions of the discharge system, detection limits (DLs) assessed for Ag, Cd, Hg, In, Pb, Tl and Zn were respectively 0.001, 0.006, 0.16, 0.093, 0.076, 0.007 and 0.018 mu g L-1, and they were on average by 10-times better as referred to earlier FLA-APGD constructions operated without the gaseous jet. In addition, the discharge was characterized by greater stability and precision of measurements (< 2%). Dynamic ranges of calibration curves covered between 3.5 (Cd, In, Zn) and over 5 orders of magnitude (Ag, Tl). The biggest weakness of the developed system turned out to be high susceptibility to matrix interferences. It was found that presence of foreign ions (at concentrations of 1-10 mg L-1) suppressed emission from studied elements on average by 20-60%. To clarify the reason for extremely high sensitivity of emission lines of Ag, Cd, Hg In, Pb, Tl and Zn, efficiency of transport of these elements from the liquid phase to the plasma was determined. It was established that from 5% to 70% of analytes presented in sample solutions were released, whereas evaporation of water was merely 5-15%. Since the FLA-APGD plasma was enriched with analytes atoms, it was concluded that such processes like electrospray formation or solution evaporation do not make a major contribution to transport of analytes from the liquid phase to the discharge, and that it was mainly due to plasma induced volatile species generation processes.