Plasma diagnostics of a new direct-current atmospheric pressure glow discharge

Complete text of publication follows. Most glow discharges have been operated in noble gases under reduced pressure; however, recently also new atmospheric pressure glow discharges (APGD) were introduced. It was shown that a stable glow discharge could be sustained in both He, Ar and Ne at pressures ranging from 1 to 760 torr. The advantages of an APGD in comparison to reduced pressure glow discharge are the ease of construction and use as well as the instrumentation and operating costs. Furthermore, APGDs offer the possibility to introduce gaseous as well as liquid samples (see e.g. R. K. Marcus et al., J. Anal. At. Spectrom., 16 (2001) 931-937) which makes them promising radiation and ionisation sources for optical emission spectrometry. In this study a new type of direct-current APGD for optical emission spectrometry was developed. The plasma is generated between two electrodes in a helium atmosphere. Currents between 15 and 50 mA could be applied at voltages ranging from 600 to 900 V. For the discharge the voltage-current characteristics were found to be normal to abnormal. The use of other gases (e.g. Ne) and their mixtures is also considered. The use of several electrode materials was investigated as well as their geometry. Hollow ones allow the introduction of the sample directly into hottest part of the plasma whereas massive ones showed slightly higher plasma stability. Spatially resolved plasma diagnostics was performed and spectroscopic measurements of several species are presented. Moreover basic plasma properties were determined, such as voltage-current-gas flow characteristics, rotational temperatures, ionisation temperatures, excitation temperatures, electron temperatures and electron number densities. Spatially resolved determinations of the excitation temperature of the discharge were carried out by the introduction of ferrocene into the plasma source and measurement of selected iron atomic emission lines (371-378 nm). The excitation temperatures were found to be in the range from 3000 to 5000 K. The radiation source described will be shown to be useful for coupling with chemical and electrochemical hydride generation as well as with gas chromatography and electrothermal evaporation for the elemental determination at the trace level.