A new, versatile, direct-current helium atmospheric-pressure glow discharge Francisco J. Andrade,* William C. Wetzel, George C.-Y. Chan, Michael R. Webb, Gerardo Gamez, Steven J. Ray and Gary M. Hieftje Received 30th May 2006, Accepted 19th September 2006 First published as an Advance Article on the web 5th October 2006 DOI: 10.1039/b607544d A novel direct current glow discharge sustained in helium at atmospheric pressure has been developed. Current–voltage behavior and spectroscopic characteristics strongly suggest that the system operates in the glow regime, in spite of the high pressure. The diffuse and extremely stable discharge is typically operated within a voltage range of 300–900 volts (in the current-controlled mode) and at currents ranging over tens to hundreds of milliamps. Spatially resolved spectroscopic measurements of some selected species are presented. Rotational temperature profiles were calculated using the OH emission spectrum, yielding values in the positive column ranging from 1300 to 1600 K. Introduction Glow discharges (GD) have become invaluable analytical sources for both optical and mass spectrometry. 1 Though their analytical application has been traditionally focused on the elemental characterization of solid samples, suitable schemes for the analysis of gases 2 and liquids 3 have also been devel- oped. Direct current (dc) GDs allow the analysis of conductive materials, while radiofrequency powering schemes must typi- cally be used for non-conductive samples. For both powering methods, depth-resolved elemental profiles can be obtained. New areas of application, such as the use of GDs for the ionization of organic compounds, have also been explored. In this case, it has been shown that by pulsing 4–6 the GD, switching the polarity of the electrodes, 7 or implementing a flowing afterglow technique, 8,9 information at both the ele- mental and molecular levels can be generated. Overall, few sources display the versatility and simplicity of GDs, which makes them extremely powerful tools for facing new chal- lenges in chemical analysis. 10 In spite of these advantages, the operating pressure of GDs, typically in the single Torr range, sometimes limits their performance. At reduced pressure, sample introduction be- comes more complex and special arrangements, such as atmo- spheric-sampling devices, 2 have been developed to partially overcome this problem. Desolvation in a low-pressure GD is inefficient, so liquid samples must usually be dried before being introduced into the discharge cell. 3 Also, the necessity of working at reduced pressure can increase instrumental com- plexity. In fast-flowing afterglow strategies, for example, the use of a high gas flow requires a significant pumping capa- city. 11 Additional limitations arise when GDs are used for the ionization of organic compounds. At reduced pressure, mole- cules generally undergo minimal collisional relaxation, which favors fragmentation. Thus, obtaining intact parent molecular ions with a GD becomes more difficult, and often requires a pulsed powering scheme and time-resolved detection. 6,12 For these reasons, there has long been an interest in developing GDs that can be operated at atmospheric pressure. One limitation to operating direct-current GDs at higher pressures lies in the glow-to-arc transition (GAT). Elevating the pressure favours the onset of instabilities on the surface of the electrodes which, in a direct current GD, will ultimately lead to an arc. 13–15 Glow discharges and arcs represent two significantly different regimes. GDs are stable, diffuse, high- voltage and low-current sources: each of these characteristics is intimately related to the analytical performance of the GD. High voltages provide electrons with considerable energy from the applied electrical field, which is then transmitted to other species without extensive heating of the gas. Arcs, on the other hand, are sustained with low voltages and high currents, resulting in high gas temperatures. This thermal process gen- erates density gradients in the gas, which lead to a filamentary and typically unstable discharge. For these reasons, avoiding the GAT is extremely important in order to preserve the desirable analytical features of GDs. Efforts to develop GDs that can be sustained at atmospheric pressure have been focused on minimizing the effect of tran- sient instabilities of the electrical field on the surface of the electrodes, either by changing the system geometry or by using alternative powering schemes. Changes to the system geometry are based on similarity laws, which state that the gap between the electrodes must be reduced as the pressure is raised, in order to maintain the stability of the glow regime. 16 At atmo- spheric pressure, sub-mm gaps are required, which has led to the development of miniaturized dc GDs. 17,18 Although these devices have shown promise, 19 many remaining problems, particularly in the area of sample introduction, must be addressed in order to fully exploit their advantages. To avoid transient instabilities at the electrodes, radiofrequency (rf) powering schemes have also been used. 20 In some cases, the rf is combined with a dielectric barrier at the surface of at least one of the electrodes, 21–27 which usually leads to a plasma Department of Chemistry, Indiana University, Bloomington, IN 47405, USA This journal is  c The Royal Society of Chemistry 2006 J. Anal. At. Spectrom., 2006, 21, 1175–1184 | 1175 PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry