A Glowing Source of Inspiration

In the world of atomic spectrometry, the inductively coupled plasma (ICP) reigns supreme. New plasma sources for atomic emission and mass spectrometry are emerging, however, which promise to be vastly simpler, cheaper, and more versatile. One example that is under development in a number of laboratories worldwide is the solution-cathode glow discharge (SCGD), an atmospheric-pressure glow discharge operated out in the ambient atmosphere. The SCGD is particularly intriguing because this high-temperature, direct-current plasma is sustained directly on top of a liquid surface – a curious property that could make the SCGD a giant-killer.

To fully appreciate the simplicity of the SCGD, it helps to be familiar with its operating principles. The SCGD uses a continuous flow of a conductive analyte solution (for example, 0.1M HNO₃) cascading from the tip of a quartz capillary into a catch-basin. Once ignited, the SCGD is sustained between the liquid surface (cathode) and a positively-biased metallic counter-electrode (anode). Because the surface of the liquid represents the cathode of the glow discharge, the SCGD directly samples the liquid via a ‘sputtering’ action, ejecting material into the plasma for analysis by atomic emission spectroscopy (AES).

Amazingly, this humble 100-Watt discharge has demonstrated analytical capabilities on a par with some conventional AES approaches. At analyte solution flow rates of 1 mL/min, limits of detection for many elements have been reported at levels near or below 1 ng/mL, with a linear response over three or four orders of magnitude; performance comparable to ICP-AES. More impressively, the SCGD has demonstrated limits of detection of 1–30 pg/mL for the alkali earth metals, significantly outstripping ICP-AES. Moreover, because the liquid surface sampled by the discharge is constantly renewed by the flowing analyte stream, the SCGD is a natural detector for chromatographic separations. Researchers have reported excellent performance with HPLC, ion chromatography, and capillary electrophoresis.

What I have described so far is relatively straightforward atomic spectrometry; however, the SCGD plasma is proving extremely versatile. For example, Shelley and coworkers have reported that biomolecules introduced into the flowing solvent stream are detected as intact molecular ions (and ion fragments) when the SCGD is analyzed by mass spectrometry (1). Researchers in the material sciences have used the SCGD to create nanoparticles directly from solution, environmental chemists have shown that the SCGD is an excellent means of disinfecting water supplies, and medical researchers are investigating the ability of plasmas to disinfect wound sites and enhance healing.

This seemingly disparate set of applications is possible because plasmas are inherently versatile; often, physical traits, such as gas temperature, can be tuned to an intended application. Open questions remain, however. For the SCGD to realize its potential, a more thorough fundamental understanding is required of the physical mechanisms that permit this highly energetic plasma to sit atop the surface of a liquid.