The annular glow discharge: a small-scale plasma for solution analysis

A recent trend in plasma spectrochemistry, and in analytical chemistry in general, has been towards miniaturization. Miniaturized plasma sources often have properties leading to low cost, potential portability, and simplified integration with separation instrumentation such as chromatographs. However, these discharges have not often performed well with solution samples. To address this shortcoming, we have developed a new, small-scale (5 x 2 mm) plasma, maintained in atmospheric-pressure helium between a tubular cathode and a rod-shaped anode (both made of steel). The discharge extends between the two electrodes and an annular glow is visible within the cathode tube. An aerosol is introduced into the plasma through the cathode, and atomic emission is observed in the near-cathode region. In this study, we observe the effects of solvent addition on the plasma in terms of its electrical and spectroscopic characteristics and how they change with solution flow rate. Given that even a robust source such as the inductively coupled plasma is different under wet and dry conditions, it is not surprising that the discharge introduced here is affected by the presence of an aerosol. However, the discharge is stable even with significant solvent loading. In this preliminary investigation, flow injection has been used to establish detection limits for several metals between 7 ppb (0.7 ng absolute) for Cd and 111 ppb (11 ng absolute) for Cu.