Role of powering geometries and sheath gas composition on operation characteristics and the optical emission in the liquid sampling-atmospheric pressure glow discharge

Characterization of the liquid sampling-atmospheric pressure glow discharge optical emission spectroscopy (LS-APGD-OES) source is described with regards to applications in low-flow separations such as capillary liquid chromatography and electrophoresis. Four powering modes are investigated, including the effects of the individual modes on current–voltage characteristics, analyte emission response, and temporal broadening of flow injection profiles. A concentric sheath gas is employed to stabilize the solution delivery at low liquid flow rates. Sheath gas composition (N₂ or He) effects analyte emission responses as well as gas phase rotational and excitation temperatures. The respective powering modes both measures of temperature, with the OH rotational gas temperatures ranging from ∼2100 to 3000 K and the Fe (I) excitation temperatures ranging from ∼2400 to 3600 K. Rotational temperature values increase slightly when helium is employed as a sheath gas as opposed to nitrogen, with the corresponding excitation temperatures increasing somewhat as well. Analytical response curves for Na and Hg in the various powering modes demonstrate good linearity, with the limits of detection for the analytes found to be on the order of ∼4–10 ppm for 5 μl injections; equating to absolute detection limits of between 20 and 45 ng. It is believed that the approach demonstrated here suggests further improvements that will permit applications in a wide variety of aqueous solution analyses where low-flow rates and limited volumes are encountered.