An atmospheric pressure glow discharge optical emission source for the direct sampling of liquid media

Abstract

A liquid sampling–atmospheric pressure glow discharge (LS-APGD) optical emission spectroscopy source is described. This device utilizes an electrolytic solution containing the analyte specimen as one of the discharge electrodes. The passage of an electrical current (either electrons or positive ions) across the solution/gas phase interface causes local heating and the volatilization of the analyte species. Collisions in the negative glow region of the plasma result in optical emission, which is characteristic of the analyte elements. Operation of this device with the analyte solution acting as either the cathode or anode is demonstrated. Typical discharge conditions include currents of 80 mA, potentials of up to 1000 V, with the i-V characteristics exhibiting ‘abnormal’ glow discharge behavior. The analyte-containing electrolyte solution is not limited to hydronium ions, as other salts (e.g., Li+ and Na+) also provide sufficient conductivity to sustain the discharge. Spatial maps of analyte emission and spectral background provide insights into the basic operation mechanism of the discharge. The use of a coaxial gas flow around the stainless-steel capillary outlet permits stable plasma operation at low liquid flow rates (0.1–0.3 mL min−1) where pulsation present in the liquid delivery system makes the discharge itself quite unstable. Analytical response curves for Hg, Mg, Na and Pb are demonstrated to have good linearity, with preliminary limits of detection determined to be in the range of 1.1–2.0 ppm (≈10 ng) for 5 µL sample volumes.