Study of a direct current atmospheric pressure glow discharge in helium with wet aerosol sample introduction systems

Abstract

In this study a He DC atmospheric pressure glow discharge (APGD) is characterized under dry conditions and with the introduction of wet aerosols. The aerosols are generated by a conventional pneumatic nebulization system using a MicroMist nebulizer placed in a double pass spray chamber according to Scott or alternatively by a custom-built drop-on-demand (DOD) system based on printer cartridges. The experiments are performed at a He gas flow of 500 mL min⁻¹ and 40 mA current. The influences of the H₂O load on the discharge rotational temperature (T_rot), excitation temperature (T_exc) and the electron number density (n_e) are determined. Temperature reductions when comparing dry and wet conditions are found to be around 500 K for the T_rot and 240–400 K for the T_exc. Detection limits for the elements Cd, Cu, Mg, Mn and Na are presented for pneumatic nebulization coupled to a continuous flow injection system. They are found to be between 10 μg L⁻¹ for Na and 140 μg L⁻¹ for Cu. In the case of the drop-on-demand system a value of 16 μg L⁻¹ for Na was obtained. The applicability of the discharge to the determination of Na in a tap water sample at concentration levels of 20 mg L⁻¹ is shown for both introduction systems and the accuracy of the results is found to be within 1.3 mg L⁻¹ as compared to the result of inductively coupled plasma optical emission spectrometry (ICP-OES).