In Situ Detection of Trace Heavy Metal Cu in Water by Atomic Emission Spectrometry of Nebulized Discharge Plasma at Atmospheric Pressure

The in situ detection of trace heavy metal is very important for human health and environmental protection. In this paper, a novel and stable nebulized discharge excited by an alternating current (AC) power supply at atmospheric pressure is employed to detect the trace metal copper by atomic emission spectrometry. Different from the previous experiments in which a conductive object was wrapped around a pneumatic nebulizer directly as a discharge electrode. Plasma is generated near needle electrodes and aerosol is introduced from above the electrode gap by a pneumatic nebulizer, which avoid damage to the fragile device. The effects of applied voltage, gas flow rate, pH value of liquid, and concentration of organic addition agents on the emission intensity of Cu I (3d104p-3d104s, 324.75 nm) are investigated for the purpose of optimizing the experiment conditions. For studying the discharge characteristics and understanding the mechanisms of metal atomic excitation, the waveforms of applied voltage and discharge current are measured, and the vibrational and rotational temperature are calculated by the spectra of N2 (C3∏u-B3∏g, Δυ = −2). In addition, gas temperature evolution of nebulized discharge is acquired and it is found that the emission intensity of Cu I (3d104p-3d104s, 324.75 nm) can be affected by applied voltage, gas flow rate, pH value of liquid, and concentration of organic addition agents. An optimized experimental condition of nebulized discharge for Cu detection is 3.59 of the pH, 5.6 kV of applied voltage, 1.68 L/min of Ar flow rate, and 2% of the ethanol. Under this condition, the limit of detection (LOD) of Cu can reach up to 0.083 mg/L.