## 1 Introduction  
In the last few decades, with the rapid development of industry and agriculture, heavy metal pollutants have been widely found in soil, water, and atmospheric particulates, and are causing serious harm for human health and destroying the ecological balance by their long-term toxicity, difficulty in degradation and accumulation along the food chain [1]. It is important to detect trace heavy metal ions, because heavy metal pollution processes are slow, and there are serious consequences when the content exceeds the critical value [2]. In the conventional methods, inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectrometry (AAS) have good sensitivity and stability in metal detection, and AES has the ability to detect multiple elements simultaneously [3; 4; 5]. However, these technologies often require large equipment, complex operation, or even vacuum operation, so they cannot satisfy demand for fast and in situ detection [6]. Against this background, researchers have begun to use atmospheric pressure discharge plasma (APDP)-AES for the analysis of metal elements. APDP is characterized by high electron temperature, high density of active species, simple equipment, and multiple discharge forms [7], which can directly detect heavy metal elements in solid, liquid, and gas particles [8; 9; 10]. Among them, the detection of heavy metal in water is the most commonly used. Electrolyte cathode discharge (ELCAD), as a way to generate APDP, was first introduced in elemental analysis by [PERSON] et al. in 1993 [11]. Based on the original ELCAD, researchers made a series of efforts to improve the detection ability of heavy metal and discharge stability, such as to change the excitation power supply and discharge form, reduce the size of the device, and add some surfactants, etc. [12]. [PERSON] et al. [13] used a new microsecond pulse energy supply driven-ELCAD for Cu element analysis, and by inhibiting the interference of N\({}_{2}\) (C\({}^{3}\)\(\uparrow\)\(\mathrm{I}\)u-\({}^{3}\)\(\uparrow\)\(\mathrm{I}\)g) and OH (A\({}^{2}\)\(\Sigma\)-X\({}^{2}\)\(\uparrow\)\(\mathrm{I}\)) on detection and changing the trigger mode of ICCD, the detection limit of Cu is 0.092 mg/L. [PERSON] et al. [14] developed direct current-APDG-AES system for direct determination of Cd in liquid, different from conventional needle electrode as anode, plasma is generated between a solid pin-type tungsten cathode and a liquid drop (50 mL) placed on a graphite disk anode, and a LOD of 0.20-0.40 mg/L is obtained for Cd with high tolerance to Ca, K, Mg, and Na (up to 200 mg/L) in solution. To further improve the detection performance, the researchers began to convert the liquid analytes into a gaseous or small particle liquid bead form to improve the excitation efficiency of the analytes [15; 16; 17; 18]. [PERSON] et al. [19] studied an original SCGD-ultrasonic nebulizer (USN) to do elemental analysis. By comparing the performance of plasma with and without USN, they found that the analytical lines' intensities of the studied metals are increased from several to 35 times, and in the novel method, the detection limits of Li and Mn are 0.08 \(\upmu\)g/L and 52 \(\upmu\)g/L, respectively. [PERSON] et al. [20] generated microplasma-dielectric barrier discharge (DBD) on the nozzle of a pneumatic nebulizer as the excitation source for simultaneous multi-element analysis and the acquired LODs of 19 elements are between 0.9 and 880 \(\upmu\)g/L (for Cd and Cr, respectively). Compared with the other optical emission spectrometry (OES), this system balanced the scope of the simultaneous detection of elements and the detection sensitivity. [PERSON] et al. [21] adopted an on-line detection approach to introduce the gas phase analytes into the discharge area by a low power DBD-tungsten coil electro-thermal vaporization-AES, and with a 10 mL sample solution, LODs of 0.8 mg/L for Cd and 24 mg/L for Zn are acquired.