## 3 Results and Discussion
### Optimization of Experimental Conditions  
The emission intensities of spectral lines can reflect the population of the excited species. The content of elements in the measured material can be expressed by the emission intensity of corresponding elements. Therefore, in order to decrease the LOD of Cu, the optimization experiment is mainly focused on the change of the Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm) emission intensity. The effects of voltage, gas flow rate, pH, and organic matter on the emission intensity of Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm) are researched, and measured with 1 mg/L Cu \({}^{2+}\) to obtain the optimal parameters. The effect of applied voltage on the emission intensity of Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm) is shown in Figure 7, which affected energy efficiency [29]. The result is obtained under the conditions of the 2.1 L/min Ar flow rate, and no organic matter and nitric acid is added. The emission intensity increases almost linearly with the increase in applied voltage from 3.8 to 6.7 kV. With the increase in applied voltage, more energetic electrons are produced in a stronger electric field, which leads to the generation of more excited atoms, and the emission intensity of Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm) increases correspondingly [25]. Figure 6: (**a**) Measurement area of plasma gas temperature; (**b**) the distribution of plasma gas temperature with horizontal position (Ar flow rate: 1.68 L/min, discharge gap: 2 mm, applied voltage: 5.6 kV). Figure 7: The effect of the applied voltage on emission intensity (Ar flow rate: 2.1 L/min, no organic matter and nitric acid added). The effect of the Ar flow rate on Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm) emission intensity is also investigated and the result is illustrated in Figure 8, because it affected the stability of the plasma and nebulization efficiency [30]. The result is obtained under the conditions of a 5.6 kV applied voltage, no organic matter and nitric acid, and a 2 mL/min solution flow rate. The emission intensity increases with the increase in the Ar flow rate, and reaches the maximum value when the flow rate of Ar is 1.68 L/min. However, when the flow rate is greater than 1.68 L/min, the emission intensity decreases gradually with the continuous increase in the Ar flow rate. Given that the liquid inlet speed is stable at 2 mL/min, the optimal gas flow rate produces a more nebulized solution in the discharge region, makes a series of sufficient reactions about the formation of Cu atoms, and promotes a more stable discharge. The pH also has a significant influence on the Cu I (3d\({}^{10}\)4p-3d\({}^{10}\)4s, 324.75 nm) emission intensity, affecting the chemical characteristic because it changes the liquid ingredient [31]. The result is obtained under the conditions of 5.6 kV applied voltage, 1.68 L/min Ar flow rate, and no organic matter added, as shown in Figure 9. The emission intensity increases with the pH value from 2.08 to 3.59 and reaches the maximum value, and when the pH value is higher than 3.59, the emission intensity gradually decreases. The result indicates that Cu ions are more favorably converted to their volatile species in a weak acidic condition, promoting the progress of reaction (2), so the emission intensity is increased [16]. Cu ions are largely unstable at near neutral pH, resulting in an enhanced hydrolysis of metal ions, so the emission intensity is reduced [23]. Therefore, 3.59 pH with HNO\({}_{3}\) is selected to improve the sensitivity of detection and the atomization efficiency of copper. Figure 8: The effect of the Ar flow rate on emission intensity (applied voltage: 5.6 kV, no organic matter and nitric acid, solution flow rate: 2 mL/min). Figure 9: The effect of the solution pH on emission intensity (applied voltage: 5.6 kV, Ar flow rate: 1.68 L/min, no organic matter added).