Table 3.

The number of selected characteristic variables by XGBoost of zinc, copper, cobalt, and nickel ions under different preprocessing methods.

Preprocessing	Number of Characteristic Variables
	Zinc	Copper	Cobalt	Nickel
Raw	57	77	78	68
Denoising	62	83	82	71
1st derivative	63	86	73	72
2nd derivative	59	75	81	87