Fig. 1. Spectra analysis of three single ions. (a) Original spectra of single ions; (b) ion proportion at wavelength point

Fig. 2. Spectral signals with different orders. (a) Original spectral signals; (b) original spectral filtering signals; (c) first-order spectral filtering signals; (d) second-order spectral filtering signals

Fig. 3. Relationship among coverage degrees and orders for different ions. (a) Co2+; (b) Cu2+

Fig. 4. Fitting plot of order and distortion degree with wavelet filtering-fractional differentiation method

Fig. 5. Non-inferior solution sets of different ions. (a) Co2+; (b) Cu2+

Fig. 6. Spectra with different differential orders. (a) Original spectra; (b) first-order spectra; (c) second-order spectra; (d) 1.4-order spectra; (e) 1.5-order spectra

Fig. 7. Wavelength point selection based on multi-index fusion feature after pretreatment

Fig. 8. Errors between predicted and actual mass concentrations of different ions. (a) Co2+; (b) Cu2+