Fig. 1. NJW cluster analysis result
(a): Original sample point;(b): Sample point after clustering

Fig. 2. Spectral determination spectra of 800 mg·kg^-1 heavy metal Pb under different water contents

Fig. 3. Spectral preprocessing result
(a): S-G smooth denoising;(b): Linear background minus background

Fig. 4. Effect of r on peak signal to noise ratio

Fig. 5. Spectrum before and after NMF algorithm processing
(a): Spectrum before NMF processing; (b): The spectrum of NMF processed by the number of end elements r=5; (c): The spectrum of NMF after the number of end elements r=10

Fig. 6. Comparison of measured and predicted values of PLSR model, EPO-PLSR model and NMF-PLSR model
(a): PLSR model; (b): EPO-PLSR model;(c): NMF-PLSR model