Fig. 1. Raw Infrared spectra of milk powder
(a): Pure milk powder; (b): Melamine-containing milk powder

Fig. 2. Spectra examples of pure milk powder and melamine-containing milk powder
a: Melamine-containing milk powder; b: Pure milk powder

Fig. 3. The classification results of PCA
+: Melamine-containing milk powder;◇: Pure milk powder

Fig. 4. The classification results of NMF
+: Melamine-containing milk powder;◇: Pure milk powder

Fig. 5. The classification results of PLS-DA
(a): Melamine-containing milk powder; (b): Pure milk powder

Fig. 6. The classification results of LDA
+: Melamine-containing milk powder; ◇: Pure milk powder

Fig. 7. The classification results of ULDA
+: Melamine-containing milk powder; ◇: Pure milk powder

Fig. 8. The transformation vector plot for ULDA