Table 2 A summary of regions of the FTIR spectrum of leather, found through PCA/PLS loadings and k-means clustering, which change as a result of time, degradation or pH of surrounding conditions [22, 24, 32]
Analysis | Result observed | Influential regions (cm−1) | Possible peak assignment |
|---|---|---|---|
PCA: full spectrum | PC1: changes over time PC2: changes over time | 2441–2501 3209–3229 | None: spectral feature O–H stretch, N–H stretch |
PC3: Differences between control and degraded | 1474–1496 | C–N–H bend | |
PLS-DA: full spectrum | Differences between control and degraded Differences between pH 3 and pH 5 | 1490–1525 1000–1080 2849–2933 2329–2370 1705–1753 | C–N–H bend C–O–C=O in tannin ester stretch C–H stretch Sulfur (possible contamination) C=O stretch |
k-means: full spectrum | Differences between pH 3 and pH 5 | 2911–2925 1738–1746 1140–1160 1019–1034 | C–H stretch C=O stretch C–O–C cyclic ether stretch C–O–C=O in tannin ester stretch |
PCA: 800–1800 cm−1 | PC1: changes over time PC2: changes over time | 1151–1196 1526–1539 989–998 | C–O–C cyclic ether stretch C–N–H bend Amide III, C-O stretch |
PC3: Differences between control and degraded PC4: Differences between control and degraded | 1684–1686 1341–1343 1030–1086 1258–1273 | C=O stretching C–N stretch, C–O stretch C-O stretch Amide III | |
PLS-DA: 800–1800 cm−1 | Differences between control and degraded | 1630–1648 1503–1531 997–1061 | C–N–H bend/Amide I C–N–H bend C–O–C=O in tannin ester stretch |
k-means: 800–1800 cm−1 | Differences between pH 3 and pH 5 | 1023–1031 1490–1511 | C–O–C = O in tannin ester stretch C–N–H bend |