Three-step identification of infrared spectra of similar tree species to Pterocarpus santalinus covered with beeswax

Abstract It is difficult to distinguish the Pterocarpus santalinus in the furniture wood market, since it is similar to other tree species such as Dalbergia louuelii and Pterocarpus tinctorius. Moreover, the features of the wood are usually covered with the beeswax on the surface, making it even harder to be identified the tree species. Extractive is important components of wood and plays a significant role in the color of wood. Benzene-alcohol extractives contain specific chemical components of wood, which is an important part to distinguish tree species. Color parameters show that the color is obviously deepened when the sample is covered with beeswax. As shown in the SEM examination, beeswax appears on the wood surface in a large area and is immersed in the vessels of wood. In this paper, with the infrared spectrum three-step identification, the differences of the chemical composition of benzene-alcohol extractive are analyzed to distinguish tree species. The results show that the functional groups of the compounds contained in beeswax are mainly composed of long-chain methyl -CH3, methylene –CH2- and ester carbonyl C O, carboxyl C–O stretching at 2956, 2917, 2850, 1740, 1468, 1378 and 721cm-1. The absorption peaks of beeswax are so strong that they weaken the peaks of benzene-alcohol extractives from three tree species. In the FT-IR and SD-IR spectra, P. santalinus and P. tinctorius have strong peaks at 1736 cm−1, 1590 cm−1, 1151 cm−1 (1149 cm−1), 1067 cm−1 (1064 cm−1), 1033 cm−1, 958 cm−1 (959 cm−1), and 835 cm-1 (833 cm-1), while D. louvelii has a weak broad peak at 1630 cm-1 and 1602 cm-1, and there is no absorption peak at other wave numbers. At 754 cm-1 and 700 cm-1, no absorption peaks are found in the other two tree species. In the Synchronous 2DCOS-IR spectra in the ranges of 900–1200cm-1,1200-1425 cm-1, and 1480-1700cm-1, the sensitivity of some functional groups in P. santalinus and D. louvelii for the temperature perturbation tends to be the same. The main difference between them is that more functional groups of P. santalinus are sensitive to temperature disturbance. For P. tinctorius, it is sensitive to temperature perturbation, in which, the ester groups C–O stretching vibration at 1151, 1168, 1202 cm-1 and aromatic ring skeleton stretching belongs to flavonoids and lignin at 1511 and 1594 cm-1. The above results indicate that the three-step identification by the infrared spectroscopy can accurately and quickly distinguish tree species.