基于近红外光谱的象牙鉴定

Ivory Identification Based on Near Infrared Spectroscopy

中国已经全面禁止了象牙交易,但象牙的走私并未因此而终止。对象牙及其制品的鉴定是打击走私一个重要环节。传统的形态学鉴定法,是基于横截面上施氏结构的有无,但有的象牙本身缺少特征纹路,有的则经过雕刻加工后纹路消失,这类样品往往无法鉴定。从分子生物学角度鉴定象牙具有更高准确性,象牙DNA含量极低,象牙中提取DNA的难度较大。已有学者采用光谱技术鉴定和区分象牙等材质,并表明采用拉曼光谱和近红外光谱的短波区域可以区分象牙及其类似物(其他动物牙齿)。采用近红外光谱的长波波段1000~1800 nm,通过建模的方式建立了象牙的鉴别方法。以非洲象(Loxodonta spp.)象牙、亚洲象(Elephas maximus)象牙、猛犸象(Mammuthus spp.)象牙为校正样品集,其他非象牙材质制品,包括抹香鲸(Physeter macrocephalus)牙、河马(Hippopotamus amphibius)牙,象牙果(Phytelephas macrocarpa),模仿成象牙的塑料制品等为验证样品集,采集了230件样品共383条近红外光谱信息。比较了象牙样品颜色和厚度等对光谱鉴定结果的影响,建议扫描具有该物质典型颜色部位,同时取样品厚度大于1 mm的部位进行扫描,否则易造成误判。基于象牙光谱4个较敏感波段,1160~1200、1430~1500、1680~1710和1720~1750 nm,以及SIMCA(soft independent modeling by class analogy)定性分析方法,建立象牙的鉴别模型。建模过程中,在平衡假阳性率和假阴性率后,推导出本模型的最佳主因子值为2和F值为0.21。采用所建立的模型鉴定样品,对象牙的识别准确率达到100%,对角制品、塑料仿制品以及象牙果仿制品等伪品的识别率也达100%,但对质地较相近的其他动物的牙齿,如野猪牙、抹香鲸牙等,则易错鉴为象牙,需要结合其他方法做进一步的鉴定。光谱模型鉴定方法具有无损、简便等特点,同时通过模型鉴别光谱图形较人为观察更加客观和高效,因此较适合作为监管部门现场执法用的快速查验初筛方法。

The trade of elephant ivory(from now on referred to as ivory)and its products has been completely banned in China,but the smuggling has not stopped.The identification of ivory is an important part of the fight against smuggling.The ivory objects can be identified visually if characteristic features,such as the Schreger pattern,on the cross-sections can be observed.However,if it does not have the features or disappears after carving,the sample is difficult to identify by morphology.When the morphological method is not adequate,molecular biology-based method is an alternative.However,since the extremely low DNA content,extracting DNA from ivory is not easy.Some scholars have distinguished ivory and the analogues(teeth of other animals)by using the Raman spectrum and the short wave region(780~1100 nm)of the near-infrared(NIR)spectrum.In this study,an identification method of ivory was established by establishing a NIR spectroscopy identification model,based on the spectrum region from 1000 to 1800 nm.Taking African elephant(Loxodonta spp.)ivory,Asian elephant(Elephas maximus)ivory and mammoth(Mammuthus spp.)ivory as the calibration set and other non-ivory products,including sperm whale(Physeter macrocephalus)teeth,hippopotamus(Hippopotamus amphibius)teeth,taguanut(phyelephas macrocarpa),ivory plastic imitation,etc.as the verification set,a total of 383 NIR spectra of 230 samples were collected.By comparing the spectral data of ivory products with different colors and thicknesses,it was found that the color and thickness of the ivory will affect the analysis results.Scanning the parts with typical color of the substance and the parts with the thickness greater than 1 mm is recommended.Based on the spectra in the regions of 1160~1200,1430~1500,1680~1710 and 1720~1750 nm,and the SIMCA(soft independent modeling by class analogy)qualitative analysis method,a calibration model for predicting ivory or non-ivory by NIR spectroscopy was developed.After balancing the false positive rate and false negative rate,it was deduced that the best principal factor of this model was 2 and the F value was 0.21.When applying the model,the recognition accuracy of ivory was 100%;all the animal horn,plastic and ivory fruit products could be accurately identified as non-ivory with 100%accuracy.However,the teeth of other animals with similar ivory textures,such as boar teeth and sperm whale teeth,tended to be mistaken for ivory by the model;consequently,further testing by other methods was required for these substances.The spectral model method is simple,nondestructive,and more objective and efficient than manual interpretation of spectrograms.Therefore,it is suitable to be used as a preliminary screening method for on-site law enforcement by regulators.