巧克力珍珠的呈色与成因属性

采用LA-ICP-MS、拉曼光谱仪、微型光纤光谱仪等分析测试仪器,对近期市场上出现的巧克力珍珠样品的化学成分、拉曼光谱、反射光谱、表面微观特征及其成因属性进行了初步研究。分析与测试结果表明,多数巧克力珍珠样品内发育有简单且着色外浅(棕黄色、棕红色)内深(灰黑色)的同心色带结构,深浅不一的棕红色色斑和灰白色色斑沿珍珠层表面相间无序分布;经低温加热漂白处理后,巧克力珍珠样品的化学成分、拉曼光谱、UV-NIS反射光谱特征均发生了不同程度的变化。其化学成分以相对富Cu和Pb、贫K为特征;1 276 cm-1处的拉曼谱峰红移至1 371 cm-1附近,1 583 cm-1处的拉曼谱峰则蓝移至1 545 cm-1附近,663 cm-1处的拉曼谱峰随之消失;在可见光波长范围内,c带(697 nm)的相对吸收强度骤减,a带向长波方向红移并与b带组合成一宽大舒缓的吸收谱带。初步证实,巧克力珍珠样品的颜色与其黑色素内共轭双键电子跃迁之间尚无直接的内在联系,主要归因于塔希提棕黑色珍珠内存在的钾色素蛋白经热变异作用所致。

Colouration and Genetic Attribute of Chocolate Pearl

The preliminary researches on the chemical compositions,Raman spectrum,reflectance spectrum,surface micro-characteristics and the genetic attribute of the chocolate pearl samples that appear in the market recently are tested by means of LA-ICP-MS,Raman Spectrometer,Micro-optical fibre and other testing instruments.The analysing and testing results show that most of the chocolate pearl samples carry the simple concentric chromatape structure with light-coloured outer layer(brownish yellow,brownish red),dark-coloured inner layer(grayish black) and the uneven coloured brownish red spots and grey spots are distributed randomly along the surface of pearl layer.After the bleaching treatment at the low temperature,the chemical compositions,Raman spectrum and UV-NIS reflectance spectrum characteristics of the chocolate pearl samples change from time to time.The chemical compositions of the chocolate pearl samples are characterized by rich Cu and Pb while lack of K.The red shift of Raman spectrum at 1 276 cm-1 is close to 1 371 cm-1 while the blue shift of Raman spectrum at 1 583 cm-1 is close to 1 545 cm-1and Raman spectrum at 663 cm-1 vanishes.It is obvious that the absorption strength of band c(697 nm) is weakened abruptly,red shift of band a to the long wave and combined with band b to form a wide absorption spectral band within the range of the optical wavelength.It is proved that there is no direct internal relation between the colour of the chocolate pearl samples and the conjugate double-bond electron transition of the melanin,mainly due to the thermal variation effect of the K pigment protein existing in the brownish black Tahitian pearl.