Single-Second Pulse LA-MC-ICP-MS Analysis of Sr Isotope: Application to the Quasi "Non-Destructive" Discrimination between Seawater Cultured Pearl and Freshwater Cultured Pearl
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摘要:
珍珠是一种广受欢迎的生物成因有机宝石。由于海水珍珠与淡水珍珠之间巨大的价格差异，市场上淡水珍珠冒充海水珍珠进行售卖的现象屡见不鲜，尤其是在某些外观特征如颜色、尺寸、大小和表面纹理等极为相似的珍珠中，如白色的“淡水Akoya珍珠”与白色的Akoya珍珠[1]。海水珍珠与淡水珍珠的鉴别方法研究存在巨大的市场需求。本研究提出，Sr同位素组成可以作为区分海水珍珠与淡水珍珠的有效工具。为了实现珍珠样品中Sr同位素组成的高效测试，本研究开发了一种基于单秒激光剥蚀的LA-MC-ICP-MS Sr同位素分析方法，使用线性回归策略进行数据校正[2]。该方法将剥蚀时间从30~60 s缩短至1 s，不仅加快了测试速度和降低成本，而且极大程度地减少了对珍珠这类宝石的损伤(图 1)。此外，本研究开发了一款用于珍珠微区原位Sr同位素分析的参考物质GIC-P，并对不同激光剥蚀束斑、频率和能量密度条件下的剥蚀行为进行了探索。总体来说，单秒脉冲Sr同位素LA-MC-ICP-MS分析方法，可在几近于“无损”的情况下满足海水珍珠与淡水珍珠的鉴定需求。
基于所开发的分析方法与标准物质，本研究首次报道了海水珍珠和淡水珍珠的Sr同位素组成。结果发现，海水珍珠的87Sr/86Sr比值基本保持在0.709 2附近，与现代海水的Sr同位素比值吻合良好[3-4]，而淡水珍珠则显示出相对较高且变化较大的87Sr/86Sr比值。从中国市场上收集到的淡水珍珠87Sr/86Sr比值范围在0.710 2~0.713 5，这与长江流域淡水样品中的Sr同位素组成呈现出高度一致性[5]。这表明珍珠中的Sr同位素组成与其养殖水体中的Sr同位素组成紧密相关，利用Sr同位素区分海水珍珠与淡水珍珠的本质在于对其养殖水体Sr同位素组成的追溯，是一种极为可靠的鉴别技术。
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关键词:
- 海水珍珠 /
- 淡水珍珠 /
- Sr同位素 /
- LA-MC-ICP-MS /
- 微区原位分析
Abstract:Pearls are popular organic gems formed by biological processes. The price differences between seawater and freshwater cultured pearls create significant market demands for robust methods to differentiate between the two types. This demand is particularly important for pearls exhibiting similar characteristics, such as colour, shape, size, and surface texture, for example, those observed in white "freshwater Akoya pearls" and Akoya pearls[1]. In this study, the utilization of Sr isotope composition is proposed as a tool for distinguishing between freshwater and seawater cultured pearls. A single-second laser ablation protocol coupled with a linear regression calibration method[2] was developed for Sr isotope analysis to differentiate pearl types (i.e., freshwater pearls or seawater pearls) while causing minimum damage with the ablation time being reduced from 30-60 s to only 1 s (Fig. 1). Additionally, a pearl reference material (RM), GIC-P, was developed for micro and in situ Sr isotope analysis. The ablation behavior, involving laser ablation spot sizes, repetition rates, and energy densities required for accurate Sr isotope analysis, was investigated. In general, the single-second pulse LA-MC-ICP-MS analytical method developed in this study minimizes sample consumption and offers identification between seawater pearls and freshwater pearls with quasi "non-destructive" capabilities.
With the proposed method and the newly developed RM, it was reported for the first time that seawater cultured pearls consistently exhibit the 87Sr/86Sr ratio around 0.709 2, consistent with modern seawater values, while freshwater cultured pearls display much higher and variable 87Sr/86Sr ratios [3-4]. The collected freshwater cultured pearls from the Chinese market yielded values ranging from 0.710 5 to 0.71 23, aligning well with the Sr isotopes composition in freshwater samples along the Yangtze River[5]. The results indicate that the Sr isotope in pearl samples depends largely on the Sr isotope composition of the ambient water environment, and discrimination between seawater cultured pearls and freshwater cultured pearls using the Sr isotopes ratio is indeed a method tracing the cultivation environment, thus being robust and reliable.
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图 1 同时具有各向同性色区和各向异性色区的俄勒冈太阳石Figure 1. Sunstone samples from Oregon with both isotropic and anisotropic colour zones -
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