Clumped isotope analysis of carbonates: comparison of two different acid digestion techniques.
Clumped isotope analysis of carbonates: comparison of two different acid digestion techniques.
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DOI：
10.1002/rcm.6609
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发表时间：
2013-07
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影响因子：
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通讯作者：
U. Wacker;J. Fiebig;B. Schoene
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文献类型：
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作者：
U. Wacker;J. Fiebig;B. Schoene
原理磷酸消化反应的动力学性质使得能够使用气源同位素比质谱仪(IRMS)对碳酸盐进行聚集同位素分析。在大多数实验室中，酸消化是在25℃的密封容器中进行的，或在90℃的普通酸浴中进行的。这里我们表明，根据所采用的消化技术，获得不同的Δ47结果。方法在25℃和90℃的密封容器中，在普通酸浴中，用104%H3PO4分别与多次复制的生物文石和NBS-19反应。样本量从4毫克到14毫克不等。在IRMS测量相对于参考气体的质量44到49的丰度之前，将聚集同位素分析的标准纯化方法应用于释放出的二氧化碳。结果随着样本量的增加，在25°C的反应中观察到系统的趋势是更低和更一致的Δ47值。我们认为，释放出的二氧化碳或反应中间体与自由水分子的二次再平衡优先发生在相对较小的样品(4-7毫克)，最终产生比>；7毫克等分值更高的Δ47值。相比之下，在90°C下用普通酸浴消化的碳酸盐，没有观察到这样的样品大小对Δ47值的影响。结论对于小于7 mg的碳酸盐样品，如果在90℃下消化，Δ-47值的测定将更加精确和准确。根据我们的结果，对于方解石和文石，25℃和90℃之间的磷酸分馏因子相差0.07‰。
RATIONALE The kinetic nature of the phosphoric acid digestion reaction enables clumped isotope analysis of carbonates using gas source isotope ratio mass spectrometry (IRMS). In most laboratories acid digestions are performed at 25°C in sealed vessels or at 90°C in a common acid bath. Here we show that different Δ47 results are obtained depending on the digestion technique employed. METHODS Several replicates of a biogenic aragonite and NBS 19 were reacted with 104% H3 PO4 in sealed vessels at 25°C and at 90°C using a common acid bath. The sample size varied between 4 mg and 14 mg. Purification methods that are standard for clumped isotope analyses were applied to the evolved CO2 before measuring the abundances of masses 44 to 49 relative to a reference gas by IRMS. RESULTS A systematic trend to lower and more consistent Δ47 values is observed for reactions at 25°C if the sample size is increased. We suggest that secondary re-equilibration of evolved CO2 or reaction intermediates with free water molecules preferentially occurs for relatively small samples (4-7 mg), finally yielding elevated Δ47 values compared with >7 mg aliquots. In contrast, no such sample size effect on Δ47 values is observed for carbonates that are digested at 90°C using the common acid bath. CONCLUSIONS The determination of Δ47 values of carbonate samples smaller than 7 mg becomes more precise and accurate if digestions are performed at 90°C. Based on our results we propose that the difference in phosphoric acid fractionation factor between 25°C and 90°C is 0.07‰ for both calcite and aragonite.