Institute of Geosciences, Goethe-University, Frankfurt am Main, Germany.
Rapid Commun Mass Spectrom. 2013 Jul 30;27(14):1631-42. doi: 10.1002/rcm.6609.
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.
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.
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.
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.
磷酸消化反应的动力学性质使得可以使用气体源同位素比质谱（IRMS）对碳酸盐进行团聚同位素分析。在大多数实验室中，酸消化在密封容器中于 25°C 或在普通酸浴中于 90°C 进行。在这里，我们表明，取决于所采用的消化技术，会得到不同的Δ47 结果。
用 104% H3 PO4 将几种生物成因方解石和 NBS 19 的重复样品在密封容器中于 25°C 和 90°C 下进行反应，使用普通酸浴。样品量在 4 毫克至 14 毫克之间变化。对所产生的 CO2 应用团聚同位素分析的标准纯化方法，然后通过 IRMS 测量相对于参考气体的质量 44 到 49 的丰度。
如果增加样品量，则在 25°C 下进行反应时会观察到较低且更一致的Δ47 值的系统趋势。我们认为，相对较小的样品（4-7 毫克）中，所产生的 CO2 或反应中间体与游离水分子的二次再平衡优先发生，最终与>7 毫克等分试样相比，产生升高的Δ47 值。相比之下，在使用普通酸浴于 90°C 下消化的碳酸盐中，不会观察到样品尺寸对Δ47 值的影响。
如果在 90°C 下进行消化，则小于 7 毫克的碳酸盐样品的Δ47 值的测定变得更加精确和准确。基于我们的结果，我们建议方解石和文石的 25°C 和 90°C 之间磷酸分馏因子的差异为 0.07‰。
