The importance of a Ni correction with ion counter in the double spike analysis of Fe isotope compositions using a 57Fe/58Fe double spike
The importance of a Ni correction with ion counter in the double spike analysis of Fe isotope compositions using a 57Fe/58Fe double spike
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DOI：
10.1002/2015gc006012
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发表时间：
2015-12
期刊：
影响因子：
3.7
通讯作者：
V. A. Finlayson;J. Konter;Lin Ma
中科院分区：
文献类型：
--
作者：
V. A. Finlayson;J. Konter;Lin Ma
本文提出了一种利用57 Fe-58 Fe双加标多接收电感耦合等离子体质谱（MC-ICP-MS）高精度测定火成岩物质铁同位素比值的新方法。样品纯化后，样品溶液中仍存在镍的近基线信号水平，对58 amu产生同量异位素干扰。为了校正干扰，监测次要的60 Ni同位素并用于从总58 amu束中减去成比例的58 Ni信号。由于约翰逊噪声以相似的幅度出现，60 Ni信号难以在法拉第检测器上精确测量。从总的58 amu波束中减去该噪声主导的信号，并且其误差在双尖峰校正期间被放大。将60 Ni离子束置于离子计数器上可以进行更精确的测量，δ 56 Fe的重现性提高了近三倍，从法拉第测量时的±0.145‰提高到0.052‰。法拉第检测器对60 Ni信号的量化较差，并且无法辨别离子计数器上可见的瞬时20 Ne 40 Ar干扰，这可能是导致再现性较差的原因。另一个考虑因素是仪器稳定性（本文定义为峰中心质量的漂移），这会影响高分辨率分析。相对于交叉进样标准品，分析出现较大漂移时，通常会产生非重复数据。在此基础上，我们提出了一种能够检测漂移影响数据的定量离群值检测方法。剔除离群值后，二级标准品单次运行的长期精密度提高到±0.046‰。平均3-4个分析进一步提高精度到0.019‰，允许区分超镁铁矿物。
We present a new method capable of measuring iron isotope ratios of igneous materials to high precision by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) using a 57Fe‐58Fe double spike. After sample purification, near‐baseline signal levels of nickel are still present in the sample solution, acting as an isobaric interference on 58 amu. To correct for the interference, the minor 60Ni isotope is monitored and used to subtract a proportional 58Ni signal from the total 58 amu beam. The 60Ni signal is difficult to precisely measure on the Faraday detector due to Johnson noise occurring at similar magnitude. This noise‐dominated signal is subtracted from the total 58 amu beam, and its error amplified during the double spike correction. Placing the 60Ni beam on an ion counter produces a more precise measurement, resulting in a near‐threefold improvement in δ56Fe reproducibility, from ±0.145‰ when measured on Faraday to 0.052‰. Faraday detectors quantify the 60Ni signal poorly, and fail to discern the transient 20Ne40Ar interference visible on the ion counter, which is likely responsible for poor reproducibility. Another consideration is instrumental stability (defined herein as drift in peak center mass), which affects high‐resolution analyses. Analyses experiencing large drift relative to bracketing standards often yield nonreplicating data. Based on this, we present a quantitative outlier detection method capable of detecting drift‐affected data. After outlier rejection, long‐term precision on individual runs of our secondary standard improves to ±0.046‰. Averaging 3–4 analyses further improves precision to 0.019‰, allowing distinction between ultramafic minerals.