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首页> 外文期刊>Geochemistry, geophysics, geosystems >The importance of a Ni correction with ion counter in the double spike analysis of Fe isotope compositions using a Fe-57/Fe-58 double spike
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The importance of a Ni correction with ion counter in the double spike analysis of Fe isotope compositions using a Fe-57/Fe-58 double spike

机译：在使用Fe-57 / Fe-58双峰的Fe同位素组成的双峰分析中，使用离子计数器进行Ni校正的重要性

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摘要

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 Fe-57-Fe-58 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 Ni-60 isotope is monitored and used to subtract a proportional Ni-58 signal from the total 58 amu beam. The Ni-60 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 Ni-60 beam on an ion counter produces a more precise measurement, resulting in a near-threefold improvement in Fe-56 reproducibility, from 0.145 when measured on Faraday to 0.052. Faraday detectors quantify the Ni-60 signal poorly, and fail to discern the transient (NeAr)-Ne-20-Ar-40 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 parts per thousand. Averaging 3-4 analyses further improves precision to 0.019 parts per thousand, allowing distinction between ultramafic minerals.
机译：我们提出了一种新的方法，该方法能够通过使用Fe-57-Fe-58双尖峰的多收集器电感耦合等离子体质谱（MC-ICP-MS）来测量火成材料的铁同位素比，从而达到高精度。样品纯化后，样品溶液中仍存在接近基线的镍信号水平，对58 amu产生等压干扰。为了纠正干扰，监测了少量的Ni-60同位素，并用于从总的58 amu束中减去成比例的Ni-58信号。由于Johnson噪声以相似的幅度出现，因此Ni-60信号很难在法拉第探测器上精确测量。从总的58 amu光束中减去此噪声为主的信号，并在双尖峰校正期间放大了其误差。将Ni-60离子束放在离子计数器上可产生更精确的测量结果，从而使Fe-56重现性提高了近三倍，从法拉第测量时的0.145提高到0.052。法拉第检测器无法很好地量化Ni-60信号，并且无法辨别离子计数器上可见的瞬态（NeAr）-Ne-20-Ar-40干扰，这很可能是造成重现性差的原因。另一个考虑因素是仪器的稳定性（此处定义为峰中心质量的漂移），它会影响高分辨率的分析。相对于包围标准，经历较大漂移的分析通常会产生不可复制的数据。基于此，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。在排除了异常之后，我们的次要标准的单个运行的长期精度提高到了每千分之0.046。平均3-4次分析可进一步将精度提高到千分之0.019，从而区分超镁铁矿石。

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