Optimizing sample and spike concentrations for isotopic analysis by double-spike ICPMS
Optimizing sample and spike concentrations for isotopic analysis by double-spike ICPMS
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DOI：
10.1039/c2ja30215b
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发表时间：
2012-01-01
影响因子：
3.4
通讯作者：
John, Seth G.
中科院分区：
文献类型：
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作者：
John, Seth G.
双尖峰技术广泛用于测量天然样品的同位素组成。为了通过双加标分析获得最准确的结果，选择合适的双加标成分、分析物浓度以及加标与天然比率 (C-spk/C-nat) 非常重要，其中 C-nat 是具有同位素天然丰度的样品或标准品的浓度，C-spk 是具有非天然同位素组成的添加加标的浓度。在这里，使用蒙特卡罗技术探索改变这些参数的影响，该技术模拟计数统计、约翰逊噪声和同量异位素干扰的误差。通常，最佳加标成分和 C-spk/C-nat 是在加标加样品总浓度 (C-spk + C-nat) 必须保持恒定的约束下计算的，以便随着双加标量的增加，样品量减少。实际上，C-spk + C-nat 没有理由保持恒定，因为只要不超过检测器限制，具有固定数量样品的分析人员可以向该样品添加任意数量的尖峰。因此，在这里，双尖峰被优化，同时允许 C-spk 和 C-nat 独立变化。对于 33 种不同的元素，这种允许 C-spk 和 C-nat 独立变化的新方法在没有同量异位素干扰的情况下使理论误差降低了约 30%。在存在同量异位素干扰的情况下，这种方法可以在准确度和精密度方面带来更大的改进。然后将理论误差与 δ Fe-56 标准品以及在海水中测量的 δ Fe-56、δ Zn-66 和 δ Cd-114 的观测误差进行比较。实际海水样品的理论误差和测量误差高度相关，使用仅包含约翰逊噪声和计数统计数据的误差模型解释了 delta Fe-56、delta Zn-66 和 delta Cd-114 中观测误差的分别为 78%、85% 和 96%。这证实了此类模型可最大限度地减少理论误差，可用于优化尖峰成分、C-spk 和 C-nat，以提高天然样品分析的准确性和精确度。
Double spike techniques are widely used for measuring the isotopic composition of natural samples. In order to achieve the most accurate results by double spike analysis, it is important to choose an appropriate double-spike composition, analyte concentration, and spike to natural ratio (C-spk/C-nat) where C-nat is the concentration of a sample or standard with a natural abundance of the isotopes and C-spk is the concentration of an added spike with an unnatural isotope composition. Here, the effect of varying these parameters is explored using a Monte Carlo technique which simulates error from counting statistics, Johnson noise, and isobaric interferences. Typically, optimal spike composition and C-spk/C-nat are calculated under the constraint that total concentration of spike plus sample (C-spk + C-nat) must remain constant, so that as the amount of double spike is increased, the amount of sample is decreased. In practice, there is no reason for C-spk + C-nat to be held constant, because an analyst with a fixed quantity of sample may add any amount of spike to this sample as long as detector limits are not exceeded. Therefore, here, double spikes are here optimized while allowing C-spk and C-nat to vary independently. For thirty three different elements, this new approach of allowing C-spk and C-nat to vary independently led to a decrease in theoretical error of up to similar to 30% in the absence of isobaric interferences. In the presence of isobaric interferences, this approach can deliver even larger improvements in accuracy and precision. Theoretical error is then compared to observed error both for delta Fe-56 standards and for delta Fe-56, delta Zn-66, and delta Cd-114 measured in seawater. Theoretical error and measured error for real seawater samples are highly correlated, with 78%, 85%, and 96% of observed error in delta Fe-56, delta Zn-66, and delta Cd-114, respectively, accounted for using an error model which includes only Johnson noise and counting statistics. This confirms that such models, which minimize theoretical error, can be used to optimize spike composition, C-spk, and C-nat in order to increase accuracy and precision for analysis of natural samples.