Double Spike-Standard Addition Technique and Its Application in Measuring Isotopes

Abstract

Double spike (DS) method has been extensively used in determining stable isotope ratios of many elements. However, challenges remain in obtaining high-precision isotope data for ultra-trace elements owing to the limitations of instrumental signal-to-noise ratios and the systematics of precision of DS-based measurements. Here, the DS-standard addition (SA) (DSSA) technique is proposed to improve measurements of isotope compositions of ultra-trace elements in natural samples. According to the U-shaped relationship between DS measurement uncertainty and the spike/ sample ratio, theoretical equations and an error propagation model (EPM) were constructed comprehensively. In our method, a spiked secondary standard solution with a high, precisely known spike/sample ratio is mixed with samples such that the mixtures have spike/sample ratios within the optimal range. The abundances of the samples relative to the added standards (sample fraction; f spl ) and the samples' isotope ratios can then be obtained exactly using a standard DS data reduction routine and the isotope binary mixing model. The accuracy and precision of the DSSA approach were verified by measurements of cadmium and molybdenum isotopes at as low as 5 ng levels. Compared with traditional DS measurements, the sample size for isotope analysis is reduced to 1/6−1/5 of the original with no loss of measurement precision. The optimal mixing range f spl = 0.15−0.5 is recommended. The DSSA method can be extended to isotope measurement of more than 33 elements where the DS method is applicable, especially for the ultra-trace elements such as platinum group and rare earth element isotopes.

High-precision measurement of Cd isotopes in ultra-trace Cd samples using double spike-standard addition MC-ICP-MS

“…According to the theory of DSSA and an optimal ratio of 111 Cd spike(spk) / 112 Cd sample(spl) provided by Tan et al 8 the recommended mixing range of the 111 Cd spk / 112 Cd spl was 2–6. 8,44 For Münster-mixed solutions, a δ 114/110 Cd of 0.489 ± 0.015‰ (2SD) (named Std1 Mu , P standard(std) = 4.132) was achieved by mixing 10.40% Münster Cd and 89.60% of NIST 3108 ( 111 Cd spk / 112 Cd spl = 4.621), and a δ 114/110 Cd value of 0.283 ± 0.021‰ (named Std2 Mu , P std = 2.066) was obtained by mixing 5.21% Münster Cd and 94.79% NIST 3108 ( 111 Cd spk / 112 Cd spl = 2.180). For Spex-mixed solutions, similar procedures were conducted.…”

“…The δ 114/110 Cd spl , δ 114/110 Cd mix (mixed solution: std + spl), P std ( 111 Cd spk / 112 Cd std ), and P mix ( 111 Cd spk / 112 Cd mix ) can be precisely achieved by DS data reduction Worksheet. For a single-standard, the δ 114/110 Cd value of an unknown sample can be solved by eqn (2): 44 δ 114/110 Cd spl = ( P std × δ 114/110 Cd mix – P mix × δ 114/110 Cd std )/( P std − P mix )…”

“…Recently, an innovated technique for analyzing isotopes in ultra-trace element samples, named a double spike-standard addition (DSSA), has been proposed. 44 The key idea of the DSSA technique is the addition of certified standard solution containing DS ( e.g. , 111 Cd– 113 Cd) with known isotope composition to a sample to boost the amount of analyte.…”

“…, 111 Cd– 113 Cd) with known isotope composition to a sample to boost the amount of analyte. This method only needs ∼1/5 of sample mass required for traditional DS while achieving comparable or better precision, 44 which sheds light on measuring Cd isotopes precisely for those naturally ultra-trace Cd samples because the Cd mass of sample for purification can be decreased significantly by adding a DS spiked standard solution. However, although the DSSA's reliability was verified through the pure Cd standard solutions and high-Cd geological reference materials ( e.g.…”

“…However, although the DSSA's reliability was verified through the pure Cd standard solutions and high-Cd geological reference materials ( e.g. , NIST 2711a and JSd-2), 44 its application to ultra-trace Cd samples needs to be further tested with more natural samples with various types of matrices to ensure the accuracy and precision is widely applicable.…”