Volume 30 Issue 1 - December 31, 2015 PDF
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Determination of 87Sr/86Sr and δ88/86Sr ratios in plant materials using MC-ICP-MS
Hou-Chun Liu1,2, Chuan-Hsiung Chung1,2, Chen-Feng You1,2,*, Yi-Hsuan Chiang1
1 Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan
2 Earth Dynamic System Research Center, National Cheng Kung University, Tainan 70101, Taiwan
 
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Growth of higher plants extracts the bio-essential metals from the soils, chemical reactions in the rhizosphere facilitate the metal uptake through roots, and the metabolic processes continuously control the behavior of metals and their translocation between the reservoirs in plants. There are a number of reasons for studying uptake of metals and the associated effects on plant growth and environments.

Recent improvements in mass spectrometers and isotopic analytical techniques provide an opportunity to investigate a variety of elements involving stable isotope fractionation during exogenic processes. The potential of these mass-dependent isotope fractionations driven by the inorganic and/or bio-essential processes have become the new isotopic tracers for source identification, and involved physiochemical processes in natural systems. Metal stable isotope fractionation of Ca, Mg, Fe, and Zn [1] in higher plants have been demonstrated for assessment of the predominant binding forms in which the elements entered the plant, and elucidating the extent and behavior of remobilization of metals within the plant [1] . Nowadays, there are numerous studies concerning the mechanisms of uptake and translocation of Sr by the plants [2-4] ; however, critical results for the effects on plant growth and Sr cycling in environments are still scant. Proper plant material standards, digestion method and chemical purification are of importance for addressing the above scientific issues.

The biggest challenges for highly accurate and precise isotopic determinations on MC-ICP-MS are the instrumental isotopic discrimination, and polyatomic and isobaric interferences. Influences of matrix induced mass bias and intensity bias, and polyatomic and isobaric interferences on MC-ICP-MS Sr isotopic determination were evaluated in detail using NIST SRM Sr solutions spiked with a series of high purity solution standards. The application of the empirical external normalization (EEN) technique combined with the standard-sample bracketing (SSB) method has been developed to correct such mass bias. On the other hand, the removal of polyatomic (e.g., Ca, Mg, and Na) and isobaric (e.g., Rb) elements are strongly relied on extraction chromatography techniques (Fig. 1). One should be noticed, plant materials contain very high levels of organic compounds, resulting in complexity in matrix separation, rather than other specimens of rocks and surface waters.

Fig. 1. Elution curve for matrix separation using Eichrom Sr resin under seawater (widely accepted standard for Sr isotope) and apple leaves matrices.
Assessments of NIST SRM 1515 apple leaves standard digestion and matrix separation suggest that the incomplete digestion of plant materials would potentially bias the extraction chromatography purification of Sr and cause large uncertainty in δ88/86Sr determinations (Fig. 2). This issue is predominantly attributable to the presence of invisible residual organic compounds in the digests. A totally digested organic sample is a prerequisite for high efficiency matrix separation of low level of Ca, Mg, and Rb content and is critical for an accurate and precise MC-ICP-MS δ88/86Sr determination. This study emphasizes that (1) a total digestion of organic containing materials, (2) purity of analyte of interest, and (3) close matrix matching to avoid differences in mass bias effect between samples and standards, are the three important factors for a precise δ88/86Sr determinations by MC-ICP-MS.
Fig. 2. Variability in NIST SRM 1515 apple leaves δ88/86Sr ratio for incompletely and totally digested specimens (sample ID from #1 to #3). The error bar represents 2σ for the individual measurement.

References:
[1] von Blanckenburg F, von Wirén N, Guelke M, Weiss DJ, Bullen TD (2009) Fractionation of metal stable isotopes by higher plants. Elements 5:375–380
[2] Rediske JH, Selders AA (1953) The absorption and translocation of strontium by plants. Plant Physiol 28:594–605
[3] Russell RS, Squire HM (1958) The absorption and distribution of strontium in plants.1. Preliminary studies in water culture. J Exp Bot 9:262–276
[4] Isermann K (1981) Uptake of Stable Strontium by Plants and Effects on Plant Growth. In: Handbook of Stable Strontium. Springer, New York, pp 65–86
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