Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 72, Issue 23, Pages 5659-5674Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2008.08.016
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Funding
- NIG-FSC [IP/715/1001]
- NERC [bgs04003, nigl010001] Funding Source: UKRI
- Natural Environment Research Council [bgs04003, nigl010001, NER/I/S/2002/00691] Funding Source: researchfish
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The determination Of accurate Sr isotope ratios in calcium phosphate matrices by laser ablation multi-collector ICP-MS is demonstrated as possible even with low Sr concentration archaeological material. Multiple on-line interference correction routines for doubly-charged REE, Ca dimers and Rb with additional calibration against TIMS-characterised materials are required to achieve this. The calibration strategy proposed uses both inorganic and biogenic apatite matrices to monitor and correct for a Ca-40-P-31-O-16 polyatomic present at levels of 0.3-1% of the non-oxide peak, which interferes on Sr-87 causing inaccuracies of 0.03-0.4% in the Sr-87/Sr-86 isotope ratio. The possibility also exists for synthetic materials to be used in this calibration. After correction for interferences total combined uncertainties of 0.04-0.15% (2SD) are achieved for analyses of 13-24 mu g of archaeological tooth enamel with Sr concentrations of ca. 100-500 ppm using MC-ICP-MS. In particular, for samples containing >300 ppm Sr, total uncertainties of similar to 0.05% are possible utilising 7-12 ng Sr. Data quality is monitored by determination of Sr-84/Sr-86 ratios. When applied to an archaeological cattle tooth this approach shows Sr-isotope variations along the length of the tooth in agreement with independent TIMS data. The Ca-40-P-31-O-16 polyatomic interference is the root cause of the bias at mass 87 during laser ablation ICP-MS analysis of inorganic and biogenic calcium phosphate (apatite) matrices. This results in inaccurate Sr-87/Sr-86 ration; even after correction of Ca dirners and doubly charged rare earth elements. This interference is essentially constant at specific ablation conditions and therefore the effect on Sr-87/Sr-86 data varies in proportion to changes in the Sr concentration of the ablated material. Complete elimination of this interference is unlikely through normal analytical mechanisms and therefore represents a limitation on the achievable accuracy of LA-(MC-)ICP-MS Sr-87/Sr-86 data without rigorous calibration to known reference materials. (C) 2008 Elsevier Ltd. All rights reserved.
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