Measurement of longitudinal sulfur isotopic variations by laser ablation MC-ICP-MS in single human hair strands
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A new method for the measurement of longitudinal variations of sulfur isotope amount ratios in single hair strands using a laser ablation system coupled to a multicollector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) is reported here for the first time. Ablation parameters have been optimized for the measurement of sulfur isotope ratios in scalp human hair strands of 80–120-μm thickness and different washing procedures have been evaluated. The repeatability of the method has been tested and the ability to measure sulfur isotopic variations in 1,000-μm-long hair segments has been evaluated. A horse hair sample previously characterized for carbon and nitrogen isotope ratios in an interlaboratory study has been characterized by LA-MC-ICP-MS to be used as an in-house standard for the bracketing of human hair strands. 34S/32S isotope amount ratios have been measured and corrected for instrumental mass bias adopting the external standardization approach using National Institute of Standards and Technology (NIST) RM8553 and full uncertainty budgets have been calculated using the Kragten approach. Results are reported as both 34S/32S isotope amount ratios and δSV-CDT values (sulfur isotopic differences relative to a reference sample expressed in the Vienna Canyon Diablo Troilite (V-CDT) scale) calculated using NIST RM8553, NIST RM8554, and NIST RM8556 to anchor results to the V-CDT scale. The main advantage of the new method versus conventional gas source isotope ratio mass spectrometry measurements is that longitudinal variations in sulfur isotope amount ratios can be resolved. Proof of concept is shown with human scalp hair strands from three individuals, two UK residents and one traveler (long periods of time abroad). The method enables monitoring of longitudinal isotope ratio variations in single hair strands. Absolute ratios are reported and δ34SV-CDT values are plotted for comparison. Slight variations of <1.2‰ were detected in the hair strands from UK residents whereas the traveler presented a variation of >5‰. Thus, the measurement of sulfur isotopic variations in hair samples has potential to be an indicator of geographical origin and recent movements and could be used in combination with isotope ratio measurements in water/foodstuffs from different geographical locations to provide important information in nutritional and geographical studies.
KeywordsMass spectrometry/ICP-MS Laser ablation Forensics Sulfur Isotope ratio MC-ICP-MS
The work described in this paper was supported by the UK Department for Innovation, Universities, and Skills as part of the National Measurement System Chemical and Biological Metrology Knowledge Base Program. The authors would like to thank the FIRMS network (http://www.forensic-isotopes.rdg.ac.uk/) for providing horse hair sample and particularly Wolfram Meier-Augenstein for helpful discussions. The authors are grateful for support from the Education and Science Council of the Principado de Asturias (grant BP07-059) towards Justo Giner Martinez-Sierra’s secondment at LGC.
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