NanoSIMS Analysis of Arsenic and Selenium in Cereal Grains

1. NanoSIMS Analysis of Arsenic and Selenium in Cereal Grains 3rd year D.Phil Department of Materials – University of Oxford Katie Moore Supervisor: Chris Grovenor

2. Motivation Why is a materials scientist looking at plants? Interdisciplinary collaborations allow critical problems in the life sciences, difficult to solve with traditional analysis techniques, to be explored with established physical science techniques.

3. The Arsenic Problem • Arsenic contamination of groundwater • Contaminated groundwater is used to irrigate rice paddy fields • Resulting in rice grain containing elevated levels of arsenic • A major problem in Bangladesh, India, China and America. • Arsenic is a toxic and carcinogenic element Ref: X. Y, Xu et al., Environ. Sci. Technol., 42(15), 2008

4. The Selenium Problem • Between 0.5 and 1 billion people worldwide may be deficient in selenium including populations in developed countries. • In the UK this is caused by a reduction in the amount of wheat imported from America and a fall in the consumption of cereals • Selenium is an important trace element Daily selenium intake in the UK is now about ½ of the reference amount M. R. Broadley et al., Proc. Nutr. Soc. (65) 2006 M. S. Fan et al., Sci. Total Environ. (389), 2008 MAFF, Food Surveillance Information Sheet, (126), 1997 Refs:

5. Agricultural Solutions • To increase Se: • Add a selenium fertiliser to the soil (practiced in Finland) • To decrease As: • Polish the grain to remove the high As parts • Both of these solutions require knowledge of where the trace elements are located in the grain. • Determining where these very low concentrations are located with sub-cellular resolution is a serious analytical challenge Ref: M. H. Eurola et al., J. Sci. Food Agric., (56), 1991

6. Secondary Ion Mass Spectrometry (SIMS) • Sample is bombarded by positively charged primary ion beam • This results in sputtering of the top few atomic layers and ejection of atoms, ions and clusters • Secondary ions are collected and mass analysed Image adapted from Ref: http://www.eaglabs.com/training/tutorials/sims_theory_tutorial/index.php

7. The NanoSIMS 50 Schematic of the NanoSIMS The Oxford NanoSIMS Ref: CAMECA, http://www.cameca.fr/doc_en_pdf/ns50_instrumentation_booklet.pdf, Instrumentation booklet, June 2007.

8. Characteristics of SIMS • SIMS • High sensitivity (down to ppb for some elements) • Detection of all elements from Hydrogen to Uranium including all isotopes • High mass resolution • NanoSIMS • High lateral resolution (50 nm) • Parallel detection of 5 ionic species Ref: CAMECA, http://www.cameca.fr/doc_en_pdf/ns50_instrumentation_booklet.pdf, Instrumentation booklet, June 2007.

9. SIMS Sample Preparation • Sample needs to be flat, conducting, and dry • Bulk chemical analysis (ICP-MS) showed • trace levels of 2.5 ppm arsenic in the rice and 17 ppm selenium in the wheat • Rice samples were • grown at Rothamsted Research • Wheat samples were grown • in a field trial in Nottingham

10. Structure of Wheat Grain 80µm Aleurone layer Starchy endosperm Cross section Embryo

11. Selenium in Wheat Grain 16O- 12C14N- 32S- 31P16O- 80Se- SE 30µm Max CN- counts: 105,000 Max selenium counts: 4 Ref: K. L. Moore et al., New Phytol., (185), 2010

12. Selenium in Wheat Grain Aleurone cell Starch grains 16O- 12C14N- 12C14N- 16O- 80Se- 80Se- 31P16O- 32S- Ref: K. L. Moore et al., New Phytol., (185), 2010

13. Selenium in Wheat Grain 16O- 12C14N- 16O- 32S- Starch grain 80Se- 31P16O- 80Se- 32S- Selenium is localised in the protein region around the starch grains Selenium hotspots are found in the aleurone cells High resolution, sub-cellular, localisation of ppm concentrations Ref: K. L. Moore et al., New Phytol., (185), 2010

14. Arsenic in Rice Grain Arsenic is localised in the sub-aleurone protein Ref: K. L. Moore et al., New Phytol., (185), 2010

15. Rice Roots – Experiment setup • Variables: • Arsenate or arsenite • With or without Fe plaque • Wild type or lsi2 mutant Hydroponically grown rice plants Lsi2 transporter Fe plaque No Fe plaque Ref: Zhao, F.J., et al., New Phytol., 181(4), 2009

16. Rice Roots – Fe Plaque 28Si- 12C14N- 31P- SE 75As- 56Fe16O- Sc Ex EP 25 µm

17. Rice Roots – Lsi2 mutant 28Si- 12C14N- 31P- SE 75As- Xy En 25 µm Colour merge: Red = As, Green = CN, Blue = Si

18. Conclusions • The NanoSIMS has successfully been used to provide a detailed analysis of the distribution of trace elements selenium and arsenic in wheat and rice respectively and the distribution of As in roots. • Selenium is localised in the protein regions around the wheat starch grains with hotspots in the bran layer • Arsenic is concentrated in the sub-aleurone protein of the rice rather than in the aleurone. • The Fe plaque has a strong adsorption affinity for As • The Lsi2 mutant blocks As uptake in the endodermis • These experiments have demonstrated the unique potential of state-of-the-art SIMS instrumentation to analyse the distribution of ppm levels of important trace elements with sub-cellular resolution

19. Acknowledgements Supervisor: Chris Grovenor NanoSIMS postdoc: Markus Schröder Collaborators: Fang-jie Zhao, Steve McGrath, Malcolm Hawkesford, Peter Shewry Root Sample Preparation: Barry Martin, Chris Hawes EPSRC: D.Phil funding IOM3: For this opportunity