Presentation on theme: "NanoSIMS Analysis of Arsenic and Selenium in Cereal Grains"— Presentation transcript:
1 NanoSIMS Analysis of Arsenic and Selenium in Cereal Grains Katie Moore3rd year D.PhilDepartment of Materials – University of OxfordSupervisor: 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 Arsenic is a toxic and carcinogenic element The Arsenic ProblemArsenic contamination of groundwaterContaminated groundwater is used to irrigate rice paddy fieldsResulting in rice grain containing elevated levels of arsenicA major problem in Bangladesh, India, China and America.Arsenic is a toxic and carcinogenic elementRef: X. Y, Xu et al., Environ. Sci. Technol., 42(15), 2008
4 Daily selenium intake in the UK is now about ½ of the reference amount The Selenium ProblemBetween 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 cerealsSelenium is an important trace elementDaily selenium intake in the UK is now about ½ of the reference amountM. R. Broadley et al., Proc. Nutr. Soc. (65) 2006M. S. Fan et al., Sci. Total Environ. (389), 2008MAFF, Food Surveillance Information Sheet, (126), 1997Refs:
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 partsBoth 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 challengeRef: 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 beamThis results in sputtering of the top few atomic layers and ejection of atoms, ions and clustersSecondary ions are collected and mass analysedImage adapted from Ref:
7 Schematic of the NanoSIMS The Oxford NanoSIMSRef: CAMECA, Instrumentation booklet, June 2007.
8 Characteristics of SIMS High sensitivity (down to ppb for some elements)Detection of all elements from Hydrogen to Uranium including all isotopesHigh mass resolutionNanoSIMSHigh lateral resolution (50 nm)Parallel detection of 5 ionic speciesRef: CAMECA, Instrumentation booklet, June 2007.
9 SIMS Sample Preparation Sample needs to be flat, conducting, and dryBulk chemical analysis (ICP-MS) showedtrace levels of 2.5 ppm arsenic in the rice and 17 ppm selenium in the wheatRice samples weregrown at Rothamsted ResearchWheat samples were grownin a field trial in Nottingham
10 Structure of Wheat Grain Aleurone layer80µmStarchyendospermCross sectionEmbryo
11 Selenium in Wheat Grain 16O-12C14N-32S-31P16O-80Se-SE30µmMax CN- counts: 105,000Max selenium counts: 4Ref: K. L. Moore et al., New Phytol., (185), 2010
12 Selenium in Wheat Grain Aleurone cellStarch grains16O-12C14N-16O-12C14N-31P16O-80Se-32S-80Se-Ref: K. L. Moore et al., New Phytol., (185), 2010
13 Selenium in Wheat Grain 16O-32S-16O-12C14N-Starchgrain31P16O-80Se-32S-80Se-Selenium is localised in the protein region around the starch grainsSelenium hotspots are found in the aleurone cellsHigh resolution, sub-cellular, localisation of ppm concentrationsRef: K. L. Moore et al., New Phytol., (185), 2010
14 Arsenic is localised in the sub-aleurone protein Arsenic in Rice GrainArsenic is localised in the sub-aleurone proteinRef: K. L. Moore et al., New Phytol., (185), 2010
15 Rice Roots – Experiment setup Variables:Arsenate or arseniteWith or without Fe plaqueWild type or lsi2 mutantHydroponically grown rice plantsLsi2 transporterFe plaqueNo Fe plaqueRef: Zhao, F.J., et al., New Phytol., 181(4), 2009
16 Rice Roots – Fe Plaque 12C14N- 28Si- 31P- 56Fe16O- 75As- SE Sc Ex EP 25 µm
17 Colour merge: Red = As, Green = CN, Blue = Si Rice Roots – Lsi2 mutant12C14N-28Si-31P-75As-SEXyEn25 µmColour merge: Red = As, Green = CN, Blue = Si
18 ConclusionsThe 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 layerArsenic is concentrated in the sub-aleurone protein of the rice rather than in the aleurone.The Fe plaque has a strong adsorption affinity for AsThe Lsi2 mutant blocks As uptake in the endodermisThese 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 SchröderCollaborators:Fang-jie Zhao, Steve McGrath,Malcolm Hawkesford, Peter ShewryRoot Sample Preparation:Barry Martin, Chris HawesEPSRC:D.Phil fundingIOM3:For this opportunity