Biological X-ray Absorption Spectroscopy Imaging

I. J. Pickering,¹ G. Hirsch,² H. H. Harris,³ M. J. George,⁴ E. Y. Sneeden,⁴ R. C. Prince,⁵ and G. N. George¹

¹Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, S7N 4R5, Canada
²Hirsch Scientific, Pacifica, CA 94044, USA
³School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia
⁴Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, CA 94309, USA
⁵ExxonMobil Research and Engineering Company, Annandale, NJ 08801, USA

Micro X-ray beam studies of biological systems such as cell cultures, intact tissues or whole organisms pose special experimental challenges. While such studies can provide unprecedented spatial and chemical information, the specimens are unusually delicate and great care must be taken to ensure the measurement is of a living system. Beam damage is of critical concern. Additionally, the elements of interest are often present at such low concentrations that they are close to the limit of fluorescence detection.

We have developed protocols at SSRL's beam lines 9-3 and 6-2 to ensure minimum exposure of the sample. We use the technique of X-ray absorption spectroscopy imaging, in which fluorescence maps are collected at two or more energies each sensitive to a different chemical species of the element, and from these quantitative concentration maps of each species are deduced. An advantage of this method over microprobe coupled with micro-XAS is that chemical contrast is obtained for the entire scanned area, rather than from selected pixels, and the dwell time at each pixel is very small. The sample is maintained under ambient conditions and additional external checks on its integrity are also applied. Tapered metal monocapillaries provide the microfocusing capabilities. Results of XAS-imaging on arsenic in a hyperaccumulating fern, on sulfur in whole cells, and on mercury in zebrafish larvae will be presented.