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Thanks for the tip. Actually, we are interested in studying the diffusion of exposed liquid in SAXS microfluidic experiments. I wonder what the mechanism for the brown color could be. I would expect dithionite to oxidize to colorless sulfates and sulfites rather than produce elemental sulfur or sulfides, but then radiation damage is a messy process. Soluble silver compounds might do something, but I'd like to avoid high Z compounds if possible. Anyone know what they put in burn paper to make it change color? Richard On Feb 8, 2011, at 8:31 AM, Bosch, Juergen wrote: Hi Richard, I assume you want to track down which parts of a crystal you already have exposed ? I think dithionite leads to brown colouring of the protein after/during exposure. At least we've observed frequently that some ingredients in our crystals turn brown over time and you could clearly see where the beam hit. I don't know how applicable this would be for small beamsizes and a couple of images, but for whole datasets it would work. James Holton probably has a build-it-yourself solution to this I expect. Jürgen - Jürgen Bosch Johns Hopkins Bloomberg School of Public Health Department of Biochemistry & Molecular Biology Johns Hopkins Malaria Research Institute 615 North Wolfe Street, W8708 Baltimore, MD 21205 Phone: +1-410-614-4742 Lab: +1-410-614-4894 Fax: +1-410-955-2926 http://web.mac.com/bosch_lab/<http://web.me.com/bosch_lab/> On Feb 8, 2011, at 6:32 AM, Richard Edward Gillilan wrote: Does anyone know of a water soluble dye that changes color upon exposure to x-rays? Preferably from clear to a dark color. Must work in the liquid state (non-frozen... so color centers are out). Doesn't matter if it is organic or inorganic. Thanks Richard Gillilan MacCHESS Cornell University Ithaca, NY