Infoscience

Journal article

XANES microspectroscopy of biominerals with photoconductive charge compensation

Specimen charging under X-ray illumination is a well known phenomenon that can seriously obstruct the analysis of insulating samples. Synchrotron X-PEEM spectromicroscopy can reach a lateral resolution of 20 nm, 1-2 orders of magnitude larger than electron microscopies, but has the added capacity to probe oxidation state through total yield X-ray absorption near edge structure (XANES) spectroscopy. This capability may be compromised, however, if specimen charging restricts electron emission, as was encountered in the study of silicified bacteria from an Icelandic hot spring microbial mat. Bacteria living in an environment containing a high concentration of dissolved silica provide nucleation sites for amorphous silicate precipitation, a process which may lead to the preservation of the cellular structure, i.e. fossilization. TEM studies of bacteria in progressive stages of mineralization showed that mineral formation was initiated in the extracellular sheath, reaching the cell interior after death. Spectromicroscopy at the Si L-edge of sectioned mineralized bacteria encountered major charging difficulties, which were relieved by simultaneously illuminating the specimen with 325 nm HeCd laser light during the analysis. The low energy light excites mobile free electrons below the work function threshold, which can offset surface positive charge. This approach allowed spectroscopy to be performed from microscopic areas, and may be applicable to a wider range of insulating samples. (C) 2001 Elsevier Science B.V. All rights reserved.

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