Specimen charging under X-ray illumination is a well known phenomenon that
can seriously obstruct the analysis of insulating samples. Synchrotron X-PE
EM 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 struc
ture (XANES) spectroscopy. This capability may be compromised, however, if
specimen charging restricts electron emission, as was encountered in the st
udy of silicified bacteria from an Icelandic hot spring microbial mat. Bact
eria living in an environment containing a high concentration of dissolved
silica provide nucleation sites for amorphous silicate precipitation, a pro
cess which may lead to the preservation of the cellular structure, i.e. fos
silization. TEM studies of bacteria in progressive stages of mineralization
showed that mineral formation was initiated in the extracellular sheath, r
eaching the cell interior after death. Spectromicroscopy at the Si L-edge o
f sectioned mineralized bacteria encountered major charging difficulties, w
hich were relieved by simultaneously illuminating the specimen with 325 nm
HeCd laser light during the analysis. The low energy light excites mobile f
ree electrons below the work function threshold, which can offset surface p
ositive charge. This approach allowed spectroscopy to be performed from mic
roscopic areas, and may be applicable to a wider range of insulating sample
s. (C) 2001 Elsevier Science B.V. All rights reserved.