Chemical and Elemental Information
The following are imaging and characterization techniques that can be used to study chemical and elemental information.
Synchrotron Infrared Hyperspectral Imaging
Unlike X-rays, noninvasive infrared photons carry energy quanta matching closely with molecular structure–specific vibrational energy levels. These molecular vibrational energy levels can be subtly perturbed by changes in the local chemical environment. Since this specific intrinsic contrast mechanism removes the requirement for specific tags commonly used in traditional optical imaging technologies, infrared absorption spectroscopy is ideally suited for exploratory analyses of the chemistry and structure of biological materials, cells, and tissues with a spatial resolution of 10 nm to 15 µm. Time-resolved infrared spectroscopy can be used to study chemical and structural changes in living biological systems, with timescales ranging from seconds to minutes, and to identify prospective regions and time-points of interest.
X-Ray Absorption and Emission Spectroscopy
Metal ions have key roles in biological structure and function. For example, they are the active sites of many enzymes and shuttle electrons in key metabolic or signaling pathways. This suite of related techniques provides both structural and electronic information on metal sites in biomolecules. X-ray absorption edge spectroscopy can reveal oxidation state and chemical states of elements in complex environments. Extended X-ray absorption fine structure (EXAFS) can determine high-resolution bond distances, revealing the local structure around a metal ion. Related techniques such as X-ray emission, resonant inelastic scattering, and X-ray Raman can provide a detailed picture of metal-ligand bonding. XAS methods provide very complementary information to data from X-ray crystallography and small-angle scattering studies. This technique also is applied to studies of biologically important ligands (e.g., carbon, nitrogen, sulfur, and chlorine) and their interactions with metals.
X-ray Fluorescence Imaging
X-ray fluorescence imaging (XRF), or x-ray spectromicroscopy, maps the distributions of elements and chemical species of interest within biological samples. Synchrotron XRF (SXRF) can provide detailed images of element speciation to a resolution of 0.5 µm per pixel, a sensitivity beyond desktop XRF, electron microprobe, or other elemental imaging techniques.