Many elements present in biological systems in small concentrations play a major role in normal and abnormal biological processes. These elements can be detected in sub-micron regions and their biochemical role can be studied by x-ray spectroscopy of the characteristic x rays produced when excited by a focussed electron beam. The usual methods for measuring the x-rays produced involves energy-dispersive spectrometry (EDS) or wavelength- dispersive spectrometry. At present, EDS provides poor elemental detection limits because of the small signal-to-background ratio resulting from its energy resolution of only 2-3%. WDS can provide lower detection limits because of its superior energy resolution (typically 0.01-0.1%) but existing wavelength dispersive spectrometers collect only a small fraction of the x-rays produced. The goal of the proposed work is to develop an improved scanning wavelength-dispersive spectrometer for use in transmission electron microscopes and to employ it in the study of the local concentration of elements present in biological tissues. The spectrometer uses a new diffractor geometry with toroidally curved surface and spherically curved planes. It could provide x-ray collection solid angles of up to 0.075 sterad when employed in a scanning transmission electron microscope such as the JEOL 100CX. Depending on the diffractor used, the scanning monochromator could cover all elements in the periodic table from fluorine through iodine. The initial studies with this spectrometer will involve the measurements of calcium in cardiac muscle and it is expected that the measurement time to obtain significant results can be reduced by as much as 7 times compared with the use of EDS. In this work, the results obtained by EDS and WDS will be compared by simultaneous measurements on the same specimens.
