Parallel-EELS mapping of calcium in cryosectioned cells

Abstract

Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) provides a high sensitivity for microanalysis of certain important biological elements whose physiological concentrations in cells are rather low. Minimum detectable concentrations for calcium obtained by EELS can be better than those obtained by energy-dispersive x-ray spectroscopy (EDXS). However, in order to detect the very small core-edge signal/background ratios encountered in EELS of biological specimens, relatively elaborate acquisition and processing methods must be employed. Application of another strategy, STEM-EELS elemental mapping, has generally been restricted to analyses where elements are present at relatively high concentrations, such as calcium in mineralized tissue or carbon, nitrogen and oxygen in organic specimens.This is because only simple methods were available for signal estimation if the data had to be processed on-the-fly. Recently there has been considerable interest in the spectrum-imaging technique where entire spectra are collected at each pixel. In the present work we have applied this technique to measure calcium in Purkinje cell dendrites of rapidly frozen mouse cerebellar cortex.