Electron probe and electron energy loss analysis in biology

Abstract

Methods, applications and limitations of quantitative electron probe analysis, X-ray mapping, electron energy loss analysis and energy filtered imaging are described, with emphasis on the analysis of thin (less than 200nm) cryosections. Energy dispersion electron probe analysis can measure reliably 5 to 10mM/Kg of biologically prevalent elements in 50nm diameter areas of 100 to 150 nm thick cryo sections during 100-300 sec counts. The minimal detectable mass (MDM) with a conventional thermionic electron source is approximately 10(-19)g Fe (100 sec count) and can be reduced to 10(-20)g through the use of a field emission gun (FEG). A spatial resolution of 8.7nm is demonstrated in two-dimensional Fourier transforms of Mo X-ray maps of stained catalase crystals. Significant biological results of quantitative electron probe analysis include the measurement of total Ca released from the Mg and K taken up by the sarcoplasmic reticulum during muscle contraction, and the demonstration that mitochondria do not contribute to the physiological regulation of cytoplasmic free Ca levels in cardiac, vascular smooth and striated muscle. Electron energy loss analysis (EELS) promises a significant improvement in sensitivity for the measurement of Ca; based on statistical errors of the measurement, 250 microM/Kg Ca should be measureable with EELS in 250 sec. through the Ca L-edge loss. The use of a doubly corrected magnetic sector spectrometer as a transmission electron microscope imaging filter outside the microscope vacuum is illustrated, and the resolution of the iron core (7.5nm) and surrounding organic shell of single ferritin molecules is demonstrated in, respectively, iron M and carbon K loss images.