Electron energy loss chemical mapping of low Z elements in biological sections.

A STEM VG HB 501 equipped with a Gatan spectrometer has been interfaced to a PDP 11-34 computer. Digital energy filtered images have been recorded with several energy windows on both sides of a characteristic level, so that the exact background can be stripped under the core loss signal for each pixel. Results concern the distribution of nitrogen (K-edge at 402 eV), oxygen (K-edge at 532 eV) and iron (L23 edge at 705 eV) in embedded sections of bone marrow. The present performances of the system allow the detection of composition variations of 1 to 2% for these elements, with a lateral accuracy of the order of 5 nm in a section of 50 nm thickness. Individual ferritin molecules distributed within the section are clearly imaged and analyzed with the characteristic iron edge.