Energy-filtered transmission electron microscopy of ferritin.

Abstract

ASTRACT The focusing properties of a magnetic-sector spectrometer are shown to be suitable for forming high-spatial-resolution, energy-filtered transmission electron microscope.images. Filtered images of ferritin molecules by using electrons scattered from the-characteristic iron M2,3 and carbon K absorption edges clearly distinguish the 75-A iron core and 120-A protein shell. The minimum detectable mass is estimated to be 0.84 x 10-20 g for Fe for an electron dose of'18 C/cm2 and 99% confidence. OP1Viewing screenThe visualization of the elemental composition of materials at ultrastructural resolution is a major aim of modem, analytical electron microscopy. Relatively high (10-20 nm) resolution images showing the distribution of some constituent elements have been obtained by imaging with characteristic x-rays produced by a scanning electron beam generated with a highbrightness field emission source (1-3). However, x-ray mapping is ultimately limited by the low-fluorescence yield ofx-ray production and low-collection efficiency of x-ray detectors. Compositional imaging at higher resolution and with greater sensitivity should be attainable by imaging with electrons that have experienced characteristic energy losses within the specimen (4-8). Furthermore, such energy-loss images contain molecular, in addition to atomic, information (9).To form energy-filtered images in otherwise conventional transmission electron microscopes, achromatic energy filters, such as the Castaing-Henry (10, 11) and Omega filters (12), have been used. These filters are complicated and require extensive modification of the microscope column that is not easily realizable on available high-vacuum microscopes. The simpler magnetic-sector spectrometer is not generally achromatic, and imaging with this type of spectrometer normally has been performed in a scanning transmission electron microscope (5,(7)(8)(9)13). We shall demonstrate, however, that although the chromatic aberration of a single-sector spectrometer is large, high-resolution transmission electron microscopic images can be produced with it and that this method is also readily adaptable to high-vacuum transmission electron microscopes. INSTRUMENTATIONThe sector used for filtered imaging is double focusing and was corrected for second-order aberrations at the energy-dispersion plane (14). Its use as a spectrometer and spectrograph has been described (14, 15). A schematic diagram of the median-plane focal properties are shown in Fig. 1 area of the specimen to be generated by changing the magnetic field in the sector. Other planes between OP1 and the magnetic sector entrance are focused to first order by the spectrometer. In particular, the object plane OP2, corresponding to the microscope viewing screen, is focused at the image plane IP2. For the sector described (14), IP2 is 23 cm behind the energy-selecting slits, and there a real energy-filtered image is formed. By placing a transmission phosphor at this plane, the filtered images can be observed with the silicon intensified target...

Quantitative energy-filtered image analysis in cytochemistry

“…De-* To whom correspondence should be addressed. dicated STEM/EELS instruments have analyzed the chemical analytical potentials of this combination in biological and non-biological materials [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]27]. The formation of elemental distribution images has been described for both combinations [5,8,13,14,17].…”

Design and first applications of a post-column imaging filter

“…The first-order aberrations and distortions can be removed by placing several round lenses plus quadrupole lenses [11], or quadrupoles alone [12] [13,14]. Image quality is then limited only by thirdorder aberrations and distortions, and recorded images of 1024 x 1024 pixels are indistinguishable from images recorded by even the most highly corrected 03A9-filters, which employ 4 magnetic sectors and 7 sextupoles [7].…”

Optimizing the window size for multiple least squares fitting of electron energy loss spectra

“…As previously shown, this approach may be particularly useful when the uncertainty of analysis is not limited by gain variations ( Wang et al ., 1992 ) and the signal‐to‐background is insufficient for application of the inverse power law models. For the detection of high local concentrations, application of the inverse power law is generally sufficient ( Egerton, 1981; Pun et al ., 1984 ), even for energy‐filtered transmission electron microscopic imaging ( Shuman & Somlyo, 1981, 1982; Kothleitner & Hofer, 1998).…”