An approach to an objective background subtraction for elemental mapping with core-edges down to 50 eV: description, evaluation and application.
To image the distribution of a specific element in a specimen with an energy filtering TEM, the element-unspecific background under the core-edge has to be subtracted. The most commonly used procedure is the three-window power-law method leading to considerable systematic errors for low-energy core-edges. Here a new method is described which can be considered as a generalized difference method. Characteristic examples for element detection in biological specimens using this method are shown. The background under the core-edge can be described by one or two pre-edge windows as a polynome of third order. This function can be deduced from specimen areas that are not known to contain the element or from a second specimen used as a standard. Control experiments showed that background subtraction for on-overlapping core-edges in the low-loss region (50-200 eV) needs two pre-edge images, whereas at higher-energy losses (> 300 eV) only one pre-edge image is necessary. With the method described, objective elemental mapping becomes possible even for edges at 50-100 eV. This was proven for the M2,3-edge of iron at 60 eV. The detection of phosphorous was possible with a signal-to-noise ratio five times higher than when using the three-window method. Preliminary data showed that it should be possible to detect calcium with only one image before the edge.
Haking
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German Cancer Research Center (DKFZ), Biomedical Structure Analysis Group, Heidelberg, Germany.
Troester
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H
Richter
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Crucifix
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C
Spring
H
H
Trendelenburg
M F
MF
D000818:Animals; D002244:Carbon / Q000032:analysis; D004273:DNA, Neoplasm / Q000032:analysis / Q000276:immunology; D003952:Diagnostic Imaging; D005293:Ferritins / Q000737:chemistry / Q000648:ultrastructure; D007089:Image Enhancement / Q000295:instrumentation / Q000379:methods*; D007075:Immunoglobulin M; D007501:Iron / Q000032:analysis; D008854:Microscopy, Electron / Q000295:instrumentation / Q000379:methods*; D014407:Tumor Cells, Cultured; D017806:Tymovirus / Q000648:ultrastructure
D004273:DNA, Neoplasm; D007075:Immunoglobulin M; D002244:Carbon; D005293:Ferritins; D007501:Iron