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 energ y filtering TEM, the element-unspecific background under the core-edge has to be subtracted. The most commonly used procedure is the three-window powe r-law method leading to considerable systematic errors for low-energy core- edges. Here a new method is described which can be considered as a generali zed difference method. Characteristic examples for element detection in bio logical specimens using this method are shown. The background under the cor e-edge can be described by one or two pre-edge windows as a polynome of thi rd order. This function can be deduced from specimen areas that are not kno wn to contain the element or from a second specimen used as a standard. Con trol experiments showed that background subtraction for on-overlapping core -edges in the low-loss region (50-200 eV) needs two pre-edge images, wherea s at higher-energy losses ( > 300 eV) only one pre-edge image is necessary. With the method described, objective elemental mapping becomes possible ev en for edges at 50-100 eV. This was proven for the M-2,M-3-edge of iron at 60 eV. The detection of phosphorus was possible with a signal-to-noise rati o five times higher than when using the three-window method. Preliminary da ta showed that it should be possible to detect calcium with only one image before the edge. (C) 1999 Elsevier Science B.V. All rights reserved.