Elemental mapping of DNA chains by energy-filtering TEM

DNA chains were detected by phosphorus mapping based on energy-filtering transmission electron microscopy (EF-TEM). The three-window method and the image-merging system were used to make phosphorus-mapping images. It was impossible to observe DNA chains without the image-merging system, because of the low signal levels of single images. It became easy to assign DNA molecules when the typical superstructure of plasmids was preserved. To preserve the structure during specimen preparation, rapid-freezing and freeze-drying methods were used. Mapping images can be obtained only when carbon-supporting films are extremely thin, as thick supporting films weaken the signals. The thickness of the supporting film, which was estimated to be < 2 nm, was the most important factor in this study. In the three-window method, the calculation to remove the background absolutely includes a few errors, because the precise spectrum was not observed. To obtain higher quality mapping images, several improvements in the hardware are suggested.