Fig. 4

The Fe L-edge EELS analysis of sample #084. a HAADF image shows the position of EELS analysis with different dwell times. To avoid collecting EELS signal from iron and iron sulfide nanoparticles, we changed measurement configuration for sample #084. Here the EELSs are measured at the same location, so the time is cumulative increase, that is, dwell times of 50, 100 and 500 ms for EELS measurement actually corresponds to 50, 150 and 650 ms, respectively. Additionally, EELS with a dwell time of 300 ms was collected independent line scanning, while EELSs with a dwell time of 20 ms were obtained by summing up four line scanning data to improve the signal-to-noise ratio. Fe²⁺ and Fe³⁺ lines represent L₃ peak positions of Fe²⁺ and Fe³⁺ reference materials, respectively. b EELS results of amorphous areas and metallic iron nanoparticle (npFe⁰). c Variations of Fe³⁺/ΣFe ratio versus the dwell time. The calculated Fe³⁺/ΣFe value is −4.5% at 20 ms. The negative value is mainly due to the quantitative method errors