New approach to obtain the correct chemical compositions by absorption correction using analytical transmission electron microscopy

抄録

Analytical transmission electron microscopy (ATEM) is a powerful tool to obtain the chemical compositions of minerals at very small areas of minerals down to the nanometer scale. Most ATEM analytical systems are capable of performing quantitative calculations of chemical compositions by absorption correction. However, an appropriate procedure of obtaining the correct chemical compositions by absorption correction has not yet been established. In this study, we analyzed thin foils of garnet and olivine with known compositions and thicknesses using two different analytical systems to evaluate the certainty of the absorption correction based on the Cliff-Lorimer equation. The results show that the absorption correction using the real sample thicknesses at the analyzed spots did not yield the correct chemical compositions. The correct quantification data were obtained when using the sample thickness t corrected to minimize the value of the function SSR(t) (sum of squared residuals) = ∑_i[n_i⁰ − ∑_j n_ij(t)]², where n_i⁰ is the ideal atomic percentage of the i site, n_ij(t) is the atomic percentage of the j ion at the i site calculated by the software, and the sum holds all the ionic sites. Also, it was confirmed that the minimum value of the SSR obtained by calculating all existing elements as oxides (oxygen is not quantified) is in most cases smaller than that obtained by quantifying all elements, including oxygen, independently. This method of using the SSR without knowing the real thickness at the analyzed spot can be applied to the analytical results with absorption correction by any software. Therefore, this SSR has the potential of becoming a universal indicator to assess the results of quantitative chemical analyses by absorption correction in ATEM.