Determination of light elements C, N, O in various minerals and synthetic compounds using X-ray microanalysis

Abstract

The goal of the study is developing of the methodology of X-ray microanalysis of light elements C, N and O which are jointly present in various minerals and synthetic compounds, including ultrafine diamonds, carbon filamentous fibers, etc. An accelerating voltage of 10 kV high enough to reduce the contribution of the sample surface to the intensity of the lines, and at the same time prevent from the overestimation of the corrections for the line absorption was used. The beam current ranged within 90 - 120 nA. The lines of nitrogen and oxygen are particularly strongly absorbed by carbon. The background intensity is measured near the line. In the region of Kα lines of C and O, the background changes linearly, while for oxygen exhibits a large slope. The shape of continuous X-ray spectrum from the sample at 10 kV in the region of the nitrogen line can be approximated by a polynomial dependence. We used a differential mode of the amplitude discrimination of the signal. It is shown that the position and shape of the carbon line depends on the type of the chemical bond: from covalent (diamond, graphite) to a more ionic bond with oxygen (carbonates). Wide in diamond and graphite Kα lines are shifted in carbonates to the shortwave region and substantially narrowed. An additional maximum appears due to admixing of the wave functions of 2p electrons of carbon to the wave functions of 2s electrons of oxygen. The errors related to the influence of the type of chemical bond on the shape of the spectra can be avoided by the analysis of the integrated intensities. In some cases, the resistance of the samples to the impact of the electron beam is increased by the raster mode of size 5-8 micron or by moving the sample within the area ~100 x 100 pm 2 . Calculation of the concentrations was carried out in the PAP program using B. L. Henke absorption coefficients. Some errors in the correction factors for the line absorption are usually corrected by selecting the absorption coefficients. The detection limit of carbon is 0.10 % wt. and for oxygen in carbonates it ranges within 0.39 - 0.90 % wt., whereas in the samples grown from nano-diamond colloids attains 0.75 wt. %.