The method of atomic emission spectrometry with excitation spectra in microwave plasma was used to determine the composition of lanthanum sulfide crystals and europium-doped gadolinium oxide, as well as elements in the melt (tin, boron, and lithium). Calibration curves for rare earth elements are nonlinear and do not provide the required accuracy of analysis. To reduce errors and linearize the calibration curves, the internal standard method was used. Molecular ions N2, N2+ and OH did not correct for changes in plasma conditions and inter-element effects. Internal standard elements were selected based on the proximity of the first ionization potential to the analytes, considering Ba, Al, Ga, and In. The use of these elements as internal standards allowed the linearization of calibration curves, achieving an analytical accuracy of 95−105%. The found total mass of the elements was 97−103% of the sample mass, with accuracy confirmed by the method of standard additions.
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