Selection of Internal Standards for Determining Rare-Earth Elements by Microwave Induced Plasma Optical Emission Spectrometry

Microwave plasma atomic emission spectrometry is used to determine the composition of lanthanum sulfide and europium-doped gadolinium oxide crystals, and also of elements in the melt (tin, boron, and lithium). Calibration dependencies for rare-earth elements were found to be nonlinear and did not meet the required accuracy. The internal standard method was used to minimize errors and linearize calibration dependences. Molecular species N₂, \({\text{N}}_{2}^{ + }\) , and OH did not affect plasma conditions or interelement influences. Internal standards were selected based on the proximity of their first ionization potential to that of the analytes; Ba, Al, Ga, and In were considered. Using these elements as internal standards enabled the linearization of the calibration dependences, achieving an recovery of 95–105%. The determined total mass of elements was 97–103% of the sample weight, with accuracy confirmed by the standard addition method.