Dual-Gate Chemometric Spectral-Line Selection with Multi-Acid Digestion for ICP-OES Determination of HREEs in Complex Ores

Reliable determination of heavy rare earth elements (HREEs) in ores with multiple components through ICP-OES analysis is limited by their complex spectral signatures and dependency on the chemical matrix of each sample. A double gate-line selection method was developed that utilizes both correlation between candidate ICP-OES line intensity values and ICP-MS reference concentration values for 24 geological samples and standard addition recovery screening. The multi acid decomposition of all samples utilized HNO3, HCl, HF, HClO4, and H2O2. The eight selected lines were Gd at 335.048 nm, Tb at 350.914 nm, Dy at 353.171 nm, Ho at 345.600 nm, Er at 369.265 nm, Tm at 313.125 nm, Yb at 369.419 nm, and Lu at 291.139 nm. Limits of detection for the blank-derived data range from 0.02 to 0.10 mg/kg; the limit of quantification ranges from 0.07 to 0.32 mg/kg. However, the lower bound of calibration standards represents 2.00 mg/kg in the solid phase. Therefore, any results that fall below this threshold will be represented as estimated (extrapolated) rather than measured (quantitatively determined). CRM recoveries for the eight target analytes fell within the range of 91.67 to 105.78 percent; however, the two CRM comparisons where the analytical result for one or more target analytes exceeded the two-sided critical T-value (two tailed) of 2.31 were Tb in OREAS 465 and Er in OREAS 461. Due to the fact that the 24 samples analyzed for ICP-OES/ICP-MS correlation analysis were also used for the initial selection of candidate ICP-OES lines, the demonstrated correlations represent internal validity versus independent prediction. Single interferent controlled experiments and matrix-matched calibration experiments were not completed.