Trace quantities of rare earth elements (REEs) in ultrapure europium oxide (Eu2O3) are measured by inductively coupled plasma mass spectrometry (ICP-MS). The signal intensity of REE decreases with increasing concentration of europium oxide. Then, the internal standard method is examined to minimize this signal suppression. Four internal standards, cadmium (111Cd+) and indium (115In+), and thallium (205TI+) and bismuth (209Bi+) are investigated, since their mass numbers are close to those of lighter (89Y+-157Gd+) and heavier (159Tb+-175Lu+) REEs, respectively. The use of internal standards o Cd and In suppress interference for lighter REEs, and Tl and Bi for heavier REEs. These results imply the interference concerned with a mass-related space charge effect. In trace analysis of REE impurities in ultrapure Eu2O3, In and Tl are suggested to be used as internal standards for lighter and heavier REEs, respectively, due to the recoveries close to 100%. Lighter REE oxide ions occurring in a plasma generally interfere with the determination of heavier REEs. However, almost all REEs are determined correctly even at ultratrace levels by a proper selection of isotope. Unfortunately, polyatomic ions from the europium oxide such as 153EuO+ and 153Eu18OH+ ions seriously interfere with the determination of 169Tm+ and 172Yb+. These interferences are reduced by using ion chromatography (IC) combined with ICP-MS detection.
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