Determination of rare earth elements in iron-rich geological samples by ICP-OES

The determination of rare earth elements (REEs) by ICP-OES in geological sa mples, containing iron at levels greater than 5% (0.5 mg/mL in final soluti on) is subjected to spectral or matrix interference. The interference is se rious in case of Lu, Gd, and Tb, but relatively less for Sm, Yb, Pr, Tm, an d Er. Dy an Ho do not suffer spectral interferences, but the iron matric su ppresses their signals almost completely at low levels (0.05 mg/mL or below ). The interference prevails on most of the emission lines with adequate se nsitivity for REEs, obviating the need for separation of iron from REEs. Ir on is qualitatively! separated by solvent extraction with methyl isobutyl k etone (MIBK) to bring its concentration in the solution under tolerance lev el. The separation is rapid, easy to manipulate, without any appreciable lo ss or addition of REEs to the matrix as compared to the separation of REEs as oxalate with calcium as carrier as recommended by some authors. The aqueous phase containing REEs is evaporated to dryness, treated with a fen drops of HClO4, and finally digested in 0.5 M HCl. The solution obtaine d is aspirated into the argon plasma for the determination of REEs. The met hod has been applied to iron-rich geological samples, international geostan dards SY-2 and SY-3, and a few pre-analyzed in-house samples after spiking; with known :amounts of iron (at levels usually found in magnetites and hem atites). The REE values obtained in international geostandard and in-house samples severe found to bt in good agreement with reported values.