LIU Danna, ZHANG Hao, FAN Xiaolong, BAO Xiangchun
The determination of Ga, Rb, and Sr in rare earth metals and their oxides by inductively coupled plasma mass spectrometry (ICP-MS) is severely affected by double-charge interferences from matrix elements such as La, Ce, Er, Yb, and Lu, compromising the accuracy of the results. This study established a method using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS). Samples were digested with nitric acid and hydrogen peroxide, and diluted with 1% (V/V) nitric acid for analysis. Rh was employed as the internal standard to correct for matrix effects, and the tandem quadrupole (MS/MS) mode was used to eliminate double-charge interferences from the rare earth matrix. The mass-to-charge ratio (m/z) of the first mass filter (Q1) was set to transmit ions at m/z of 69, 85, and 88. Ammonia (NH3) was introduced into the collision/reaction cell. Under these conditions, the analyte ions (69Ga+, 85Rb+, 88Sr+) do not react with NH3, while the interfering double-charged ions (e.g., 138La++, 138Ce++, 170Er++, 170Yb++, 176Yb++, 176Lu++) undergo reaction with NH3, forming product ions with m/z different from the analytes. The second mass filter (Q2) was then set to m/z of 69, 85, and 88, allowing only the unaffected analyte ions to pass through to the detector, thus effectively eliminating the mass spectral interference. The NH3 flow rate was optimized, and the optimal value was found to be 1.80 mL/min. The calibration curves showed excellent linearity with correlation coefficients all greater than 0.999 9. The limits of detection (LODs) for Ga, Rb, and Sr ranged from 0.003 8 to 0.026 μg/g, and the limits of quantification (LOQs) were between 0.012 and 0.084 μg/g. This method was applied to determine Ga, Rb, and Sr in synthetic neodymium metal, synthetic samarium metal, CeO2, Er2O3, and Yb2O3 samples. The relative standard deviations (RSDs,n=11) ranged from 3.8% to 13.5%, and the spike recoveries were in the range of 88% to 114%.
