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Reagents
Two hundred fifty grams of high-purity La2O3 (Alfa Aesar, REacton, powder, Lot No. U12A037, informative purity value: 99.999%) was purchased from FUJIFILM Wako Pure Chemicals Corporation (Osaka, Japan). Hydrochloric acid, HNO3, HF, and H2SO4 (Ultrapur-100 grade) were purchased from Kanto Chemicals Co. Inc. (Tokyo, Japan). A 50% diethyl oxalate solution was prepared by dissolving diethyl oxalate (JIS analytical grade, FUJIFILM Wako Pure Chemicals Corporation, Osaka, Japan) in ethanol (JIS analytical grade, FUJIFILM Wako Pure Chemicals Corporation, Osaka, Japan). A 2% oxalic acid solution was prepared by dissolving oxalic acid (JIS analytical grade, FUJIFILM Wako Pure Chemicals Corporation, Osaka, Japan) in water. A NIST SRM 3127a La standard solution was used to determine La in the filtrate, washing solution, and residues resulting from mechanical loss. A Co solution intended as an internal standard for La determination with ICP-OES was prepared by dilution of JCSS Co standard solution. The Co standard solution (Co: 1000 mg dm−3) was purchased from Kanto Chemicals Co. Inc. The NMIJ CRM standard solutions (3635-a Y), NIST SRM standard solutions (3148a Sc, 3110 Ce, 3142a Pr, 3135a Nd, 3147a Sm, 3117a Eu, 3118a Gd, 3157a Tb, 3115a Dy, 3123a Ho, 3116a Er, 3160a Tm, 3166a Yb, and 3130a Lu), and NIST traceable standard solutions of SPEX XSTC-331 (Th) were used for the impurity analysis using ICP-MS/MS.
Measurement Procedure of Metallic Impurities by ICP-MS/MS
Approximately 1.5 g of La2O3 was weighed in a 30-ml quartz crucible for impurity analysis. The sample in the quartz crucible was ignited at 900°C for 2 h using an electric furnace and then cooled in a silica gel desiccator for 1 h in the balance room. Subsequently, 1.2 g of La2O3 was precisely weighed in a 200-ml borosilicate glass beaker. Twenty milliliters of HNO3 (1 + 2) was added to the sample in the beaker. The sample was heated at 120°C on a hot plate until La2O3 was completely dissolved. After complete dissolution, the solution was cooled down to ambient temperature and then diluted as a stock solution to 1 kg with 0.2 mol L−1 HNO3 in a high-density polyethylene bottle (mass fraction of La; ca. 1 g kg−1). Three sub-sample solutions (mass fraction of La of 10 mg kg−1) were prepared for the impurity analysis by diluting the stock solution. Impurities were measured by ICP-MS/MS using the calibration curve method. Calibration standard solutions were prepared with mass fractions of 0 ng kg−1, 50 ng kg−1, 100 ng kg−1, and 250 ng kg−1. Table 1 lists the operating conditions for ICP-MS/MS. The ICP-MS/MS instrument was tuned before measurement in accordance with the manufacturer’s recommendations.
TABLE 1
Instrumental conditions of Thermo Fisher iCAP TQ ICP-MS/MS for impurity analysis.
| ICP source operation parameters | |
|---|---|
| RF power | 1550 W |
| Ar plasma gas flow | 16 L/min |
| Ar auxiliary gas flow | 1.0 L/min |
| Nebulizer gas flow | 1.12 L/min |
| Nebulizer | Borosilicate glass concentric nebulizer |
| Sample uptake rate | 0.3 ml min−1 |
| Spray chamber | Borosilicate glass cyclonic spray chamber |
| Sampling cone and skimmer cone | Ni sampling cone, Ni skimmer cone |
| Spectrometer operating parameters | |
| Measured isotopes and measurement modes | Single Quadrupole Mass Spectrometry mode |
| 89Y, 141Pr, 142Ce, 143Nd, 145Nd, 146Nd, 147Sm, 149Sm, 151Eu, 157Gd, 159Tb, 161Dy, 163Dy, 165Ho, 166Er, 167Er, 169Tm, 172Yb, 175Lu, 232Th | |
| H2 gas collision reaction mode; H2 gas flow 10 ml min−1 | |
| 45Sc (45; m/z at QMS1, 45: m/z at QMS2) | |
| Dwell time | 100 ms |
| Number of sweeps | Ten times |
| Measurement repetition | Five times |
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