Anal. Chem. 1994,66,431-439
Determination of Ultratrace Neodymium in High-Purity Lanthanum Compounds by High-Accuracy Isotope Dilution Inductively Coupled Plasma Mass Spectrometric Analysis with Chemical Preconcentration Ellyn S. Beary' and Paul J. Paulsen Center for Analytical Chemistry, National Institute of Standards and Technology, Building 222, Room A 339, Gaithersburgl Maryland 20899 Direct, accurate quantitationof ultratraceNd in La compounds was not possible by ICPMS. The La/Nd ratio of >106required chemical separations to provide a suitable sample solution for instrumental analysis. Separation of Nd from the La matrix is problematic since the two elements are close in mass and similar in chemicalbehavior. The ICPMSin a semiquantitative survey mode proved to be a valuable tool in developing the required separations. Nd was quantifiedusing isotope dilution which requires neither 100%recovery nor absolute isolation of the Nd, resulting in considerable flexibility in the design of preconcentrationprocedures. Nanogram per gram quantities of Nd in high-purityLa compounds were determined using this procedure. The inductively coupled plasma (ICP) as an ion source for mass spectrometric (MS) detection has been investigated for more than a decade.' ICPMS has been established as a valuable analytical tool and is ideally suited for the verification of chemical purity in both solution samples and solids. All analytical techniques depend on high-quality ,chemically pure materials for calibration. In addition, the success of certain manufacturing processes directly relates to the availability of suitable high-purity starting materials; such as the production of semiconductors, superconductors, and optical fibers. Purity analyses of a heavy metal fluoride glass and its precursors are addressed in this paper. This glass, a proprietary combination of Zr, Ba, La, Al, and Na as fluorides, was used in the production of an infrared optical fiber. Theoretically, this low loss optical fiber as applied to long-distance communications systems could surpass the performance of commercial silica fibers by an order of magnitude.2 However, cationic contamination such as Fe,2+ C O , ~Ni,2+ + C U , ~and + Nd3+in the fluoride glass has absorption bands that seriously degrade the theoretical performance of the optical fiber.3 Tolerable concentrations of such impurities in the fluoride fiber are in the picogram per gram range. The ICPMS analyses reported here were used to assess overall chemical purity and to quantify the ultratrace Nd contamination in related manufacturing materials. Graphite furnace atomicabsorption has been used to determine the levels of Fe, Co, Ni, and Cu contamination in similar material^.^!^ (1)
Houk, R. S.; Fassel, V. A,; Flesch, G. D.; Svec, H.J.; Gray, A. L.; Taylor, C . E.Anal. Chem. 1980, 52, 2283-2289.
(2) Beary,E.S.;Paulsen,P. J.;Rains,T.C.;Ewing,K. J.;Jaganathan, J.;Aggarwal, I. J . Cryst. Growth 1990, 106, 51-60. (3) Drexhage, M. G . ; Moynihan, C. T. S c i . A m . 1988, 110-116.
This article not subject to U.S. Copyright. Published 1994 by the American Chemical Society
The evaluation of materials for chemical purity requires that the absence of contaminants be confirmed, and neither high-accuracy quantitation nor identification of ultratrace impurities is necessary. Routine ICPMS semiquantitative analyses provide data on about 70 elements from mass 6 to mass 240. Semiquantitative procedures using both internal and external standards were used for the assessment of total purity of the La precursors. Many elements are reported at the detection limit and their presence cannot be verified. However, the absence of total impurities verifies chemical purity. For example, total contamination below 0.1 mg/g is 99.99% pure. In addition, the comprehensive analytical information provided by ICPMS semiquantitative analyses easily provides a unique profile of the material analyzed. Combinations of contaminants as well as their relative concentrations (as opposed to the less accurate absolute concentrations) can serve as a signature or fingerprint and identify the source of contamination, thus simplifying purification procedures. Lanthanum fluoride was the primary source of the Nd contamination. Control of its purity was critical in order to meet the subnanogram per gram tolerance for Nd in the glass fiber. The accurate quantification of Nd at the nanogrampicogram per gram level in high-purity La compounds was an analytical challenge. Laser ablationS and ETV6 sampling techniques are available for direct solid analysis using mass spectrometry; however, the absence of appropriate solid standards prevents accurate quantitation. The more accurate quantitative methods for ICPMS use internal standards and require solution sampling. However, solution samples with greater than 0.1-1% w/v total solids will clog the sampling and skimmer cones between the ICP torch and the mass spectrometer. Suitable matrix dilutions to reduce the solids content of the analytical sample also dilute the ultratrace analyte, often to a level below the instrumental detection limit. Many researchers have addressed the reduction of high solids content of solution samples for ICPMS using a variety of online and offline separations proced~res.~-l~ This work describes the development of such procedures as applied to (4) (5) (6) (7)
Jaganathan, J.; Aggarwal, I. Appl. Specrrosc. 1993, 47, 190-191. Arrowsmith P. Anal. Chem. 1987, 19, 1437-1444. Shibata, N.; Fudagawa, N.; Kubota, M . Anal. Chem. 1991, 63, 636-640. Plantz, M. R.; Fritz, J. S.; Smith, F. S.; Houk, R. S. Anal. Chem. 1989,61, 149-1 53.
(8) Sturgeon, R. E.; Berman, S. S.; Willie, S. N.; Desaulniers, J. A. AMI. Chem. 1981,53, 2337-2340. (9) Beauchemin, D.; Berman, S. S. Anal. Chem. 1989, 61, 1857-1862.
Analytical Chemistry, Vol, 66, No. 4, February 15, 1994 431
both semiquantitativeand quantitative analyses of high-purity La compounds. Direct and accurate quantification of Nd at the subnanogram per gram level in high-purity La compounds was not possible by ICPMS using solution sampling. For a solution containing 0.1% ' total solids, accurate quantification at about 100 ng of Nd/g of La was expected, which is several orders of magnitude higher than the subnanogram per gram goal. The high concentration, not the presence of La, prevented the direct quantitation of Nd in these samples. ICPMS was used in a semiquantitative survey mode to develop a separation that significantly reduced the La/Nd ratio, resulting in the preconcentration of Nd in the analytical sample as presented to the instrument. The degree of the La/Nd reduction ultimately determined the analytical measurement limit. The high-accuracy quantitation of Nd at the nanogram per gram level was accomplished using ICPMS isotope dilution combined with chemical preconcentration.
EXPER IMENTAL SECTION Materials and Reagents. All of the mineral acids and highpurity water used in this work were prepared and analyzed at the National Institute of Standards and Technology (NIST)." (CAUTION: Mineral acids, such as H N 0 3 and HC104, are irritants to eyes, skin, and mucous membranes. In addition, perchlorates can form explosive mixtures in combination with carbonaceous material.12) Cation-exchange resin was used for the rare earth element (REE) separation. Bio-Rad analytical-grade, AG 50W X 4, 200-400 mesh in the hydrogen form with a capacity of 1.1 mequiv/mL resin bed was purchased. It was converted to the ammonium form using high-purity NH40H prepared by bubbling NH3 gas intoquartzsubboilingdistilled water. The resin was contained in plastic columns of 1.65 cm X 12 cm resin height. The resulting volume of the resin bed was about 25 cm3. a-HIBA (2-hydroxy-2-methyl-propanoic acid) was used to elute the REE in this separations scheme. Reagent-grade a-HIBA was purified by sublimation at about 30 OC. This procedure is effectivein removing organic impurities. In these experiments, the 0.25 mol/L a-HIBA was prepared by dissolving the appropriate amount of purified reagent in high-purity water. The pH was adjusted to about 5 with high-purity NH40H. A large batch of solution was prepared for use throughout the experiment since both the concentration and the pH of the solution affect the elution rate of the REE. Instrumentation. The inductively coupled plasma mass spectrometer (ICPMS) used for this work was a VG Plasmaquad. The diffusion pumps on this instrument have been replaced with turbomolecular pumps; a 330 L/s pump on the quadrupole and a 1500 L/s pump for the lens region. The operating parameters for the ICP were an rf power of 1.3 kW and a coolant and auxiliary argon flow of 16 and 0.5 L/min, respectively. The nebulizer argon flow rate was controlled by a flow controller to approximately 0.75 L/min. The flow rate was optimized to provide a LaO+/La+ ratio of
