Anal. Chem. 2000, 72, 3504-3511
An ICP-OES Method with 0.2% Expanded Uncertainties for the Characterization of LiAlO2 Marc L. Salit,* Robert D. Vocke, and W. Robert Kelly
National Institute of Standards and Technology, Chemical Science and Technology Laboratory, Analytical Chemistry Division, 100 Bureau Drive, M/S 8391, Gaithersburg, Maryland 20899-8391
An improved inductively coupled plasma-optical emission spectrometry (ICP-OES) method1 has been applied to the determination of Li and Al mass fractions and the Li/Al amount-of-substance ratio in representative samples of LiAlO2. This ICP-OES method has uncertainty on the order of 0.2%,2,3 comparable to the best analytical methods. This method is based on several strategies, which are detailed in this work. The mean measured mass fractions of Li and Al in eight samples were 0.10151 ( 0.00016 ((0.16%) and 0.41068 ( 0.00056 ((0.14%), and the mean Li/Al amount-of-substance ratio was 0.9793 ( 0.0017 ((0.17%). The uncertainty is dominated by sample handling and heterogeneitysabout a factor of 2 larger than the ICP-OES instrumental uncertainties, which were 0.04% for Al and 0.07% for Li. In the spring of 1997, our laboratory took on a project to characterize a ceramic material, LiAlO2, for its Li isotope ratio, Li and Al mass fractions, and Li/Al amount-of-substance ratio. Pellets of this material are intended for use as a target in commercial light water reactors for the manufacture of 3H.4,5 Stringent engineering requirements for the bulk composition of the target material and the mass of 6Li/unit length required both analytical method development and the generation of a quality control material for process control of production analytical measurements. The 6Li/unit-length-of-rod specification is a critical parameter in the reactor core design and is the main motivation for these measurements. The 6Li acts as a “burnable poison” in the reactor core, capturing neutrons as it is consumed to make 3H. The presence of 6Li depletes the neutron flux in the reactor, and its concentration must be managed to avoid “poisoning” the nuclear chain reaction. The 6Li/unit-length specification is designed to permit the required neutron flux to be maintained as both the 6Li (1) Salit, M. L.; Turk, G. C. Anal. Chem. 1998, 70, 3184-3190. (2) Unless otherwise stated, all uncertainties reported in this work are expanded uncertainties, calculated according to procedures outlined in the ISO Guide to the Expression of Uncertainty in Measurements“GUM.”3 These uncertainties represent a 95% confidence interval and are estimated from both statistically evaluated uncertainty componentssType Asand uncertainty components evaluated by other meanssType B. All uncertainty components are expressed as standard deviations and are combined as variances. (3) Guide to the Expression of Uncertainty in Measurement, 1st ed.; ISO: Switzerland, 1993. (4) Pincus, W. TVA Plant to Supply Nuclear Bomb Material Tritium. Washington Post, December 23, 1998, A07. (5) TVA Approves Plan to Make Weapons Material. Washington Post, December 9, 1999, A13.
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and the reactor fuel are “burned.” The Li mass fraction, the 6Li/ 7Li ratio, and the material linear density are all required to establish the 6Li/unit length. While the Li mass fraction is required primarily for the 6Li/ unit-length specification, it is also needed, along with the Al mass fraction, to verify a mass balance specification (to ensure material purity, sum of all determined species > 99%). The Li/Al amountof-substance ratio is specified to be 0.98 + 0.02/-0.06 to ensure chemical and mechanical stability of the ceramic material. The analytical requirements arose from the need for reliable assessment against these specifications and from the need to establish a process-control material. These requirements include small uncertainties and traceability to SI. SI traceability ensures measurement comparability between different laboratories, different techniques, and measurements made at different times. Coupled with small uncertainty, traceability ensures a control material which is useful for control of accuracy over time, not merely variability assessment. Small uncertainty is assured through precision measurement approaches, coupled with management of bias; traceability is assured by calibration with primary materials and a rigorous uncertainty budget. This analysis required precise and accurate sample handling, isotope ratio measurement, and assays. The target expanded uncertaintiesstaking into account potential biasessfor the isotope ratio, assays, and amountof-substance ratio were on the order of 0.2%. ICP-OES Analysis. A Li assay with uncertainty of a few parts per thousand would conventionally be performed with an isotopedilution Thermal Ionization Mass Spectrometry (TIMS) procedure, and a comparable Al assay would be performed with gravimetry. A high-performance ICP-OES method recently developed in our laboratory provides an expedient alternative to these methods for the determination of the Li and Al mass fractions and the Li/Al amount-of-substance ratio.1 The ICP-OES procedure is an extension of the approach developed for the analysis of single-element spectrometric solutions which are issued as NIST Standard Reference Materials (SRMs) for experiment control and instrument calibration. Potential biases in this measurement are controlled by several strategies, and the measurement yields relative expanded uncertainties at the parts-per-thousand level, as opposed to the more typical 1% level expected from ICP-OES. The multielement ICP-OES procedures for the bulk composition (Li and Al mass fractions) and amount-of-substance ratio measurements were developed for this application. ICP-OES measurements compare the number of emitting species in the 10.1021/ac0000877 CCC: $19.00
© 2000 American Chemical Society Published on Web 06/30/2000
Table 1. Li Sample Preparation for TIMS Isotope Ratio Measurement: Ion Chromatography Separation cation separation column AG 50W-X8, 100-200 mesh, 5-mL resin bed, 0.007-m i.d. elution scheme columns conditioned 10 mL of 0.5 mol/L HCl in volume fraction of 80% methanol samples evaporated to dryness, redissolved, in 1 mL of 0.5 mol/L HCl in volume fraction of 80% methanol then loaded onto conditioned columns wash 2 mL of H2O, discarded 16 mL of 0.5 mol/L HCl in volume fraction of 80% methanol added to columns, discarded Li eluted and collected 72 mL of 0.5 mol/L HCl in volume fraction of 80% methanol eluted fraction evaporated to dryness with HClO4 and H2O conversion anion column AG 1-X8, 4-mL resin bed conversion to LiOH columns conditioned NH4OH samples loaded with 2 mL of NH4OH elute and collect LiOH with 40 mL of NH4OH dry solution to appropriate mass fraction ∼100 Li µg/g solution
EXPERIMENTAL SECTION Sample Preparation. All reagents used in this analysis were of high purity. Eight hollow cylinders of LiAlO2, approximately 2.5 cm in length and 0.8 cm in diameter, with a wall thickness of 0.15 cm and weighing ∼2.5 g, were each crushed in polypropylene bags and the chips transferred to clean 50-mL Nalgene polycarbonate bottles. LiAlO2 samples of ∼0.3 g (∼5 chips) were taken from each of the eight samples, weighed, and placed into eight individual Carius tubes (internal volume, ∼25 mL). Ten milliliters of HCl was added to each tube. Two reagent blanks were prepared in the same manner as the samples. The contents of each tube were frozen in a solid CO2-CHCl3-CCl4 mixture and the tube sealed with an O2 natural gas torch. After being warmed to room temperature, the tubes were placed inside steel shells along with 20 g of solid CO2 for external pressurization, the caps were tightened to effect a gastight seal, and the tubes were heated to 240 °C in an oven for approximately 15 h. The safe handling of Carius tubes is discussed in Kelly et al.6 and references therein.7,8,9 After the dissolution procedure, no solid material was observed, and no light scattering was observed when the solution was laser illuminated. The tubes were opened and their contents quantitatively transferred to 125-mL polycarbonate Nalgene bottles and diluted with water to a total weight of 100 g. Aliquots of these solutions were used for the isotopic analyses and for the Li and Al assays. Li Isotope Ratio. Lithium isotope ratios (6Li/7Li) were measured on a TIMS of NIST design and construction. Thermal ionization sources fractionate the isotopes in a sample, typically
enriching the ion signal in the lighter isotope (6Li). Fractionation is managed by loading samples and an isotopic reference material (IRM) in the same chemical form and measuring them in exactly the same manner, permitting absolute lithium isotopic ratios to be determined by calibration with the IRM. The certified IRM used in this study was IRMM 016 (Li2CO3, Institute for Reference Materials and Measurements, European Commission Joint Research Centre, Geel, Belgium). Chemical Form of Li. The standard and samples were prepared using ion exchange chromatography to separate Li from the other cations present, and Li was converted to LiOH, then to Li3PO4, before analysis. Care was taken to ensure complete recovery of Li from the ion exchange column so that no fractionation occurred in the separation. The separation is detailed in Table 1. Solutions were prepared from the eight samples and the calibrant, IRM 016, in identical fashion. Mass Spectrometry Measurements. The isotopic analysis of Li followed a modified positive-ion rhenium triple filament procedure derived from that of Sahoo and Masuda.10With this technique, only the central ionizing filament is heated to ablate and ionize the Li ions, while the side filaments held the Li in the form of Li3PO4. Details of the sample loading, instrument description, and operating conditions are in Table 2. After heating and conditioning the sample for 1 h, baseline and sample beam currents were measured. The run order of the samples and standard was randomized and repeated. ICP-OES Measurement. The ICP-OES instrument used in this experiment is a Perkin-Elmer Optima 3000 XL, an axial-view ICP with a free-running 40 MHz RF generator, solid-state array detection, and an integrated autosampler.11 Relevant measurement parameters are reported in Table 3. The ICP operating parameters are the default conditions, while spectroscopic measurement parameters have been selected for precise measurement of spectral intensities. Aluminum and lithium are determined at commonly used analytical transitions of the atomic spectrum; thus, excitation energy is inversely proportional to the wavelength. Manganese, the internal standard, is also measured with a commonly used
(6) Kelly, W. R.; Paulsen, P. J.; Murphy, K. E.; Vocke, R. D.; Chen, L.-T. Anal. Chem. 1994, 66, 2505-2513. (7) Gordon, C. L. J Research NBS 1943, 30, 107-111. (8) Gordon, C. L.; Schlecht, W. G.; Wichers, E. J. Res. Natl. Bur. Stand. (U.S.) 1944, 33, 363-381. (9) Gordon, C. L.; Schlecht, W. G.; Wichers, E. J. Res. Natl. Bur. Stand. (U.S.) 1944, 33, 457-470.
(10) Sahoo, S. K.; Masuda, A. Int. J. Mass Spectrom. Ion Processes 1995, 151, 189-196. (11) To adequately describe experimental procedures, it is occasionally necessary to identify commercial products by manufacturer’s name or label. In no instance does such identification imply endorsement by the National Institute of Standards and Technology, nor does it imply that the particular products or equipment are necessarily the best available for that purpose.
observed volume of the source between the samples and the standards, not the mass fraction of the species. Because the Li atomic weight in the samples is deliberately perturbed, and the atomic weight of Li is variable in nature, unbiased ICP-OES mass fraction determination requires knowledge of both the sample and the calibration standard atomic weight, which were provided by the TIMS measurement of the isotope ratio, also required for characterization of anticipated material performance as a reactant for 3H production.
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line of the atomic spectrum. Mn is a convenient internal standard: the excitation energies of the Al and Mn lines are well matched; Mn was not present in the samples at levels relevant to its use as an internal standard; and Mn is typically well-behaved in the ICP, often used as a performance diagnostic element. No evidence of spectral interference was observed when the wavelengths selected were examined with high-purity single-element solutions. Line selection and sample preparation (including selection of internal standard mass fraction) were done such that the three spectral lines could be measured simultaneously and with high signal to noise. The dynamic range of the spectral lines must be approximately matched to that of the spectrometer to permit simultaneous integration. The approximately 0.3 g/100 g sample preparations were diluted about 30-fold with 50 µg/g Mn to produce the solutions for analysis. A single measurement of a solution required approximately 5 min, with sufficient delay to accommodate complete flushing of the sample-input system with the new solution. The run order of samples, calibrants, and blanks was randomized to minimize potential bias arising from temporal effects. This same random measurement order was repeated 10 times, permitting precise measurement of the signal ratios. Data processing was performed external to the instrument software (which reports backgroundcorrected intensities), in a spreadsheet program of our own design. ICP-OES Samples and Experiment Design. All sample handlingsdilutions and addition of internal standardswas performed gravimetrically, with relative uncertainty from weighing of better than 0.1%. Operations where the weighing uncertainty is poorer than 0.1% were performed in multiple stages, with each stage having adequate precision to ensure the desired aggregate uncertainty. Internal-standard addition was accomplished by including the internal standard in a common diluent used for all samples, blanks, and calibrants. In future work, the internal standard for ICP-OES will be added to the Carius tubes by weight, eliminating potential bias associated with the quantitative transfer and subsequent dilutions. ICP-OES Calibrant Preparation. Four Al primary solutions were prepared from two samples, each of two different high-purity Al metal samples, reported to be >99.999% purity, exclusive of gases. The Al metal was dissolved in high-purity HCL, with HNO3 added dropwise until visible reaction occurred. These solutions were heated in covered Teflon beakers overnight, at which point clear solutions were obtained. The four Li primary solutions were prepared from high-purity lithium carbonate, Li2CO3, SRM 924a. This material was coulometrically assayed for CO32- as well as analyzed for impurities. The isotopic abundance of Li in this material was measured and a Li atomic weight reported on the certificate. The Li2CO3 was dissolved in dilute HNO3. ICP-OES calibrants were prepared from these primary solutions, at mass fractions spanning the anticipated mass fraction of analytes in the samples. Eight multielement calibration solutions were prepared, from the eight primary solutions. Aliquots of two primary solutions (one Li and one Al) were weighed into a vessel and diluted, and a final weight was recorded for them. The same diluent, ∼50 µg/g Mn in 2% HNO3, was used for the calibrants, samples, and blanks. The exact mass fraction of Mn in the diluent 3506 Analytical Chemistry, Vol. 72, No. 15, August 1, 2000
Table 2. TIMS Operating Conditions Sample Loading outgas at 4.2 A, 35 min 24 h in HEPA filtered air filament loading and drying 2.5 µL of 0.25 mol/L H3PO4 and 500 ng of Li loaded on Re side filaments, mixed, dried at 1.2 A for 5 min, repeated 3 times filament conditioning
Instrument Configuration and Operation geometry 90° sector, 30-cm radius of curvature source slit 0.101 mm collector slit 1.524 mm ion detection Faraday cup 7Li ion current during 3 × 10-11 A, decaying to 0.5 × 10-11 data collection ionizing filament heating increase 0.5 A every 2 min to 1.5 A, then protocol increase 0.1 A every 1 min to filament temperature of 1800 °C (by optical pyrometer), final filament current 3.9-4.1 A instrument pressure
