INAA Services at Washington University - Description of Analytical ProcedureThe most recent and complete published description of our analytical procedure is given in: Korotev R. L. (1991) "Geochemical stratigraphy of two regolith cores from the central highlands of the Moon," Proc. Lunar Planet. Sci., Vol. 21 (ed. V. L. Sharpton and G. Ryder), 229-289, Lunar and Planetary Institute, Houston. Below is the relevant text from that paper.
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II.B. Analytical MethodsI have determined the concentrations of 20-25 chemical elements by INAA (instrumental neutron activation analysis) using the following procedures. Samples and standards were encapsulated in high-purity silica tubing, usually with an outside diameter of 4 mm (5 mm for some particles) and an inside diameter of ~3 mm (T21 Suprasil, Heraeus-Amersil Inc., Buford, Georgia). Batches of about 50-70 tubes were irradiated with a thermal neutron flux of 3.9 x 1013 cm-2 s-1 in the second reflector ring of the University of Missouri Research Reactor (Columbia, Missouri). All <1 mm fines samples were irradiated for 48 hours. During irradiation the sample package was rotated about its cylindrical axis at a rate of 0.1 rpm (revolution per minute). After irradiation, the tubes were washed in 6 M HNO3 for 15 minutes. Samples were radioassayed by gamma-ray spectrometry in the tubes in which they were irradiated using four high-purity Ge detectors with horizontal cryostats and an ND9900 spectroscopy system (Nuclear Data Inc., now Canberra Nuclear Products Group, Schaumburg, Illinois) and associated MicroVAX II computer (Digital Equipment Corporation). During radioassay the sample tubes were spun about the cylindrical axis at a rate of 10 rpm to minimize possible geometry effects. Gamma-ray spectra were acquired at count rates of 5-12 kcounts/s over the energy range of 30-1800 keV and accumulated digitally into 8192 channels at 0.22 keV/channel. The FWHM for the 1332 keV peak of 60Co ranged from 1.75-1.90 keV under operating conditions for the four detectors and sample-to-detector distances were typically 5-10 cm. Spectral data were reduced using new versions of the TEABAGS programs of Lindstrom and Korotev (1982) which Lindstrom and I have totally rewritten to take full advantage of the computing capacity of the MicroVAX. Elemental standards for the <1 mm fines are AN-G (IWG-GIT Greenland anorthosite; Govindaraju, 1980) for Na and Ca, synthetic multielement standards for Cr, Ni, Zr, Ir, and Au, and NBS 1633a (coal flyash) for all other elements (Korotev, 1987a). For the particles, fragments of synthetic glass standards were used. Blank contributions from the silica tubes, which are each typically ~0.7 g in mass, are negligible for all elements when sample masses exceed 1 mg. The <1 mm fines from 60009 and 60010 were each irradiated in a different batch with its own set of standards. The two batches were sealed into a single can for irradiation. Both batches were radioassayed simultaneously (two detectors per batch), with each sample radio-assayed only once for approximately 1.3 hours between 7 and 10 days following irradiation. The <1 mm fines from 60001-7 were divided into six batches of approximately 59 stratigraphically consecutive samples each. One sample of surface soil 60601 was included in each batch, along with the standards. The 60001-7 samples were analyzed in three experiments in which two batches were irradiated together and radio-assayed simultaneously. Two radioassays were done on each sample, the first between 7 and 10 days following irradiation for about 1.7 hours and the second between 3 and 4 weeks following irradiation for about 2 hours. For these samples, data for 22 elements are reported. Because the 60009/10 samples did not receive a second radioassay, analytical data for some elements determined via nuclides with long half lives (Ni, Sr, Zr, Eu, Tb, Hf, Ta, and Ir) are not as precise as those for the 60001-7 samples, which received a second radioassay. Data for Sr and Zr in 60009/10 are too imprecise to be of value and are not reported. All remaining <1 mm fines samples (surface and trench soils) were analyzed in a separate experiment and received three 2-6 hour radioassays; only for these samples are data for Cs, Ce, and Nd reported (25 elements total). The >1 mm particles were irradiated in four different experiments with durations of 48 to 120 hours. Each particle received one or two radioassays of several hours at 6-10 days and another at 3-4 weeks following irradiation.
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Present ProceduresOur present procedures our similar to those given above, with the following modifications.
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