An improved chromatographic procedure for uranium (U) and thorium (Th) separation using TRU(SPEC) resins in acidic media was investigated and applied to precise 234U/230Th age determination analyses in environmental samples. The abundance and isotopic ratios were measured by thermal ionization mass spectrometry (TIMS) using a high purity mixed 233U/236U/229Th spike, which was calibrated against an international obsidian standard (A-THO). The associated instrumental mass fractionation was corrected using 233U/236U and 235U/238U ratios. The fractionation factors (α) were generally less than 3 permil. The TRU(SPEC) technique dramatically reduces the time spent for separation, by more than half, and avoids several complicated steps, including Fe(OH)3 precipitation and ammonium titration, which are required by traditional ion-exchange methods. Furthermore, this method allows both abundance and isotopic composition to be analyzed in the same Re-Re-Re filament loading, which significantly reduces the time required for machine runs and sample preparation. The high selectivity and capacity of TRU(SPEC) also allows column sizes to be reduced, thus minimizing acid consumption. Eliminating the Fe(OH)3, ammonium, HBr and acetone steps results in lower chemical blanks. This is particularly advantageous when analyzing sulphides and iron-bearing minerals, because the conventional technique is marred by ferric iron interference, causing low yields and poor ionization. The TRU(SPEC) resins produce excellent yields for both U and Th (>98%) with no detectable interference. Six blank determinations showed consistently low 238U (<0.030 ng) and 232Th (<0.020 ng) values. Twenty-seven duplicated analyses of a CBNM U isotopic standard, IRM 72/5, yielded a mean 235U/238U value of 0.99296 ± 0.0018 after fractionation correction, which agrees well with the certified value of 0.99299. Analyses of duplicated sulphides, A-THO, and a mammoth tooth agree with results derived from the conventional method. We have systematically applied the TRU(SPEC) procedure to precise age determination analyses of pyrite, anhydrite, and carbonates.
|Number of pages||12|
|Journal||Journal of the Geological Society of China|
|Publication status||Published - 1999 Aug|
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