Stable isotope analysis of Nd by double spike thermal ionization mass spectrometry

We report a high-precision stable isotopic analysis of Nd using double spike thermal ionization mass spectrometry (DS-TIMS). The analysis protocol of the DS-TIMS technique is refined to include internal normalization and iterative calculation to achieve high precision and minimize systematic bias during data analysis. Nd stable isotope ratios are reported in a standard epsilon notation relative to JNdi-1, a common isotopic reference material for the radiogenic Nd-143/Nd-144 ratio. With this technique, the epsilon Nd-146, parts par 10(4) difference of the Nd-146/Nd-144 ratio relative to that of JNdi-1, can be measured with a typical precision of +/- 0.3 epsilon units. The precision and accuracy of this refined DS-TIMS technique are demonstrated by 2 analyses. First, Nd isotope fractionation during the cation exchange column chromatography is investigated. A known amount of JNdi-1 is eluted from a column filled with a cation exchange resin by using HIBA as a solute and is sequentially collected into 7 fractions. The Nd stable isotopic composition of the 7 fractions systematically changed from epsilon Nd-146 = 4.60 during the early stage of elution to epsilon Nd-146 = 9.44 during the late stage of elution in a mass-dependent manner. Using the elution data, the Nd isotope fractionation factor between the cation exchange resin and the solute (HIBA) at room temperature is determined as alpha(146,) (resin-HIBA) = 0.999964 +/- 0.000003. Second, Nd stable isotopic variation among commercial high-purity Nd oxide reagents is investigated. The epsilon Nd-146 of 10 commercial Nd oxide reagents and La Jolla Nd vary significantly from -2.36 to 0.23. The stable isotopic composition of La Jolla Nd (epsilon Nd-146 = -1.97 +/- 0.23) is clearly different from that of JNdi-1. The isotopic variation of the reagents agrees with the mass-dependent isotopic fractionation trend. The Nd stable isotopic variation among the reagents may be caused by isotope fractionation during the reagent production process and/or may be inherited from the source materials of the reagents. (C) 2012 Elsevier B.V. All rights reserved.