| An analytical protocol for high-precision simultaneous determination of 147Sm/144Nd and 143Nd/144Nd ratios in geological minerals using LA-MC-ICP-MS is presented here. To improve the precision and accuracy of in situ Sm-Nd isotope analyses in geological samples, high analytical sensitivity and minimized non-linear mass dependent fractionation are desirable. In this study, we investigated the effects of three different cone combinations and the addition of N2 with the guard electrode either grounded or floating on the performance of Nd isotope analyses. Signal enhancements (2.5-3 folds) were observed for the Jet sample cone+X skimmer cone and for the standard sample cone+X skimmer cone compared to the standard sample cone+H skimmer cone with addition of 3 or 6 ml min-1 N2 which would benefit the accurate in situ determination of Sm-Nd isotopes in low Sm/Nd samples. The influence of a guard electrode on the LA-MC-ICP-MS sensitivity was minimal for dry plasma conditions but became significant in the presence of N2. The addition of 3 or 6 ml min-1 N2 into the carrier gas increased the sensitivity for Nd by a factor of 1.6 with the GE-on mode for the Jet sample cone+X skimmer cone. However, in the GE-off mode, the addition of 2-3 ml min-1 N2 decreased the signal intensity of Nd by a factor of two to three. The makeup gas flow rate was found to have a significant effect on the mass bias stability of Nd isotopes, and this effect was different for different cone combinations. To stabilize the mass bias, measuring at elevated makeup gas flow rates was required for the combination of the X skimmer cone with the standard sample cone or Jet sample cone, at the expense of sensitivity. In notable contrast, the mass bias was stable for the combination of the standard sample cone and H skimmer cone under their corresponding optimum makeup gas flow rates. However, we found that neodymium isotopes exhibited a large non-linear component at elevated makeup gas flow rates that could not be corrected by using the accepted mass fractionation laws. The addition of 3-6 ml min-1 nitrogen to the central gas flow in laser ablation MC-ICP-MS was found to not only significantly enlarge the mass bias stability zone, but also in suppressing this non-linear mass dependent fractionation for the combination of X skimmer cone with Jet sample cone or standard sample cone. The addition of 3-6 ml min-1 nitrogen also have a good effect on instrumental stability and reducing elemental fractionation. Our results indicate that it is very possible to obtain 147Sm/144Nd and 143Nd/144Nd ratios simultaneously with good accuracy and precision using high-sensitive cones and adding small amounts of nitrogen to the central gas flow. While it is possible to obtain accurate Nd isotopes without using any standards, for accurate 147Sm/144Nd ratios, it is important to use matrix-matched standard crrections.The accuracy and precision of the developed Sm-Nd isotope analytical method of using Jet sample cone+X skimmer cone with the addition of nitrogen has been demonstrated by analyzing a series of reference glasses (JNdi-1 glass, LREE glass, NIST 610) and minerals (apatite, monazite, titanite, eudialyte, ferromanganese nodule and Co-rich crust). Sm-Nd isotopic compositions for these samples are consistent with those values obtained by published values. Futrermore, the obtained Sm-Nd isochron age by application of improved Sm-Nd isotopic measurements for monazites of Huai’an Complex (The Trans-North ChinaOrogen) shows good agreement on U-Pb age. The well compatible results demonstrate that our developed analytical technique as an important tool for providing high-quality in situ Sm-Nd isotope measurements is feasible and robustness. |
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