Improved in situ Hf isotope ratio analysis of zircon using newly designed X skimmer cone and jet sample cone in combination with the addition of nitrogen by laser ablation multiple collector ICP-MS

Abstract

The effect of three different cone combinations on the performance of laser ablation MC-ICP-MS (Neptune plus) for the in situ Hf isotope analysis of zircon were investigated. The signal sensitivities of Hf, Yb and Lu were improved by a factor of 1.4 and 2.5, respectively, with using the X skimmer cone + standard sampler cone and the X skimmer cone + Jet sample cone compared to the standard arrangement (H skimmer cone + standard sample cone). However, when using the high-sensitivity Jet sample cone, the instrumental mass fractionation for hafnium displayed a large non-linear component that could not be corrected using the normal mass fractionation laws. The magnitude of this non-linear mass fractionation was strongly related to the central gas flow rate. The in situ Hf isotope analysis of zircon standards 91500 and Mud Tank using the Jet cone displayed large deviations (410–470 ppm) at the optimum central gas flow rate for Hf, which seriously deteriorated the performance of the Jet cone. The addition of 4 ml min⁻¹ nitrogen to the central gas flow in laser ablation MC-ICP-MS was found to not only increase the sensitivity of Hf by a factor of 2.1, but also suppress this non-linear mass fractionation. The determined Yb/Hf and Lu/Hf ratios at their corresponding optimum makeup gas flow rates for Hf intensity were found to be reduced by factors of 2 and 1.3 in the presence of nitrogen, respectively, which would benefit the accurate in situ determination of Hf isotopes in high-content Yb and Lu samples. Compared to the standard arrangement, the corresponding precision (2σ) of ¹⁷⁶Hf/¹⁷⁷Hf for single spot analysis of zircon standard 91500 was improved from 224 ppm to 50 ppm by using the newly designed X-skimmer cone and Jet sample cone in combination with the nitrogen addition technique. The determined ¹⁷⁶Hf/¹⁷⁷Hf ratios are in excellent agreement with published values in five reference zircon standards (91500, GJ-1, Mud Tank, Penglai and Plešovice). Our first Hf isotopic results from zircon standard M257 (0.281544 ± 0.000018; 2SD, n = 151) showed that it was fairly homogeneous in Hf isotopes. These results clearly demonstrate that the present analytical method has the potential to become an important tool for the pursuit of high-quality in situ Hf isotope data for zircons.