Effect of Gas Flow Rates in Laser Ablation System on Accuracy and Precision of Zircon U-Pb Dating Analysis by LA-ICP-MS

Despite zircon U-Pb dating analysis by LA-ICP-MS receiving wide acceptance, it remains a challenge to obtain results with high accuracy and precision. It is known that gas flow rates of LA system can affect the signal stability of ICP-MS and thus result in impacts on analytical uncertainty of zircon U-Pb dating. However, the exact effects and mechanism of gas flow rates on zircon U-Pb dating analysis are still unclear.

To thoroughly understand the influence of gas flow rates on the analytical uncertainty of zircon U-Pb dating, and to provide valuable information to propose a reliable and robust LA-ICP-MS approach for zircon U-Pb dating analysis.

By applying zircon standard samples of Harvard 91500 and Plešovice as researching subjects, ICP-MS connected to a 193nm nanosecond laser ablation system was used to investigate the influence of gas flow rate settings on accuracy and precision of U-Pb dating analysis. RESULTS: With fixed make-up gas (Ar) of 1.0L/min, the average ²⁰⁶Pb/²³⁸U ages of Harvard 91500 were found to increase from 1002.0±10.4Ma (1σ) to 1083.0±6.8Ma (1σ) with increasing carrier gas (He) from 0.2 to 1.2L/min. Thus, it was clear that the sample aerosol transportation efficiency can greatly affect the analytical accuracy of zircon U-Pb dating. Furthermore, when the He flow rate was higher than 0.8L/min, the analytical accuracy and precision of zircon U-Pb dating decreased due to the increased signal intensity oscillations and formation of oxides from the introduction of large particles of sample aerosols. The comparison of the data of Plešovice obtained under 0.95/0.8, 0.80/0.8 and 0.8/0.6L/min for He/Ar gas flow rate patterns indicated that there were no significant differences in U/Pb weighted average age. However, the relative deviation of 1σ single-point analysis was the smallest (1.4%) when the Ar and He flow rates were both 0.8L/min.

The analytical accuracy and precision of zircon U-Pb dating by LA-ICP-MS can be improved by optimizing the gas flow rate setting of carrier gas and make-up gas, and highly recommending 0.8L/min of both Ar and He.