| Laser ablation inductively coupled plasma mass spectrometry?LA-ICP-MS?hasbecome a powerful technique to directly determine trace elements and isotope ratios in various solid materials.The source and degree of elemental fractionation and matrix effect are the remaining challenges in LA-ICP-MS.For example,the U-Pb geochronologic analysis of accessory minerals has played an important role in Earth and solar system science in constraining the ages of a wide variety of rocks and minerals.However,the significant matrix effects observed between different accessory minerals and the lack of high-quality standards for many minerals of interest are the major limitations of its geochronological applications.In this study,the elemental fractionation behavior in nanosecond and femtosecond LA-ICP-MS were studied.The water vapor-assisted LA-ICP-MS U-Pb dating method has been developed to the analysis of zircon,monazite,xenotime and titanite with NIST 610 glass as a reference material.1.Investigation of laser ablation related elemental fractionation and mass load effectin the ICP.The ICP plasma and laser ablation related fractionation behaviors were studied in this study.The elemental ratio of U/Th ratios decreased from 1.8 to close 1 with decreasing crater diameter from 55?m to 15?m at the beginning ablation with a high makeup gas flow rate.While,the U/Th ratios remained at 1 with decreasing crater diameter from 55?m to 15?m with a low makeup gas flow rate.This indicates that mass load has a significant effect on elemental vaporization behavior in the plasma.The Cu/Ca,Zn/Ca,Au/Ca and Pb/Ca ratios are significantly increased with increasing crater diameter from15?m to 55?m?increasing mass load of the ICP?at the beginning ablation and reduced rapidly with time,followed by a constancy and a subsequent increase.The mass load induced matrix effects in the ICP is significantly alleviated with increasing single spot ablation time and decreasing crater diameter.Whereas,the laser ablation induced fractionation becomes most pronounced for increased ablation time and smaller crater diameter.Thus,such change patterns of element/Ca and U/Th ratios are caused by a gradual transfer of dominant elemental fractionation source from plasma to laser ablation site.2.The influence of carrier gases He and Ar on signal intensities in 193 nm excimer LA-ICP-MS analysis.In this study,the influence of two carrier gases?He and Ar?on signal intensities was reassessed using a local aerosol extraction strategy and a commonly used cylinder ablation cell.he experimental results showed that the sampling position in a common ablation cell strongly affected the signal enhancement factors of refractory lithophile elements when using He instead of Ar as the carrier gas.Furthermore,the signal intensities obtained using the local aerosol extraction strategy at a sample distance of 1mm with an extremely high ablation location-related gas flow rate(approximately 10 m s-1)were enhanced only slightly?by approximately 1–1.3 times?for all measured elements when using He instead of Ar as the carrier gas.The results indicate that the size of the aerosol particles or agglomerates produced by laser ablation decreased considerably as the ablation location-related gas flow rate increased.To gain information on the mechanisms that are involved in the improvement of signal sensitivity of using Ar as the carrier gas at high-velocity ablation location,small amount of water vapor was also added into the plasma in both a normal ablation cell and local aerosol extraction strategy.Similar signal intensities of the entire mass range elements were obtained in wet plasma using Ar and He as carrier gases.Under wet plasma conditions or with a high ablation location-related gas flow rate,U-Pb age dating of zircon with similar accuracy was obtained using Ar instead of He as the carrier gas.3.Comparison of signal intensities and elemental fractionation in 257 nm femtosecond LA-ICP-MS using He and Ar as carrier gas.The signal intensities and elemental fractionation at different ambient gas conditions?He,Ar and pre-mixed He-Ar mixture?were investigated using 257 nm femtosecond?fs?-LA-ICP-MS.The signal intensities of all elements obtained in pre-mixed He-Ar mixture is similar to the sensitivities acquired in pure helium.Meanwhile,the calculated elemental fractionation indices for all elements generally remain constant and more close to 1 than in pure He or Ar at different spot sizes.The technique of using the pre-mixed He-Ar mixture as carrier gas has been successfully used for the determination of major and trace elements in USGS and MPI-DING glasses by using fs-LA-ICP-MS.In addition,the U-Pb results of monazite by fs-LA-ICP-MS using NIST 610 silicate glass as a non-matrix-matched calibration standard in scanning ablation mode under Ar and He-Ar mixture atmosphere show excellent agreement with the reference value.4.Water vapor-assisted“universal”non-matrix-matched analytical method for the in situ U-Pb dating of zircon,monazite,titanite and xenotime by LA-ICP-MS.In this study,we investigated the effects of the addition of oxygen,nitrogen and water vapor before and after the ablation cell on the accuracy of the U-Pb dating of different minerals?e.g.,zircon,monazite,titanite and xenotime?by LA-ICP-MS.We found that the addition of water vapor,unlike that of oxygen and nitrogen,before the ablation cell can significantly suppress the matrix effects on U-Pb dating.The deviations of the measured 206Pb/238U ratios in these accessory minerals were significantly reduced from 10-24%to less than 1-2%when using NIST 610 glass as an external standard.This can be attributed to the suppression of elemental fractionation in both the laser ablation and ICP ionization processes by the presence of water vapor.The developed water vapor-assisted LA-ICP-MS U-Pb dating method has been successfully applied to the analysis of zircon,monazite,xenotime and titanite with NIST 610 glass as a reference material in both the 193 nm excimer laser and 213 nm Nd:YAG laser ablation systems. |
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