|Graduate School of Science||Earth and Planetary Sciences||Professor|
Cosmo- and Geo-chemistry Group : The geochemistry is the one of the key research field to derive the direct "evidence" of the growth history of the Earth and the Solar System. We are trying to understand the early sequence of the solar system, meteorites, planetesimals or the Earth’s core, and also to decode the crustal growth sequence and the origin and evolution of the life through the elemental and isotopic analyses of various samples including rock, mineral, fluid and biochemical tissues. To achieve this, we are developing a state-of-the-art technique for the elemental analysis using the high power lasers and the mass spectrometry. Among the technique, combination of the femtosecond laser and the ICP-mass spectrometry has a potential to become a significant and the most sensitive tool for the elemental and isotopic analyses for various solid samples. The newly developed system has blossomed to become the key technique to provide the most precise chronological data for both the rock and mineral samples. Moreover, the present analytical technique is likely to become a method of choice for many chemists or biochemists because it is much more versatile and user friendly and efficient method for the elemental analysis for industrial or biochemical samples. New analytical techniques can become a major tool to open up the non-traditional research field.
Overview of your research
Atmospheric pressure inductively coupled plasmas (ICPs) are flame like electrical discharge that have revolutionised the practice of elemental and isotopic ratio analysis. The high temperatures available at atmospheric pressure in the plasmas make them very efficient excitation and ionisation sources. The ionisation efficiency to the singly charged state for most elements is approaching >20% (Gray, 1985; Niu and Houk, 1996; Montaser, 1998), indicative of very attractive characteristics as ion sources (Adams, 1988; Montaser, 1998; Taylor, 2001). The mass spectrometer utilising an atmospheric pressure plasma as an ion source (ICP-MS) is likely to become a significant and cost-effective tool for many isotopic measurements. It is also much more versatile, permitting isotopic analysis for the elements which have been thought intractable, and allowing the in-situ isotopic analysis on small grains. Recent progress in the ICP-MS technique is dramatic improvements in precision and accuracy of the isotopic ratio measurements. This was achieved by a coupling of the powerful ICP ion source and a high sensitivity magnetic sector mass spectrometer coupled with multiple collector arrays. Several isotopic geochronometers, including Rb-Sr, Nb-Zr, Lu-Hf, Hf-W and U-Pb isotopic systematics, have been developed to decode the early growth history of the Earth. Precise chronological data produced by the multiple collector-ICPMS technique can provide us key information about the accretional sequence of the Earth, timing of core-mantle segregation, and/or geochemical evolution of the continental crust. Another important application achieved by the multiple collector-ICPMS technique is stable isotope geochemistry using heavy elements. Series of isotopic study have been made so fat five years, and the data obtained here suggested that isotopic fractionation was common for heavy elements including Ca, Fe, Zn or Sr. This indicates that almost all elements in the periodic table can be applied for the stable isotope study. Another important extension of the capability of the ICP-MS technique is to accept dry sample aerosols generated by a laser ablation technique, and this has opened up a completely new research field in the Earth Sciences where high spatial resolution is required. Laser ablation utilizing UV-light (frequency quadrupled 266-nm or quintupled 213-nm Nd:YAG laser, or ArF Excimer laser operating at 193-nm DUV wavelength) can provide precise and reproducible signal intensity data from a small sampling area, and is now the most widely used system for the LA-ICPMS technique. This is well demonstrated in zircon U-Pb chronology. The resulting precision in 238U-206Pb ages determination is now almost comparable to that achieved by the ion microprobe technique which has been thought to provide "bench mark" age data. However, even with the UV lasers, serious elemental fractionation occurs during laser ablation. It is recognised that the differences in elemental volatility, or time-dependent changes in particle size distribution can cause elemental fractionation. Although many efforts have been made to reduce the elemental fractionation, the complexity in the physical and chemical processes related to the laser ablation of solid samples has prevented a detailed understanding of the elemental and isotopic fractionation induced by laser ablation, and therefore, the reduction of elemental fractionation through the analysis was still a key issue to improve the data quality. Faced with this problem, we have measured aspect ratio of the ablation pit obtained by fs-laser ablation and conventional nanosecond laser (ArF Excimer laser operating at 193 nm wavelength) ablation techniques, and all the analyses were made using the same ablation cell geometry and the identical ICPMS instrumental setup.
Fields of research (key words)
Geochemistry, Cosmochemistry, Analytical Chemistry