Analytical Sciences

Evaluation of the Analytical Capability of NIR Femtosecond Laser Ablation-Inductively Coupled Plasma Mass Spectrometry

Takafumi HIRATA and Yoshiaki KON

Laboratory for Planetary Sciences, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 152-8551, Japan

A laser ablation-inductively coupled plasma-mass spectrometric (LA-ICPMS) technique utilizing a titanium-sapphire (TiS) femtosecond laser (fs-laser) has been developed for elemental and isotopic analysis. The signal intensity profile, depth of the ablation pit and level of elemental fractionation were investigated in order to evaluate the analytical capability of the present fs-laser ablation-ICPMS technique. The signal intensity profile of ⁵⁷Fe, obtained from iron sulfide (FeS₂), demonstrated that the resulting signal intensity of ⁵⁷Fe achieved by the fs-laser ablation was almost 4-times higher than that obtained by ArF excimer laser ablation under a similar energy fluence (5 J/cm²). In fs-laser ablation, there is no significant difference in a depth of the ablation pit between glass and zircon material, while in ArF laser ablation, the resulting crater depth on the zircon crystal was almost half the level than that obtained for glass material. Both the thermal-induced and particle size-related elemental fractionations, which have been thought to be main sources of analytical error in the LA-ICPMS analysis, were measured on a Harvard 91500 zircon crystal. The resulting fractionation indexes on the ²⁰⁶Pb/²³⁸U (f^Pb/U) and ²³⁸U/²³²Th (f^U/Th) ratios obtained by the present fs-laser ablation system were significantly smaller than those obtained by a conventional ArF excimer laser ablation system, demonstrative of smaller elemental fractionation. Using the present fs-laser ablation technique, the time profile of the signal intensity of ⁵⁶Fe and the isotopic ratios (⁵⁷Fe/⁵⁴Fe and 56Fe/54Fe) have been measured on a natural pyrite (FeS₂) sample. Repeatability in signal intensity of ⁵⁶Fe achieved by the fs-laser ablation system was significantly better than that obtained by ArF excimer laser ablation. Moreover, the resulting precision in ⁵⁷Fe/⁵⁴Fe and ⁵⁶Fe/⁵⁴Fe ratio measurements could be improved by the fs-laser ablation system. The data obtained here clearly demonstrate that, even with the fundamental wavelength (NIR operating at 780 nm), the fs-laser ablation system has the potential to become a significant tool for in-situ elemental and isotopic analysis of geochemical samples including heavy minerals and metallic materials.

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