Determination of Titanium Isotopes in Rutiles with High Spatial Resolution by Femtosecond Laser Ablation Multi-collector Inductively Coupled Plasma–Mass Spectrometry (ICP-MS)

Researchers at the China University of Geosciences have developed a new method for determining titanium isotopes in rutiles with great precision and high spatial resolution. The study, published in Spectrochimica Acta Part B: Atomic Spectroscopy, employed femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS) to analyze rutiles—a common form of titanium dioxide (TiO₂) belonging to the group of oxide minerals—and investigate the isotopic fractionation of titanium (1).

Crystallized Carbon Hexagonal System | Image Credit: © ktsdesign - stock.adobe.com

fs-LA-MC-ICP-MS is an advanced analytical technique that involves using a femtosecond laser to ablate rutile samples with high precision and spatial resolution. The ablated material is then transported to an inductively coupled plasma (ICP) source, where it is ionized and introduced into a mass spectrometer. The multi-collector system in the mass spectrometer allows for the simultaneous measurement of multiple isotopes of titanium, providing valuable information on isotopic fractionation. This technique enables researchers to investigate the composition and isotopic ratios of titanium in rutile samples, aiding in the understanding of geological processes and serving as a potential geochemical tracer.

The crystallization of iron-titanium (Fe-Ti) oxides can lead to significant variations in titanium isotopes during magmatic differentiation. Rutile (TiO₂), as a prominent Ti-rich mineral in igneous, metamorphic, and sedimentary rocks, offers valuable insights into geological processes. The newly developed method utilizing fs-LA-MC-ICP-MS enables researchers to determine the mass-dependent isotope fractionation of titanium in rutiles with exceptional precision and spatial resolution.

To enhance the sensitivity of titanium signals and enable low-frequency ablation and high-resolution analysis, a high sensitivity cone combination was utilized. However, the researchers encountered signal fluctuations that commonly occur at low ablation frequencies. To address this issue, a signal smoothing device was employed, ensuring stable and accurate measurements.

The results demonstrated that adjustments in laser parameters and the use of wet plasma conditions effectively mitigated bias towards higher δ⁴⁹Ti values. With these optimizations, the method achieved a spatial resolution of approximately 10 μm horizontally and ≈4 μm vertically. The accuracy of δ⁴⁹Ti results was maintained with a precision of ±0.010%. Long-term measurements of a rutile sample showcased excellent reproducibility, validating the reliability of the method.

Comparison of measurements from nine natural rutile crystals using both solution MC-ICP-MS and fs-LA-MC-ICP-MS yielded consistent δ⁴⁹Ti values, further confirming the robustness of the newly developed technique. Notably, a significant variation in δ⁴⁹Ti (up to ≈0.294%) among the 12 rutile samples was observed, highlighting the potential of titanium isotopes in rutiles as valuable geochemical tracers.

This pioneering method provides unprecedented insights into titanium isotopes within rutile crystals, allowing for a better understanding of geological processes and their implications. The identified homogeneous rutile crystals, such as USA75, Bra12, Sco2, and Bra6, can serve as essential reference materials for future in situ measurement of titanium isotope ratios.

The study establishes a powerful analytical approach for investigating titanium isotopes in rutiles, opening new avenues for geochemical research and advancing our understanding of Earth's geological history.

Reference

(1) Liu, H.; Zhang, W.; Deng, Z.; Hu, Z.; Schiller, M.; Bizzarro, M.; Liu, Y.; Luo, T.; Feng, Y. Feng, L. Determination of titanium isotopes in rutiles with high spatial resolution by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry. Spectrochimica Acta Part B: At. Spectrosc. 2023, 202, 106646. DOI:10.1016/j.sab.2023.106646